Glucose

Glucose (also called dextrose) is a simple sugar with the molecular formula C6H12O6. Glucose is the most abundant monosaccharide,[3] a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using energy from sunlight. There it is used to make cellulose in cell walls, which is the most abundant carbohydrate.[4] In energy metabolism, glucose is the most important source of energy in all organisms. Glucose for metabolism is partially stored as a polymer, in plants mainly as starch and amylopectin and in animals as glycogen. Glucose circulates in the blood of animals as blood sugar. The naturally occurring form of glucose is D-glucose, while L-glucose is produced synthetically in comparably small amounts and is of lesser importance.

Glucose, as intravenous sugar solution, is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system.[5] The name glucose derives through the French from the Greek γλυκός, which means "sweet," in reference to must, the sweet, first press of grapes in the making of wine.[6][7] The suffix "-ose" is a chemical classifier, denoting a sugar.

D-Glucose
Alpha-D-glucopyranose-2D-skeletal

α-D-glucopyranose (chair form)
Alpha-D-Glucopyranose

Haworth projection of α-D-glucopyranose
D-glucose-chain-2D-Fischer

Fischer projection of D-glucose
Names
Pronunciation /ˈɡluːkoʊz/, /ˈɡluːkoʊs/
Preferred IUPAC name
D-Glucose
Systematic IUPAC name
(2R,3S,4R,5R)-2,3,4,5,6-Pentahydroxyhexanal
Other names
Blood sugar
Dextrose
Corn sugar
D-Glucose
Grape sugar
Identifiers
3D model (JSmol)
3DMet B01203
Abbreviations Glc
1281604
ChEBI
ChEMBL
ChemSpider
EC Number 200-075-1
83256
KEGG
MeSH Glucose
RTECS number LZ6600000
UNII
Properties
C6H12O6
Molar mass 180.156 g·mol−1
Appearance White powder
Density 1.54 g/cm3
Melting point α-D-glucose: 146 °C (295 °F; 419 K)
β-D-glucose: 150°C (302°F; 423 K)
909 g/L (25 °C (77 °F))
−101.5×10−6 cm3/mol
8.6827
Thermochemistry
218.6 J K−1 mol−1[1]
209.2 J K−1 mol−1[1]
−1271 kJ/mol[2]
2,805 kJ/mol (670 kcal/mol)
Pharmacology
B05CX01 (WHO) V04CA02 (WHO), V06DC01 (WHO)
Hazards
Safety data sheet ICSC 08655
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oilHealth code 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g., sodium chlorideReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogenSpecial hazards (white): no codeNFPA 704 four-colored diamond
1
0
0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

History

Glucose was first isolated from raisins in 1747 by the German chemist Andreas Marggraf.[8][9] Glucose was discovered in grapes by Johann Tobias Lowitz in 1792 and recognized as different from cane sugar (sucrose). Glucose is the term coined by Jean Baptiste Dumas in 1838, which has prevailed in the chemical literature. Friedrich August Kekulé proposed the term dextrose (from Latin dexter = right), because in aqueous solution of glucose, the plane of linearly polarized light is turned to the right. In contrast, D-fructose (a ketohexose) and L-glucose turn linearly polarized light to the left. The earlier notation according to the rotation of the plane of linearly polarized light (d and l-nomenclature) was later abandoned in favor of the D- and L-notation, which refers to the absolute configuration of the asymmetric center farthest from the carbonyl group, and in concordance with the configuration of D- or L-glyceraldehyde.[10][11]

Since glucose is a basic necessity of many organisms, a correct understanding of its chemical makeup and structure contributed greatly to a general advancement in organic chemistry. This understanding occurred largely as a result of the investigations of Emil Fischer, a German chemist who received the 1902 Nobel Prize in Chemistry for his findings.[12] The synthesis of glucose established the structure of organic material and consequently formed the first definitive validation of Jacobus Henricus van 't Hoff's theories of chemical kinetics and the arrangements of chemical bonds in carbon-bearing molecules.[13] Between 1891 and 1894, Fischer established the stereochemical configuration of all the known sugars and correctly predicted the possible isomers, applying van 't Hoff's theory of asymmetrical carbon atoms. The names initially referred to the natural substances. Their enantiomers were given the same name with the introduction of systematic nomenclatures, taking into account absolute stereochemistry (e.g. Fischer nomenclature, D/L nomenclature).

For the discovery of the metabolism of glucose Otto Meyerhof received the Nobel Prize in Physiology or Medicine in 1922.[14] Hans von Euler-Chelpin was awarded the Nobel Prize in Chemistry along with Arthur Harden in 1929 for their "research on the fermentation of sugar and their share of enzymes in this process".[15][16] In 1947, Bernardo Houssay (for his discovery of the role of the pituitary gland in the metabolism of glucose and the derived carbohydrates) as well as Carl and Gerty Cori (for their discovery of the conversion of glycogen from glucose) received the Nobel Prize in Physiology or Medicine.[17][18][19] In 1970, Luis Leloir was awarded the Nobel Prize in Chemistry for the discovery of glucose-derived sugar nucleotides in the biosynthesis of carbohydrates.[20]

Chemical properties

With six carbon atoms, it is classed as a hexose, a subcategory of the monosaccharides. D-Glucose is one of the sixteen aldohexose stereoisomers. The D-isomer, D-glucose, also known as dextrose, occurs widely in nature, but the L-isomer, L-glucose, does not. Glucose can be obtained by hydrolysis of carbohydrates such as milk sugar (lactose), cane sugar (sucrose), maltose, cellulose, glycogen, etc. It is commonly commercially manufactured from cornstarch by hydrolysis via pressurized steaming at controlled pH in a jet followed by further enzymatic depolymerization.[21] Unbonded glucose is one of the main ingredients of honey. All forms of glucose are colorless and easily soluble in water, acetic acid, and several other solvents. They are only sparingly soluble in methanol and ethanol.

Structure and nomenclature

Glucose is a monosaccharide with formula C6H12O6 or H-(C=O)-(CHOH)5-H, whose five hydroxyl (OH) groups are arranged in a specific way along its six-carbon back. Glucose is usually present in solid form as a monohydrate with a closed pyran ring (dextrose hydrate). In aqueous solution, on the other hand, it is an open-chain to a small extent and is present predominantly as α- or β-pyranose, which partially mutually merge by mutarotation. From aqueous solutions, the three known forms can be crystallized: α-glucopyranose, β-glucopyranose and β-glucopyranose hydrate.[22] Glucose is a building block of the disaccharides lactose and sucrose (cane or beet sugar), of oligosaccharides such as raffinose and of polysaccharides such as starch and amylopectin, glycogen or cellulose. The glass transition temperature of glucose is 31 °C and the Gordon-Taylor constant (an experimentally determined constant for the prediction of the glass transition temperature for different mass fractions of a mixture of two substances)[23] is 4.5.[24]

Different forms and projections of D-Glucose in comparison
Natta projection Haworth projection
D-Glucose Keilstrich Alpha-D-Glucofuranose
α-D-glucofuranose
Beta-D-Glucofuranose
β-D-glucofuranose
Alpha-D-Glucopyranose
α-D-glucopyranose
Beta-D-Glucopyranose
β-D-glucopyranose
α-D-Glucopyranose in (1) Tollens/Fischer- (2) Haworth-projection (3) chair conformation (4) stereochemical view
Alpha glucose views

Open-chain form

Glucose Fisher to Haworth
Glucose can exist in both a straight-chain and ring form.

In its fleeting open-chain form, the glucose molecule has an open (as opposed to cyclic) and unbranched backbone of six carbon atoms, C-1 through C-6; where C-1 is part of an aldehyde group H(C=O)-, and each of the other five carbons bears one hydroxyl group -OH. The remaining bonds of the backbone carbons are satisfied by hydrogen atoms -H. Therefore, glucose is both a hexose and an aldose, or an aldohexose. The aldehyde group makes glucose a reducing sugar giving a positive reaction with the Fehling test.

Each of the four carbons C-2 through C-5 is a stereocenter, meaning that its four bonds connect to four different substituents. (Carbon C-2, for example, connects to -(C=O)H, -OH, -H, and -(CHOH)4H.) In D-glucose, these four parts must be in a specific three-dimensional arrangement. Namely, when the molecule is drawn in the Fischer projection, the hydroxyls on C-2, C-4, and C-5 must be on the right side, while that on C-3 must be on the left side.

The positions of those four hydroxyls are exactly reversed in the Fischer diagram of L-glucose. D- and L-glucose are two of the 16 possible aldohexoses; the other 14 are allose, altrose, galactose, gulose, idose, mannose, and talose, each with two enantiomers, “D-” and “L-”.

It is important to note that the linear form of glucose makes up less than 0.02% of the glucose molecules in a water solution. The rest is one of two cyclic forms of glucose that are formed when the hydroxyl group on carbon 5 (C5) bonds to the aldehyde carbon 1 (C1).

Cyclic forms


From left to right: Haworth projections and ball-and-stick structures of the α- and β- anomers of D-glucopyranose (top row) and D-glucofuranose (bottom row)

Alpha-D-Glucopyranose
Alpha-D-glucose-from-xtal-1979-3D-balls
Beta-D-Glucopyranose
Beta-D-glucose-from-xtal-3D-balls
Alpha-D-Glucofuranose
Alpha-D-Glucofuranose Molekülbaukasten 9134 (crop)
Beta-D-Glucofuranose
Beta-D-Glucofuranose Molekülbaukasten 9136 (crop)

In solutions, the open-chain form of glucose (either "D-" or "L-") exists in equilibrium with several cyclic isomers, each containing a ring of carbons closed by one oxygen atom. In aqueous solution however, more than 99% of glucose molecules, at any given time, exist as pyranose forms. The open-chain form is limited to about 0.25% and furanose forms exists in negligible amounts. The terms "glucose" and "D-glucose" are generally used for these cyclic forms as well. The ring arises from the open-chain form by an intramolecular nucleophilic addition reaction between the aldehyde group (at C-1) and either the C-4 or C-5 hydroxyl group, forming a hemiacetal linkage, -C(OH)H-O-.

The reaction between C-1 and C-5 yields a six-membered heterocyclic system called a pyranose, which is a monosaccharide sugar (hence "–ose") containing a derivatised pyran skeleton. The (much rarer) reaction between C-1 and C-4 yields a five-membered furanose ring, named after the cyclic ether furan. In either case, each carbon in the ring has one hydrogen and one hydroxyl attached, except for the last carbon (C-4 or C-5) where the hydroxyl is replaced by the remainder of the open molecule (which is -(C(CH2OH)HOH)-H or -(CHOH)-H, respectively).

The ring-closing reaction makes carbon C-1 chiral, too, since its four bonds lead to -H, to -OH, to carbon C-2, and to the ring oxygen. These four parts of the molecule may be arranged around C-1 (the anomeric carbon) in two distinct ways, designated by the prefixes "α-" and "β-". When a glucopyranose molecule is drawn in the Haworth projection, the designation "α-" means that the hydroxyl group attached to C-1 and the -CH2OH group at C-5 lies on opposite sides of the ring's plane (a trans arrangement), while "β-" means that they are on the same side of the plane (a cis arrangement). Therefore, the open-chain isomer D-glucose gives rise to four distinct cyclic isomers: α-D-glucopyranose, β-D-glucopyranose, α-D-glucofuranose, and β-D-glucofuranose. These five structures exist in equilibrium and interconvert, and the interconversion is much more rapid with acid catalysis.

Widely proposed arrow-pushing mechanism for acid-catalyzed dynamic equilibrium between the α- and β- anomers of D-glucopyranose


Chair conformations of α- (left) and β- (right) D-glucopyranose

ALPHA-D-Glucopyranose V.1
BETA-D-Glucopyranose V.1

The other open-chain isomer L-glucose similarly gives rise to four distinct cyclic forms of L-glucose, each the mirror image of the corresponding D-glucose.

The rings are not planar, but are twisted in three dimensions. The glucopyranose ring (α or β) can assume several non-planar shapes, analogous to the "chair" and "boat" conformations of cyclohexane. Similarly, the glucofuranose ring may assume several shapes, analogous to the "envelope" conformations of cyclopentane.

In the solid state, only the glucopyranose forms are observed, forming colorless crystalline solids that are highly soluble in water and acetic acid but poorly soluble in methanol and ethanol. They melt at 146 °C (295 °F) (α) and 150 °C (302 °F) (β), and decompose at higher temperatures into carbon and water.

Rotational isomers

Each glucose isomer is subject to rotational isomerism. Within the cyclic form of glucose, rotation may occur around the O6-C6-C5-O5 torsion angle, termed the ω-angle, to form three staggered rotamer conformations called gauche-gauche (gg), gauche-trans (gt) and trans-gauche (tg).[25] There is a tendency for the ω-angle to adopt a gauche conformation, a tendency that is attributed to the gauche effect.

Mutarotation

Mutarotation D-Glucose V.1
Mutarotation: D-glucose molecules exist as cyclic hemiacetals that are epimeric (= diastereomeric) to each other. The epimeric ratio α:β is 36:64. In the α-D-glucopyranose (left), the blue-labeled hydroxy group is in the axial position at the anomeric center, whereas in the β-D-glucopyranose (right) the blue-labeled hydroxy group is in equatorial position at the anomeric center.

Mutarotation consists of a temporary reversal of the ring-forming reaction, resulting in the open-chain form, followed by a reforming of the ring. The ring closure step may use a different -OH group than the one recreated by the opening step (thus switching between pyranose and furanose forms), or the new hemiacetal group created on C-1 may have the same or opposite handedness as the original one (thus switching between the α and β forms). Thus, though the open-chain form is barely detectable in solution, it is an essential component of the equilibrium.

The open-chain form is thermodynamically unstable, and it spontaneously isomerizes to the cyclic forms. (Although the ring closure reaction could in theory create four- or three-atom rings, these would be highly strained, and are not observed in practice.) In solutions at room temperature, the four cyclic isomers interconvert over a time scale of hours, in a process called mutarotation.[26] Starting from any proportions, the mixture converges to a stable ratio of α:β 36:64. The ratio would be α:β 11:89 if it were not for the influence of the anomeric effect.[27] Mutarotation is considerably slower at temperatures close to 0 °C (32 °F).

Optical activity

Whether in water or in the solid form, D-(+)-glucose is dextrorotatory, meaning it will rotate the direction of polarized light clockwise as seen looking toward the light source. The effect is due to the chirality of the molecules, and indeed the mirror-image isomer, L-(−)-glucose, is levorotatory (rotates polarized light counterclockwise) by the same amount. The strength of the effect is different for each of the five tautomers.

Note that the D- prefix does not refer directly to the optical properties of the compound. It indicates that the C-5 chiral center has the same handedness as that of D-glyceraldehyde (which was so labeled because it is dextrorotatory). The fact that D-glucose is dextrorotatory is a combined effect of its four chiral centers, not just of C-5; and indeed some of the other D-aldohexoses are levorotatory.

The conversion between the two anomers can be observed in a polarimeter, since pure α- D- glucose has a specific rotation angle of +112.2 ° · ml · dm−1 · g−1, pure β- D- glucose of +17.5 ° · ml · dm−1 · g−1.[28] When equilibrium has been reached after a certain time due to mutarotation, the angle of rotation is +52.7 ° · ml · dm−1 · g−1.[28] By adding acid or base, this transformation is much accelerated. The equilibration takes place via the open-chain aldehyde form.

Isomerisation

In dilute sodium hydroxide or other dilute bases, the monosaccharides mannose, glucose and fructose interconvert (via a Lobry de Bruyn–Alberda–van Ekenstein transformation), so that a balance between these isomers is formed. This reaction proceeds via an enediol:

Glucose-Fructose-Mannose-isomerisation

Glucose-Fructose-Mannose-isomerisation

Biochemical properties

Metabolism of common monosaccharides and some biochemical reactions of glucose
Metabolism of common monosaccharides, and related reactions

Glucose is the most abundant monosaccharide. Glucose is also the most widely used aldohexose in most living organisms. One possible explanation for this is that glucose has a lower tendency than other aldohexoses to react nonspecifically with the amine groups of proteins.[29] This reaction—glycation—impairs or destroys the function of many proteins,[29] e.g. in glycated hemoglobin. Glucose's low rate of glycation can be attributed to its having a more stable cyclic form compared to other aldohexoses, which means it spends less time than they do in its reactive open-chain form.[29] The reason for glucose having the most stable cyclic form of all the aldohexoses is that its hydroxy groups (with the exception of the hydroxy group on the anomeric carbon of D-glucose) are in the equatorial position. Presumably, glucose is the most abundant natural monosaccharide because it is less glycated with proteins than other monosaccharides.[29][30] Another hypothesis is that glucose, being the only D-aldohexose that has all five hydroxy substituents in the equatorial position in the form of β-D-glucose, is more readily accessible to chemical reactions,[31] for example, for esterification[32] or acetal formation.[33] For this reason, D-glucose is also a highly preferred building block in natural polysaccharides (glycans). Polysaccharides that are composed solely of Glucose are termed glucans.

Glucose is produced by plants through the photosynthesis using sunlight, water and carbon dioxide and can be used by all living organisms as an energy and carbon source. However, most glucose does not occur in its free form, but in the form of its polymers, i.e. lactose, sucrose, starch and others which are energy reserve substances, and cellulose and chitin, which are components of the cell wall in plants or fungi and arthropods, respectively. These polymers are degraded to glucose during food intake by animals, fungi and bacteria using enzymes. All animals are also able to produce glucose themselves from certain precursors as the need arises. Nerve cells, cells of the renal medulla and erythrocytes depend on glucose for their energy production.[34] In adult humans, there are about 18 g of glucose,[35] of which about 4 g are present in the blood.[36] Approximately 180 to 220 g of glucose are produced in the liver of an adult in 24 hours.[35]

Many of the long-term complications of diabetes (e.g., blindness, kidney failure, and peripheral neuropathy) are probably due to the glycation of proteins or lipids.[37] In contrast, enzyme-regulated addition of sugars to protein is called glycosylation and is essential for the function of many proteins.[38]

Uptake

Ingested glucose initially binds to the receptor for sweet taste on the tongue in humans. This complex of the proteins T1R2 and T1R3 makes it possible to identify glucose-containing food sources. Glucose mainly comes from food - about 300 g per day are produced by conversion of food,[39] but it is also synthesized from other metabolites in the body's cells. In humans, the breakdown of glucose-containing polysaccharides happens in part already during chewing by means of amylase, which is contained in saliva, as well as by maltase, lactase and sucrase on the brush border of the small intestine. Glucose is a building block of many carbohydrates and can be split off from them using certain enzymes. Glucosidases, a subgroup of the glycosidases, first catalyze the hydrolysis of long-chain glucose-containing polysaccharides, removing terminal glucose. In turn, disaccharides are mostly degraded by specific glycosidases to glucose. The names of the degrading enzymes are often derived from the particular poly- and disaccharide; inter alia, for the degradation of polysaccharide chains there are amylases (named after amylose, a component of starch), cellulases (named after cellulose), chitinases (named after chitin) and more. Furthermore, for the cleavage of disaccharides, there are maltase, lactase, sucrase, trehalase and others. In humans, about 70 genes are known that code for glycosidases. They have functions in the digestion and degradation of glycogen, sphingolipids, mucopolysaccharides and poly(ADP-ribose). Humans do not produce cellulases, chitinases and trehalases, but the bacteria in the gut flora do.

In order to get into or out of cell membranes of cells and membranes of cell compartments, glucose requires special transport proteins from the major facilitator superfamily. In the small intestine (more precisely, in the jejunum),[40] glucose is taken up into the intestinal epithelial cells with the help of glucose transporters[41] via a secondary active transport mechanism called sodium ion-glucose symport via the sodium/glucose cotransporter 1.[42] The further transfer occurs on the basolateral side of the intestinal epithelial cells via the glucose transporter GLUT2,[42] as well as their uptake into liver cells, kidney cells, cells of the islets of Langerhans, nerve cells, astrocytes and tanyocytes.[43] Glucose enters the liver via the vena portae and is stored there as a cellular glycogen.[44] In the liver cell, it is phosphorylated by glucokinase at position 6 to glucose-6-phosphate, which can not leave the cell. With the help of glucose-6-phosphatase, glucose-6-phosphate is converted back into glucose exclusively in the liver, if necessary, so that it is available for maintaining a sufficient blood glucose concentration. In other cells, uptake happens by passive transport through one of the 14 GLUT proteins.[42] In the other cell types, phosphorylation occurs through a hexokinase, whereupon glucose can no longer diffuse out of the cell.

The glucose transporter GLUT1 is produced by most cell types and is of particular importance for nerve cells and pancreatic β-cells.[42] GLUT3 is highly expressed in nerve cells.[42] Glucose from the bloodstream is taken up by GLUT4 from muscle cells (of the skeletal muscle[45] and heart muscle) and fat cells.[46] GLUT14 is formed exclusively in testes.[42] Excess glucose is broken down and converted into fatty acids, which are stored as triacylglycerides. In the kidneys, glucose in the urine is absorbed via SGLT1 and SGLT2 in the apical cell membranes and transmitted via GLUT2 in the basolateral cell membranes.[47] About 90% of kidney glucose reabsorption is via SGLT2 and about 3% via SGLT1.[48]

Biosynthesis

In plants and some prokaryotes, glucose is a product of photosynthesis.[49] Glucose is also formed by the breakdown of polymeric forms of glucose like glycogen (in animals and mushrooms) or starch (in plants). The cleavage of glycogen is termed glycogenolysis, the cleavage of starch is called starch degradation.[50]

The metabolic pathway that begins with molecules containing two to four carbon atoms (C) and ends in the glucose molecule containing six carbon atoms is called gluconeogenesis and occurs in all living organisms. The smaller starting materials are the result of other metabolic pathways. Ultimately almost all biomolecules come from the assimilation of carbon dioxide in plants during photosynthesis.[51] The free energy of formation of α-D-glucose is 917.2 kilojoules per mole.[52] In humans, gluconeogenesis occurs in the liver and kidney,[53] but also in other cell types. In the liver about 150 g of glycogen are stored, in skeletal muscle about 250 g.[54] However, the glucose released in muscle cells upon cleavage of the glycogen can not be delivered to the circulation because glucose is phosphorylated by the hexokinase, and a glucose-6-phosphatase is not expressed to remove the phosphate group. Unlike for glucose, there is no transport protein for glucose-6-phosphate. Gluconeogenesis allows the organism to build up glucose from other metabolites, including lactate or certain amino acids, while consuming energy. The renal tubular cells can also produce glucose.

Glucose Degradation

Glucose metabolism
Glucose metabolism and various forms of it in the process
Glucose-containing compounds and isomeric forms are digested and taken up by the body in the intestines, including starch, glycogen, disaccharides and monosaccharides.
Glucose is stored in mainly the liver and muscles as glycogen. It is distributed and used in tissues as free glucose.

In humans, glucose is metabolised by glycolysis[55] and the pentose phosphate pathway.[56] Glycolysis is used by all living organisms,[57][58] with small variations, and all organisms generate energy from the breakdown of monosaccharides.[57] In the further course of the metabolism, it can be completely degraded via oxidative decarboxylation, the Krebs cycle (synonym citric acid cycle) and the respiratory chain to water and carbon dioxide. If there is not enough oxygen available for this, the glucose degradation in animals occurs anaerobic to lactate via lactic acid fermentation and releases less energy. Muscular lactate enters the liver through the bloodstream in mammals, where gluconeogenesis occurs (Cori cycle). With a high supply of glucose, the metabolite acetyl-CoA from the Krebs cycle can also be used for fatty acid synthesis.[59] Glucose is also used to replenish the body's glycogen stores, which are mainly found in liver and skeletal muscle. These processes are hormonally regulated.

In other living organisms, other forms of fermentation can occur. The bacterium Escherichia coli can grow on nutrient media containing glucose as the sole carbon source.[52] In some bacteria and, in modified form, also in archaea, glucose is degraded via the Entner-Doudoroff pathway.[60]

Use of glucose as an energy source in cells is by either aerobic respiration, anaerobic respiration, or fermentation. The first step of glycolysis is the phosphorylation of glucose by a hexokinase to form glucose 6-phosphate. The main reason for the immediate phosphorylation of glucose is to prevent its diffusion out of the cell as the charged phosphate group prevents glucose 6-phosphate from easily crossing the cell membrane.[61] Furthermore, addition of the high-energy phosphate group activates glucose for subsequent breakdown in later steps of glycolysis. At physiological conditions, this initial reaction is irreversible.

In anaerobic respiration, one glucose molecule produces a net gain of two ATP molecules (four ATP molecules are produced during glycolysis through substrate-level phosphorylation, but two are required by enzymes used during the process).[62] In aerobic respiration, a molecule of glucose is much more profitable in that a maximum net production of 30 or 32 ATP molecules (depending on the organism) through oxidative phosphorylation is generated.[63]

Click on genes, proteins and metabolites below to link to respective articles. [§ 1]

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GlycolysisGluconeogenesis_WP534go to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to WikiPathwaysgo to articlego to Entrezgo to article
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Glycolysis and Gluconeogenesis edit
GlycolysisGluconeogenesis_WP534go to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to WikiPathwaysgo to articlego to Entrezgo to article
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GlycolysisGluconeogenesis_WP534go to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to articlego to WikiPathwaysgo to articlego to Entrezgo to article
  1. ^ The interactive pathway map can be edited at WikiPathways: "GlycolysisGluconeogenesis_WP534".

Tumor cells often grow comparatively quickly and consume an above-average amount of glucose by glycolysis,[64] which leads to the formation of lactate, the end product of fermentation in mammals, even in the presence of oxygen. This effect is called the Warburg effect. For the increased uptake of glucose in tumors various SGLT and GLUT are overly produced.[65][66]

In yeast, ethanol is fermented at high glucose concentrations, even in the presence of oxygen (which normally leads to respiration but not to fermentation). This effect is called the Crabtree effect.

Energy source

Glucose catabolism intermediates de
Diagram showing the possible intermediates in glucose degradation; Metabolic pathways orange: glycolysis, green: Entner-Doudoroff pathway, phosphorylating, yellow: Entner-Doudoroff pathway, non-phosphorylating

Glucose is a ubiquitous fuel in biology. It is used as an energy source in organisms, from bacteria to humans, through either aerobic respiration, anaerobic respiration (in bacteria), or fermentation. Glucose is the human body's key source of energy, through aerobic respiration, providing about 3.75 kilocalories (16 kilojoules) of food energy per gram.[67] Breakdown of carbohydrates (e.g., starch) yields mono- and disaccharides, most of which is glucose. Through glycolysis and later in the reactions of the citric acid cycle and oxidative phosphorylation, glucose is oxidized to eventually form carbon dioxide and water, yielding energy mostly in the form of ATP. The insulin reaction, and other mechanisms, regulate the concentration of glucose in the blood. The physiological caloric value of glucose, depending on the source, is 16.2 kilojoules per gram[68] and 15.7 kJ/g (3.74 kcal/g), respectively.[69] The high availability of carbohydrates from plant biomass has led to a variety of methods during evolution, especially in microorganisms, to utilize the energy and carbon storage glucose. Differences exist in which end product can no longer be used for energy production. The presence of individual genes, and their gene products, the enzymes, determine which reactions are possible. The metabolic pathway of glycolysis is used by almost all living beings. An essential difference in the use of glycolysis is the recovery of NADPH as a reductant for anabolism that would otherwise have to be generated indirectly.[70]

Glucose supplies almost all the energy for the brain,[71] so its availability influences psychological processes. When glucose is low, psychological processes requiring mental effort (e.g., self-control, effortful decision-making) are impaired.[72][73][74][75] In the brain, which is dependent on glucose as the major source of energy, the glucose concentration is usually 4 to 6 mM (5 mM equals 90 mg / dL),[35] but decreases to 2 to 3 mM when fasting.[76] Confusion occurs below 1 mM and coma at lower levels.[76]

The glucose in the blood is called blood sugar. Blood sugar levels are regulated by glucose-binding nerve cells in the hypothalamus.[77] In addition, glucose in the brain binds to glucose receptors of the reward system in the nucleus accumbens.[77] The binding of glucose to the sweet receptor on the tongue induces a release of various hormones of energy metabolism, either through glucose or through other sugars, leading to an increased cellular uptake and lower blood sugar levels.[78] Artificial sweeteners do not lower blood sugar levels.[78]

The blood sugar content of a healthy person in the short-time fasting state, e.g. after overnight fasting, is about 70 to 100 mg/dl of blood (4 to 5.5 mM). In blood plasma, the measured values are about 10-15 % higher. In addition, the values in the arterial blood are higher than the concentrations in the venous blood since glucose is absorbed into the tissue during the passage of the capillary bed. Also in the capillary blood, which is often used for blood sugar determination, the values are sometimes higher than in the venous blood. The glucose content of the blood is regulated by the hormones insulin, incretin and glucagon.[77][79] Insulin lowers the glucose level, glucagon increases it.[35] Furthermore, the hormones adrenaline, thyroxine, glucocorticoids, somatotropin and adrenocorticotropin lead to an increase in the glucose level.[35] In addition, there is also a hormone-independent regulation, which is referred to as glucose autoregulation.[80] After food intake the blood sugar concentration increases. Values over 180 mg/dl in venous whole blood are pathological and are termed hyperglycemia, values below 40 mg/dl are termed hypoglycaemia.[81] When needed, glucose is released into the bloodstream by glucose-6-phosphatase from glucose-6-phosphate originating from liver and kidney glycogen, thereby regulating the homeostasis of blood glucose concentration.[53][34] In ruminants, the blood glucose concentration is lower (60 mg/dL in cattle and 40 mg/dL in sheep), because the carbohydrates are converted more by their gut flora into short-chain fatty acids.[82]

Some glucose is converted to lactic acid by astrocytes, which is then utilized as an energy source by brain cells; some glucose is used by intestinal cells and red blood cells, while the rest reaches the liver, adipose tissue and muscle cells, where it is absorbed and stored as glycogen (under the influence of insulin). Liver cell glycogen can be converted to glucose and returned to the blood when insulin is low or absent; muscle cell glycogen is not returned to the blood because of a lack of enzymes. In fat cells, glucose is used to power reactions that synthesize some fat types and have other purposes. Glycogen is the body's "glucose energy storage" mechanism, because it is much more "space efficient" and less reactive than glucose itself.

As a result of its importance in human health, glucose is an analyte in glucose tests that are common medical blood tests.[83] Eating or fasting prior to taking a blood sample has an effect on analyses for glucose in the blood; a high fasting glucose blood sugar level may be a sign of prediabetes or diabetes mellitus.[84]

The glycemic index is an indicator of the speed of resorption and conversion to blood glucose levels from ingested carbohydrates, measured as the area under the curve of blood glucose levels after consumption in comparison to glucose (glucose is defined as 100).[85] The clinical importance of the glycemic index is controversial,[85][86] as foods with high fat contents slow the resorption of carbohydrates and lower the glycemic index, e.g. ice cream.[86] An alternative indicator is the insulin index,[87] measured as the impact of carbohydrate consumption on the blood insulin levels. The glycemic load is an indicator for the amount of glucose added to blood glucose levels after consumption, based on the glycemic index and the amount of consumed food.

Precursor

Organisms use glucose as a precursor for the synthesis of several important substances. Starch, cellulose, and glycogen ("animal starch") are common glucose polymers (polysaccharides). Some of these polymers (starch or glycogen) serve as energy stores, while others (cellulose and chitin, which is made from a derivative of glucose) have structural roles. Oligosaccharides of glucose combined with other sugars serve as important energy stores. These include lactose, the predominant sugar in milk, which is a glucose-galactose disaccharide, and sucrose, another disaccharide which is composed of glucose and fructose. Glucose is also added onto certain proteins and lipids in a process called glycosylation. This is often critical for their functioning. The enzymes that join glucose to other molecules usually use phosphorylated glucose to power the formation of the new bond by coupling it with the breaking of the glucose-phosphate bond.

Other than its direct use as a monomer, glucose can be broken down to synthesize a wide variety of other biomolecules. This is important, as glucose serves both as a primary store of energy and as a source of organic carbon. Glucose can be broken down and converted into lipids. It is also a precursor for the synthesis of other important molecules such as vitamin C (ascorbic acid). In living organisms, glucose is converted to several other chemical compounds that are the starting material for various metabolic pathways. Among them, all other monosaccharides[88] such as fructose (via the polyol pathway),[42] mannose (the epimer of glucose at position 2), galactose (the epimer at position 4), fucose, various uronic acids and the amino sugars are produced from glucose.[44] In addition to the phosphorylation to glucose-6-phosphate, which is part of the glycolysis, glucose can be oxidized during its degradation to glucono-1,5-lactone. Glucose is used in some bacteria as a building block in the trehalose or the dextran biosynthesis and in animals as a building block of glycogen. Glucose can also be converted from bacterial xylose isomerase to fructose. In addition, glucose metabolites produce all nonessential amino acids, sugar alcohols such as mannitol and sorbitol, fatty acids, cholesterol and nucleic acids.[88] Finally, glucose is used as a building block in the glycosylation of proteins to glycoproteins, glycolipids, peptidoglycans, glycosides and other substances (catalyzed by glycosyltransferases) and can be cleaved from them by glycosidases.

Pathology

Diabetes

Diabetes is a metabolic disorder where the body is unable to regulate levels of glucose in the blood either because of a lack of insulin in the body or the failure, by cells in the body, to respond properly to insulin. Each of these situations can be caused by persistently high elevations of blood glucose levels, through pancreatic burnout and insulin resistance. The pancreas is the organ responsible for the secretion of the hormones insulin and glucagon.[89] Insulin is a hormone that regulates glucose levels, allowing the body's cells to absorb and use glucose. Without it, glucose cannot enter the cell and therefore cannot be used as fuel for the body's functions.[90] If the pancreas is exposed to persistently high elevations of blood glucose levels, the insulin-producing cells in the pancreas could be damaged, causing a lack of insulin in the body. Insulin resistance occurs when the pancreas tries to produce more and more insulin in response to persistently elevated blood glucose levels. Eventually, the rest of the body becomes resistant to the insulin that the pancreas is producing, thereby requiring more insulin to achieve the same blood glucose-lowering effect, and forcing the pancreas to produce even more insulin to compete with the resistance. This negative spiral contributes to pancreatic burnout, and the disease progression of diabetes.

To monitor the body's response to blood glucose-lowering therapy, glucose levels can be measured. Blood glucose monitoring can be performed by multiple methods, such as the fasting glucose test which measures the level of glucose in the blood after 8 hours of fasting. Another test is the 2-hour glucose tolerance test (GTT) – for this test, the person has a fasting glucose test done, then drinks a 75-gram glucose drink and is retested. This test measures the ability of the person's body to process glucose. Over time the blood glucose levels should decrease as insulin allows it to be taken up by cells and exit the blood stream.

Overweight and fatty liver

An increased intake of glucose leads to obesity and, in consequence, partly to the metabolic syndrome with non-alcoholic fatty liver disease,[91] but not the consumption of glucose as part of a normal calorie intake.

Hypoglycemia management

Soluţie glucoză 5%
Glucose, 5% solution for infusions

Individuals with diabetes or other conditions that result in low blood sugar often carry small amounts of sugar in various forms. One sugar commonly used is glucose, often in the form of glucose tablets (glucose pressed into a tablet shape sometimes with one or more other ingredients as a binder), hard candy, or sugar packet.

Sources

Glucose 2
Glucose tablets

Most dietary carbohydrates contain glucose, either as their only building block (as in the polysaccharides starch and glycogen), or together with another monosaccharide (as in the hetero-polysaccharides sucrose and lactose).[92] Unbounded glucose is one of the main ingredients of honey.

Table 1. Sugar content of selected common plant foods (g/100g)[93]
Food Item Total
CarbohydrateA
including
"dietary fiber"
Total
Sugars
Free
Fructose
Free
Glucose
Sucrose Fructose/
Glucose
Ratio
Sucrose
as a % of
Total Sugars
Fruits              
Apple 13.8 10.4 5.9 2.4 2.1 2.0 19.9
Apricot 11.1 9.2 0.9 2.4 5.9 0.7 63.5
Banana 22.8 12.2 4.9 5.0 2.4 1.0 20.0
Fig, dried 63.9 47.9 22.9 24.8 0.9 0.93 0.15
Grapes 18.1 15.5 8.1 7.2 0.2 1.1 1
Navel orange 12.5 8.5 2.25 2.0 4.3 1.1 50.4
Peach 9.5 8.4 1.5 2.0 4.8 0.9 56.7
Pear 15.5 9.8 6.2 2.8 0.8 2.1 8.0
Pineapple 13.1 9.9 2.1 1.7 6.0 1.1 60.8
Plum 11.4 9.9 3.1 5.1 1.6 0.66 16.2
Vegetables              
Beet, Red 9.6 6.8 0.1 0.1 6.5 1.0 96.2
Carrot 9.6 4.7 0.6 0.6 3.6 1.0 77
Red Pepper, Sweet 6.0 4.2 2.3 1.9 0.0 1.2 0.0
Onion, Sweet 7.6 5.0 2.0 2.3 0.7 0.9 14.3
Sweet Potato 20.1 4.2 0.7 1.0 2.5 0.9 60.3
Yam 27.9 0.5 traces traces traces na traces
Sugar Cane 13–18 0.2 – 1.0 0.2 – 1.0 11–16 1.0 high
Sugar Beet 17–18 0.1 – 0.5 0.1 – 0.5 16–17 1.0 high
Grains              
Corn, Sweet 19.0 6.2 1.9 3.4 0.9 0.61 15.0

^A The carbohydrate figure is calculated in the USDA database and does not always correspond to the sum of the sugars, the starch, and the "dietary fiber".

All data with a unit of g (gram) are based on 100 g of a food item.

Commercial production

Glucose is produced industrially from starch by enzymatic hydrolysis using glucose amylase or by the use of acids. The enzymatic hydrolysis has largely displaced the acid-catalyzed hydrolysis.[94] The result is glucose syrup (enzymatically with more than 90% glucose in the dry matter)[94] with an annual worldwide production volume of 20 million tonnes (as of 2011).[95] This is the reason for the former common name "starch sugar". The amylases most often come from Bacillus licheniformis[96] or Bacillus subtilis (strain MN-385),[96] which are more thermostable than the originally used enzymes.[96][97] Starting in 1982, pullulanases from Aspergillus niger were used in the production of glucose syrup to convert amylopectin to starch (amylose), thereby increasing the yield of glucose.[98] The reaction is carried out at a pH of 4.6-5.2 and a temperature of 55-60 °C.[8] Corn syrup has between 20% and 95% glucose in the dry matter.[99][100] The Japanese form of the glucose syrup, Mizuame, is made from sweet potato or rice starch.[101] Maltodextrin contains about 20% glucose.

Many crops can be used as the source of starch. Maize,[94] rice,[94] wheat,[94] cassava,[94] potato,[94] barley,[94] sweet potato,[102] corn husk and sago are all used in various parts of the world. In the United States, corn starch (from maize) is used almost exclusively. Some commercial glucose occurs as a component of invert sugar, a roughly 1:1 mixture of glucose and fructose that is produced from sucrose. In principle, cellulose could be hydrolysed to glucose, but this process is not yet commercially practical.[22]

Conversion to fructose

In the USA almost exclusively corn (more precisely: corn syrup) is used as glucose source for the production of isoglucose, which is a mixture of glucose and fructose, since fructose has a higher sweetening power — with same physiological calorific value of 374 kilocalories per 100 g. The annual world production of isoglucose is eight million tonnes (as of 2011).[95] When made from corn syrup, the final product is high fructose corn syrup (HFCS).

Commercial usage

Relativesweetness
Relative Sweetness of various sugars in comparison with sucrose[103]

Glucose is mainly used for the production of fructose and in the production of glucose-containing foods. In foods, it is used as a sweetener, humectant, to increase the volume and to create a softer mouthfeel.[94] Various sources of glucose, such as grape juice (for wine) or malt (for beer), are used for fermentation to ethanol during the production of alcoholic beverages. Most soft drinks in the US use HFCS-55 (with a fructose content of 55% in the dry mass), while most other HFCS-sweetened foods in the US use HFCS-42 (with a fructose content of 42% in the dry mass).[104] In the neighboring country Mexico, on the other hand, cane sugar is used in the soft drink as a sweetener, which has a higher sweetening power.[105] In addition, glucose syrup is used, inter alia, in the production of confectionery such as candies, toffee and fondant.[106] Typical chemical reactions of glucose when heated under water-free conditions are the caramelization and, in presence of amino acids, the maillard reaction.

In addition, various organic acids can be biotechnologically produced from glucose, for example by fermentation with Clostridium thermoaceticum to produce acetic acid, with Penicilium notatum for the production of araboascorbic acid, with Rhizopus delemar for the production of fumaric acid, with Aspergillus niger for the production of gluconic acid, with Candida brumptii to produce isocitric acid, with Aspergillus terreus for the production of itaconic acid, with Pseudomonas fluorescens for the production of 2-ketogluconic acid, with Gluconobacter suboxydans for the production of 5-ketogluconic acid, with Aspergillus oryzae for the production of kojic acid, with Lactobacillus delbrueckii for the production of lactic acid, with Lactobacillus brevis for the production of malic acid, with Propionibacter shermanii for the production of propionic acid, with Pseudomonas aeruginosa for the production of pyruvic acid and with Gluconobacter suboxydans for the production of tartaric acid.[107]

Analysis

Specifically, when a glucose molecule is to be detected at a certain position in a larger molecule, nuclear magnetic resonance spectroscopy, X-ray crystallography analysis or lectin immunostaining is performed with concanavalin A reporter enzyme conjugate (that binds only glucose or mannose).

Classical qualitative detection reactions

These reactions have only historical significance:

Fehling Test

The Fehling test is a classic method for the detection of aldoses.[108] Due to mutarotation, glucose is always present to a small extent as an open-chain aldehyde. By adding the Fehling reagents (Fehling (I) solution and Fehling (II) solution), the aldehyde group is oxidized to a carboxylic acid, while the Cu2+ tartrate complex is reduced to Cu+ and forming a brick red precipitate (Cu2O).

Tollens Test

In the Tollens test, after addition of ammoniacal AgNO3 to the sample solution, Ag+ is reduced by glucose to elemental silver.[109]

Barfoed test

In Barfoed's test,[110] a solution of dissolved copper acetate, sodium acetate and acetic acid is added to the solution of the sugar to be tested and subsequently heated in a water bath for a few minutes. Glucose and other monosaccharides rapidly produce a reddish color and reddish brown copper(I) oxide (Cu2O).

Nylander's Test

As a reducing sugar, glucose reacts in the Nylander's test.[111]

Other tests

Upon heating a dilute potassium hydroxide solution with glucose to 100 °C, a strong reddish browning and a caramel-like odor develops.[112] Concentrated sulfuric acid dissolves dry glucose without blackening at room temperature forming sugar sulfuric acid.[112] In a yeast solution, alcoholic fermentation produces carbon dioxide in the ratio of 2.0454 molecules of glucose to one molecule of CO2.[112] Glucose forms a black mass with stannous chloride.[112] In an ammoniacal silver solution, glucose (as well as lactose and dextrin) leads to the deposition of silver. In an ammoniacal lead acetate solution, white lead glycoside is formed in the presence of glucose, which becomes less soluble on cooking and turns brown.[112] In an ammoniacal copper solution, yellow copper oxide hydrate is formed with glucose at room temperature, while red copper oxide is formed during boiling (same with dextrin, except for with an ammoniacal copper acetate solution).[112] With Hager's reagent, glucose forms mercury oxide during boiling.[112] An alkaline bismuth solution is used to precipitate elemental, black-brown bismuth with glucose.[112] Glucose boiled in an ammonium molybdate solution turns the solution blue. A solution with indigo carmine and sodium carbonate destains when boiled with glucose.[112]

Instrumental Quantification

Refractometry and Polarimetry

In concentrated solutions of glucose with a low proportion of other carbohydrates, its concentration can be determined with a polarimeter. For sugar mixtures, the concentration can be determined with a refractometer, for example in the Oechsle determination in the course of the production of wine.

Photometric enzymatic methods in solution

The enzyme glucose oxidase (GOx) converts glucose into gluconic acid and hydrogen peroxide while consuming oxygen. Another enzyme, peroxidase, catalyzes a chromogenic reaction (Trinder reaction)[113] of phenol with 4-aminoantipyrine to a purple dye.

Photometric Test Strip Method

The test strip method employs the above-mentioned enzymatic conversion of glucose to gluconic acid to form hydrogen peroxide. The reagents are immobilised on a polymer matrix, the so-called test strip, which assumes a more or less intense color. This can be measured reflectometrically at 510 nm with the aid of an LED-based handheld photometer. This allows for routine blood sugar determination by laymen. In addition to the reaction of phenol with 4-aminoantipyrine, new chromogenic reactions have been developed that allow photometry at higher wavelengths (550 nm, 750 nm).[114]

Amperometric glucose sensor

The electroanalysis of glucose is also based on the enzymatic reaction mentioned above. The produced hydrogen peroxide can be amperometrically quantified by anodic oxidation at a potential of 600 mV.[115] The GOx is immobilised on the electrode surface or in a membrane placed close to the electrode. Precious metals such as platinum or gold are used in electrodes, as well as carbon nanotube electrodes, which e.g. are doped with boron.[116] Cu-CuO nanowires are also used as enzyme-free amperometric electrodes. This way a detection limit of 50 µmol/L has been achieved.[117] A particularly promising method is the so-called "enzyme wiring". In this case, the electron flowing during the oxidation is transferred directly from the enzyme via a molecular wire to the electrode.[118]

Other sensory methods

There are a variety of other chemical sensors for measuring glucose.[119][120] Given the importance of glucose analysis in the life sciences, numerous optical probes have also been developed for saccharides based on the use of boronic acids,[121] which are particularly useful for intracellular sensory applications where other (optical) methods are not or only conditionally usable. In addition to the organic boronic acid derivatives, which often bind highly specifically to the 1,2-diol groups of sugars, there are also other probe concepts classified by functional mechanisms which use selective glucose-binding proteins (e.g. concanavalin A) as a receptor. Furthermore, methods were developed which indirectly detect the glucose concentration via the concentration of metabolised products, e.g. by the consumption of oxygen using fluorescence-optical sensors.[122] Finally, there are enzyme-based concepts that use the intrinsic absorbance or fluorescence of (fluorescence-labeled) enzymes as reporters.[119]

Copper iodometry

Glucose can be quantified by copper iodometry.[123]

Chromatographic methods

In particular, for the analysis of complex mixtures containing glucose, e.g. in honey, chromatographic methods such as high performance liquid chromatography and gas chromatography[123] are often used in combination with mass spectrometry.[124][125] Taking into account the isotope ratios, it is also possible to reliably detect honey adulteration by added sugars with these methods.[126] Derivatisation using silylation reagents is commonly used.[127] Also, the proportions of di- and trisaccharides can be quantified.

In vivo analysis

Glucose uptake in cells of organisms is measured with 2-deoxy-D-glucose or fluorodeoxyglucose.[76] (18F)fluorodeoxyglucose is used as a tracer in positron emission tomography in oncology and neurology,[128] where it is by far the most commonly used diagnostic agent.[129]

References

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Blood sugar level

The blood sugar level, blood sugar concentration, or blood glucose level is the amount of glucose present in the blood of humans and other animals. Glucose is a simple sugar and approximately 4 grams of glucose are present in the blood of a 70-kilogram (150 lb) human at all times. The body tightly regulates blood glucose levels as a part of metabolic homeostasis. Glucose is stored in skeletal muscle and liver cells in the form of glycogen; in fasted individuals, blood glucose is maintained at a constant level at the expense of glycogen stores in the liver and skeletal muscle.In humans, glucose is the primary source of energy, and is critical for normal function, in a number of tissues, particularly the human brain which consumes approximately 60% of blood glucose in fasted, sedentary individuals. Glucose can be transported from the intestines or liver to other tissues in the body via the bloodstream. Cellular glucose uptake is primarily regulated by insulin, a hormone produced in the pancreas.Glucose levels are usually lowest in the morning, before the first meal of the day, and rise after meals for an hour or two by a few millimoles.

Blood sugar levels outside the normal range may be an indicator of a medical condition. A persistently high level is referred to as hyperglycemia; low levels are referred to as hypoglycemia. Diabetes mellitus is characterized by persistent hyperglycemia from any of several causes, and is the most prominent disease related to failure of blood sugar regulation. There are different methods of testing and measuring blood sugar levels.

The intake of alcohol causes an initial surge in blood sugar, and later tends to cause levels to fall. Also, certain drugs can increase or decrease glucose levels.

Carbohydrate

A carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula Cm(H2O)n (where m may be different from n). This formula holds true for monosaccharides. Some exceptions exist; for example, deoxyribose, a sugar component of DNA, has the empirical formula C5H10O4. The carbohydrates are technically hydrates of carbon; structurally it is more accurate to view them as aldoses and ketoses.

The term is most common in biochemistry, where it is a synonym of 'saccharide', a group that includes sugars, starch, and cellulose. The saccharides are divided into four chemical groups: monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Monosaccharides and disaccharides, the smallest (lower molecular weight) carbohydrates, are commonly referred to as sugars. The word saccharide comes from the Greek word σάκχαρον (sákkharon), meaning "sugar". While the scientific nomenclature of carbohydrates is complex, the names of the monosaccharides and disaccharides very often end in the suffix -ose, as in the monosaccharides fructose (fruit sugar) and glucose (starch sugar) and the disaccharides sucrose (cane or beet sugar) and lactose (milk sugar).

Carbohydrates perform numerous roles in living organisms. Polysaccharides serve for the storage of energy (e.g. starch and glycogen) and as structural components (e.g. cellulose in plants and chitin in arthropods). The 5-carbon monosaccharide ribose is an important component of coenzymes (e.g. ATP, FAD and NAD) and the backbone of the genetic molecule known as RNA. The related deoxyribose is a component of DNA. Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, fertilization, preventing pathogenesis, blood clotting, and development.They are found in a wide variety of natural and processed foods. Starch is a polysaccharide. It is abundant in cereals (wheat, maize, rice), potatoes, and processed food based on cereal flour, such as bread, pizza or pasta. Sugars appear in human diet mainly as table sugar (sucrose, extracted from sugarcane or sugar beets), lactose (abundant in milk), glucose and fructose, both of which occur naturally in honey, many fruits, and some vegetables. Table sugar, milk, or honey are often added to drinks and many prepared foods such as jam, biscuits and cakes.

Cellulose, a polysaccharide found in the cell walls of all plants, is one of the main components of insoluble dietary fiber. Although it is not digestible, insoluble dietary fiber helps to maintain a healthy digestive system by easing defecation. Other polysaccharides contained in dietary fiber include resistant starch and inulin, which feed some bacteria in the microbiota of the large intestine, and are metabolized by these bacteria to yield short-chain fatty acids.

Carbohydrate metabolism

Various biochemical processes responsible for the metabolic formation, breakdown, and interconversion of carbohydrates in living organisms.

Carbohydrates are central to many essential metabolic pathways. Plants synthesize carbohydrates from carbon dioxide and water through photosynthesis, allowing them to store energy absorbed from sunlight internally. When animals and fungi consume plants, they use cellular respiration to break down these stored carbohydrates to make energy available to cells. Both animals and plants temporarily store the released energy in the form of high energy molecules, such as ATP, for use in various cellular processes.Although humans consume a variety of carbohydrates, digestion breaks down complex carbohydrates into a few simple monomers (monosaccharides) for metabolism: glucose, fructose, and galactose. Glucose constitutes about 80% of the products, and is the primary structure that is distributed to cells in the tissues, where it is broken down or stored as glycogen. In aerobic respiration, the main form of cellular respiration used by humans, glucose and oxygen are metabolized to release energy, with carbon dioxide and water as byproducts. Most of the fructose and galactose travel to the liver, where they can be converted to glucose.Some simple carbohydrates have their own enzymatic oxidation pathways, as do only a few of the more complex carbohydrates. The disaccharide lactose, for instance, requires the enzyme lactase to be broken into its monosaccharide components, glucose and galactose.

Cellular respiration

Cellular respiration is a set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy in the process, as weak so-called "high-energy" bonds are replaced by stronger bonds in the products. Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. Cellular respiration is considered an exothermic redox reaction which releases heat. The overall reaction occurs in a series of biochemical steps, most of which are redox reactions themselves. Although cellular respiration is technically a combustion reaction, it clearly does not resemble one when it occurs in a living cell because of the slow release of energy from the series of reactions.

Nutrients that are commonly used by animal and plant cells in respiration include sugar, amino acids and fatty acids, and the most common oxidizing agent (electron acceptor) is molecular oxygen (O2). The chemical energy stored in ATP (its third phosphate group is weakly bonded to the rest of the molecule and is cheaply broken allowing stronger bonds to form, thereby transferring energy for use by the cell) can then be used to drive processes requiring energy, including biosynthesis, locomotion or transportation of molecules across cell membranes.

Diabetes mellitus

Diabetes mellitus (DM), commonly referred to as diabetes, is a group of metabolic disorders in which there are high blood sugar levels over a prolonged period. Symptoms of high blood sugar include frequent urination, increased thirst, and increased hunger. If left untreated, diabetes can cause many complications. Acute complications can include diabetic ketoacidosis, hyperosmolar hyperglycemic state, or death. Serious long-term complications include cardiovascular disease, stroke, chronic kidney disease, foot ulcers, and damage to the eyes.Diabetes is due to either the pancreas not producing enough insulin, or the cells of the body not responding properly to the insulin produced. There are three main types of diabetes mellitus:

Type 1 DM results from the pancreas' failure to produce enough insulin due to loss of beta cells. This form was previously referred to as "insulin-dependent diabetes mellitus" (IDDM) or "juvenile diabetes". The cause is unknown.

Type 2 DM begins with insulin resistance, a condition in which cells fail to respond to insulin properly. As the disease progresses, a lack of insulin may also develop. This form was previously referred to as "non insulin-dependent diabetes mellitus" (NIDDM) or "adult-onset diabetes". The most common cause is a combination of excessive body weight and insufficient exercise.

Gestational diabetes is the third main form, and occurs when pregnant women without a previous history of diabetes develop high blood sugar levels.Prevention and treatment involve maintaining a healthy diet, regular physical exercise, a normal body weight, and avoiding use of tobacco. Control of blood pressure and maintaining proper foot care are important for people with the disease. Type 1 DM must be managed with insulin injections. Type 2 DM may be treated with medications with or without insulin. Insulin and some oral medications can cause low blood sugar. Weight loss surgery in those with obesity is sometimes an effective measure in those with type 2 DM. Gestational diabetes usually resolves after the birth of the baby.As of 2015, an estimated 415 million people had diabetes worldwide, with type 2 DM making up about 90% of the cases. This represents 8.3% of the adult population, with equal rates in both women and men. As of 2014, trends suggested the rate would continue to rise. Diabetes at least doubles a person's risk of early death. From 2012 to 2015, diabetes resulted in approximately 1.5 to 5.0 million deaths each year. The global economic cost of diabetes in 2014 was estimated to be US$612 billion. In the United States, diabetes cost nearly US$245 billion in 2012.

Enzyme activator

Enzyme activators are molecules that bind to enzymes and increase their activity. They are the opposite of enzyme inhibitors. These molecules are often involved in the allosteric regulation of enzymes in the control of metabolism. An example of an enzyme activator working in this way is fructose 2,6-bisphosphate, which activates phosphofructokinase 1 and increases the rate of glycolysis in response to the hormone insulin. In some cases, when a substrate binds to one catalytic subunit of an enzyme, this can trigger an increase in the substrate affinity as well as catalytic activity in the enzyme's other subunits, and thus the substrate acts as an activator.

Fructose

Fructose, or fruit sugar, is a simple ketonic monosaccharide found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into blood during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847. The name "fructose" was coined in 1857 by the English chemist William Allen Miller. Pure, dry fructose is a sweet, white, odorless, crystalline solid, and is the most water-soluble of all the sugars.

Fructose is found in honey, tree and vine fruits, flowers, berries, and most root vegetables.

Commercially, fructose is derived from sugar cane, sugar beets, and maize. Crystalline fructose is the monosaccharide, dried, ground, and of high purity. High-fructose corn syrup is a mixture of glucose and fructose as monosaccharides. Sucrose is a compound with one molecule of glucose covalently linked to one molecule of fructose. All forms of fructose, including fruits and juices, are commonly added to foods and drinks for palatability and taste enhancement, and for browning of some foods, such as baked goods. About 240,000 tonnes of crystalline fructose are produced annually.Excessive consumption of fructose may contribute to insulin resistance, obesity, elevated LDL cholesterol and triglycerides, leading to metabolic syndrome, type 2 diabetes and cardiovascular disease. The European Food Safety Authority stated that fructose is preferable over sucrose and glucose in sugar-sweetened foods and beverages because of its lower effect on postprandial blood sugar levels, and also noted that "high intakes of fructose may lead to metabolic complications such as dyslipidaemia, insulin resistance, and increased visceral adiposity". Further, the UK’s Scientific Advisory Committee on Nutrition in 2015 disputed the claims of fructose causing metabolic disorders, stating that "there is insufficient evidence to demonstrate that fructose intake leads to adverse health outcomes independent of any effects related to its presence as a component of total and free sugars."

Glucagon

Glucagon is a peptide hormone, produced by alpha cells of the pancreas. It works to raise the concentration of glucose and fatty acids in the bloodstream, and is considered to be the main catabolic hormone of the body. It is also used as a medication to treat a number of health conditions. Its effect is opposite to that of insulin, which lowers the extracellular glucose.The pancreas releases glucagon when the concentration of insulin (and indirectly glucose) in the bloodstream falls too low. Glucagon causes the liver to convert stored glycogen into glucose, which is released into the bloodstream. High blood-glucose levels, on the other hand, stimulate the release of insulin. Insulin allows glucose to be taken up and used by insulin-dependent tissues. Thus, glucagon and insulin are part of a feedback system that keeps blood glucose levels stable. Glucagon increases energy expenditure and is elevated under conditions of stress. Glucagon belongs to the secretin family of hormones.

Gluconeogenesis

Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. From breakdown of proteins, these substrates include glucogenic amino acids (although not ketogenic amino acids); from breakdown of lipids (such as triglycerides), they include glycerol, odd-chain fatty acids (although not even-chain fatty acids, see below); and from other steps in metabolism they include pyruvate and lactate.

Gluconeogenesis is one of several main mechanisms used by humans and many other animals to maintain blood glucose levels, avoiding low levels (hypoglycemia). Other means include the degradation of glycogen (glycogenolysis) and fatty acid catabolism.

Gluconeogenesis is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrates, gluconeogenesis takes place mainly in the liver and, to a lesser extent, in the cortex of the kidneys. In ruminants, this tends to be a continuous process. In many other animals, the process occurs during periods of fasting, starvation, low-carbohydrate diets, or intense exercise. The process is highly endergonic until it is coupled to the hydrolysis of ATP or GTP, effectively making the process exergonic. For example, the pathway leading from pyruvate to glucose-6-phosphate requires 4 molecules of ATP and 2 molecules of GTP to proceed spontaneously. Gluconeogenesis is often associated with ketosis. Gluconeogenesis is also a target of therapy for type 2 diabetes, such as the antidiabetic drug, metformin, which inhibits glucose formation and stimulates glucose uptake by cells. In ruminants, because dietary carbohydrates tend to be metabolized by rumen organisms, gluconeogenesis occurs regardless of fasting, low-carbohydrate diets, exercise, etc.

Glucose-6-phosphate dehydrogenase deficiency

Glucose-6-phosphate dehydrogenase deficiency (G6PDD) is an inborn error of metabolism that predisposes to red blood cell breakdown. Most of the time, those who are affected have no symptoms. Following a specific trigger, symptoms such as yellowish skin, dark urine, shortness of breath, and feeling tired may develop. Complications can include anemia and newborn jaundice. Some people never have symptoms.It is an X-linked recessive disorder that results in defective glucose-6-phosphate dehydrogenase enzyme. Red blood cell breakdown may be triggered by infections, certain medication, stress, or foods such as fava beans. Depending on the specific mutation the severity of the condition may vary. Diagnosis is based on symptoms and supported by blood tests and genetic testing.Avoiding triggers is important. Treatment of acute episodes may include medications for infection, stopping the offending medication, or blood transfusions. Jaundice in newborns may be treated with bili lights. It is recommended that people be tested for G6PDD before certain medications, such as primaquine, are taken.About 400 million people have the condition globally. It is particularly common in certain parts of Africa, Asia, the Mediterranean, and the Middle East. Males are affected more often than females. In 2015 it is believed to have resulted in 33,000 deaths. Carriers of the G6PDD allele may be partially protected against malaria.

Glucose tolerance test

The glucose tolerance test is a medical test in which glucose is given and blood samples taken afterward to determine how quickly it is cleared from the blood. The test is usually used to test for diabetes, insulin resistance, impaired beta cell function, and sometimes reactive hypoglycemia and acromegaly, or rarer disorders of carbohydrate metabolism. In the most commonly performed version of the test, an oral glucose tolerance test (OGTT), a standard dose of glucose is ingested by mouth and blood levels are checked two hours later. Many variations of the GTT have been devised over the years for various purposes, with different standard doses of glucose, different routes of administration, different intervals and durations of sampling, and various substances measured in addition to blood glucose.

Glycogen

Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in humans, animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body.

Glycogen functions as one of two forms of long-term energy reserves, with the other form being triglyceride stores in adipose tissue (i.e., body fat). In humans, glycogen is made and stored primarily in the cells of the liver and skeletal muscle. In the liver, glycogen can make up from 5–6% of the organ's fresh weight and the liver of an adult weighing 70 kg can store roughly 100–120 grams of glycogen. In skeletal muscle, glycogen is found in a low concentration (1–2% of the muscle mass) and the skeletal muscle of an adult weighing 70 kg stores roughly 400 grams of glycogen. The amount of glycogen stored in the body—particularly within the muscles and liver—mostly depends on physical training, basal metabolic rate, and eating habits. Small amounts of glycogen are also found in other tissues and cells, including the kidneys, red blood cells, white blood cells, and glial cells in the brain. The uterus also stores glycogen during pregnancy to nourish the embryo.Approximately 4 grams of glucose are present in the blood of humans at all times; in fasted individuals, blood glucose is maintained constant at this level at the expense of glycogen stores in the liver and skeletal muscle. Glycogen stores in skeletal muscle serve as a form of energy storage for the muscle itself; however, the breakdown of muscle glycogen impedes muscle glucose uptake, thereby increasing the amount of blood glucose available for use in other tissues. Liver glycogen stores serve as a store of glucose for use throughout the body, particularly the central nervous system. The human brain consumes approximately 60% of blood glucose in fasted, sedentary individuals.Glycogen is the analogue of starch, a glucose polymer that functions as energy storage in plants. It has a structure similar to amylopectin (a component of starch), but is more extensively branched and compact than starch. Both are white powders in their dry state. Glycogen is found in the form of granules in the cytosol/cytoplasm in many cell types, and plays an important role in the glucose cycle. Glycogen forms an energy reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of triglycerides (lipids). As such it is also found as storage reserve in many parasitic protozoa.

Glycolysis

Glycolysis (from glycose, an older term for glucose + -lysis degradation) is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+. The free energy released in this process is used to form the high-energy molecules ATP (adenosine triphosphate) and NADH (reduced nicotinamide adenine dinucleotide). Glycolysis is a sequence of ten enzyme-catalyzed reactions. Most monosaccharides, such as fructose and galactose, can be converted to one of these intermediates. The intermediates may also be directly useful. For example, the intermediate dihydroxyacetone phosphate (DHAP) is a source of the glycerol that combines with fatty acids to form fat.

Glycolysis is an oxygen-independent metabolic pathway. The wide occurrence of glycolysis indicates that it is an ancient metabolic pathway. Indeed, the reactions that constitute glycolysis and its parallel pathway, the pentose phosphate pathway, occur metal-catalyzed under the oxygen-free conditions of the Archean oceans, also in the absence of enzymes.In most organisms, glycolysis occurs in the cytosol. The most common type of glycolysis is the Embden–Meyerhof–Parnas (EMP pathway), which was discovered by Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas. Glycolysis also refers to other pathways, such as the Entner–Doudoroff pathway and various heterofermentative and homofermentative pathways. However, the discussion here will be limited to the Embden–Meyerhof–Parnas pathway.The glycolysis pathway can be separated into two phases:

The Preparatory/Investment Phase – wherein ATP is consumed

The Pay Off Phase – wherein ATP is produced.

High-fructose corn syrup

High-fructose corn syrup (HFCS), also known as glucose-fructose, isoglucose and glucose-fructose syrup, is a sweetener made from corn starch. As in the production of conventional corn syrup, the starch is broken down into glucose by enzymes. To make HFCS, the corn syrup is further processed by glucose isomerase to convert some of its glucose into fructose. HFCS was first marketed in the early 1970s by the Clinton Corn Processing Company, together with the Japanese Agency of Industrial Science and Technology where the enzyme was discovered in 1965.As a sweetener, HFCS is often compared to granulated sugar, but manufacturing advantages of HFCS over sugar include that it is easier to handle and more cost-effective. The United States Food and Drug Administration has determined that HFCS is a safe ingredient for food and beverage manufacturing, where "HFCS 42" refers to 42% and "HFCS 55" to 55% fructose composition in manufacturing, respectively. HFCS 42 is mainly used for processed foods and breakfast cereals, whereas HFCS 55 is used mostly for production of soft drinks.There is debate over whether HFCS presents greater health risks than other sweeteners. The number of uses and exports of HFCS from American producers have grown steadily during the early 21st century.

Hyperglycemia

Hyperglycemia (also spelled hyperglycaemia or hyperglycæmia), also known as high blood sugar, is a condition in which an excessive amount of glucose circulates in the blood plasma. This is generally a blood sugar level higher than 11.1 mmol/l (200 mg/dl), but symptoms may not start to become noticeable until even higher values such as 15–20 mmol/l (~250–300 mg/dl). A subject with a consistent range between ~5.6 and ~7 mmol/l (100–126 mg/dl) (American Diabetes Association guidelines) is considered slightly hyperglycemic, while above 7 mmol/l (126 mg/dl) is generally held to have diabetes. For diabetics, glucose levels that are considered to be too hyperglycemic can vary from person to person, mainly due to the person's renal threshold of glucose and overall glucose tolerance. On average however, chronic levels above 10–12 mmol/L (180–216 mg/dl) can produce noticeable organ damage over time.

Hypoglycemia

Hypoglycemia, also known as low blood sugar, is when blood sugar decreases to below normal levels. This may result in a variety of symptoms including clumsiness, trouble talking, confusion, loss of consciousness, seizures or death. A feeling of hunger, sweating, shakiness and weakness may also be present. Symptoms typically come on quickly.The most common cause of hypoglycemia is medications used to treat diabetes mellitus such as insulin and sulfonylureas. Risk is greater in diabetics who have eaten less than usual, exercised more than usual or have drunk alcohol. Other causes of hypoglycemia include kidney failure, certain tumors, such as insulinoma, liver disease, hypothyroidism, starvation, inborn error of metabolism, severe infections, reactive hypoglycemia and a number of drugs including alcohol. Low blood sugar may occur in otherwise healthy babies who have not eaten for a few hours.The glucose level that defines hypoglycemia is variable. In people with diabetes, levels below 3.9 mmol/L (70 mg/dL) is diagnostic. In adults without diabetes, symptoms related to low blood sugar, low blood sugar at the time of symptoms and improvement when blood sugar is restored to normal confirm the diagnosis. Otherwise, a level below 2.8 mmol/L (50 mg/dL) after not eating or following exercise may be used. In newborns, a level below 2.2 mmol/L (40 mg/dL), or less than 3.3 mmol/L (60 mg/dL) if symptoms are present, indicates hypoglycemia. Other tests that may be useful in determining the cause include insulin and C peptide levels in the blood.Among people with diabetes, prevention is by matching the foods eaten with the amount of exercise and the medications used. When people feel their blood sugar is low, testing with a glucose monitor is recommended. Some people have few initial symptoms of low blood sugar, and frequent routine testing in this group is recommended. Treatment of hypoglycemia is by eating foods high in simple sugars or taking dextrose. If a person is not able to take food by mouth, an injection of glucagon may help. The treatment of hypoglycemia unrelated to diabetes includes treating the underlying problem as well and a healthy diet. The term "hypoglycemia" is sometimes incorrectly used to refer to idiopathic postprandial syndrome, a controversial condition with similar symptoms that occur following eating but with normal blood sugar levels.

Insulin

Insulin (from Latin insula, island) is a peptide hormone produced by beta cells of the pancreatic islets; it is considered to be the main anabolic hormone of the body. It regulates the metabolism of carbohydrates, fats and protein by promoting the absorption of carbohydrates, especially glucose from the blood into liver, fat and skeletal muscle cells. In these tissues the absorbed glucose is converted into either glycogen via glycogenesis or fats (triglycerides) via lipogenesis, or, in the case of the liver, into both. Glucose production and secretion by the liver is strongly inhibited by high concentrations of insulin in the blood. Circulating insulin also affects the synthesis of proteins in a wide variety of tissues. It is therefore an anabolic hormone, promoting the conversion of small molecules in the blood into large molecules inside the cells. Low insulin levels in the blood have the opposite effect by promoting widespread catabolism, especially of reserve body fat.

Beta cells are sensitive to glucose concentrations, also known as blood sugar levels. When the glucose level is high, the beta cells secrete insulin into the blood; when glucose levels are low, secretion of insulin is inhibited. Their neighboring alpha cells, by taking their cues from the beta cells, secrete glucagon into the blood in the opposite manner: increased secretion when blood glucose is low, and decreased secretion when glucose concentrations are high. Glucagon, through stimulating the liver to release glucose by glycogenolysis and gluconeogenesis, has the opposite effect of insulin. The secretion of insulin and glucagon into the blood in response to the blood glucose concentration is the primary mechanism of glucose homeostasis.If beta cells are destroyed by an autoimmune reaction, insulin can no longer be synthesized or be secreted into the blood. This results in type 1 diabetes mellitus, which is characterized by abnormally high blood glucose concentrations, and generalized body wasting. In type 2 diabetes mellitus the destruction of beta cells is less pronounced than in type 1 diabetes, and is not due to an autoimmune process. Instead there is an accumulation of amyloid in the pancreatic islets, which likely disrupts their anatomy and physiology. The pathogenesis of type 2 diabetes is not well understood but patients exhibit a reduced population of islet beta-cells, reduced secretory function of islet beta-cells that survive, and peripheral tissue insulin resistance. Type 2 diabetes is characterized by high rates of glucagon secretion into the blood which are unaffected by, and unresponsive to the concentration of glucose in the blood. Insulin is still secreted into the blood in response to the blood glucose. As a result, the insulin levels, even when the blood sugar level is normal, are much higher than they are in healthy persons.

The human insulin protein is composed of 51 amino acids, and has a molecular mass of 5808 Da. It is a dimer of an A-chain and a B-chain, which are linked together by disulfide bonds. Insulin's structure varies slightly between species of animals. Insulin from animal sources differs somewhat in effectiveness (in carbohydrate metabolism effects) from human insulin because of these variations. Porcine insulin is especially close to the human version, and was widely used to treat type 1 diabetics before human insulin could be produced in large quantities by recombinant DNA technologies.The crystal structure of insulin in the solid state was determined by Dorothy Hodgkin. It is on the WHO Model List of Essential Medicines, the most important medications needed in a basic health system.

Maltose

Maltose ( or ), also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) bond. In the isomer isomaltose, the two glucose molecules are joined with an α(1→6) bond. Maltose is the two-unit member of the amylose homologous series, the key structural motif of starch. When beta-amylase breaks down starch, it removes two glucose units at a time, producing maltose. An example of this reaction is found in germinating seeds, which is why it was named after malt. Unlike sucrose, it is a reducing sugar.

Starch

Starch or amylum is a polymeric carbohydrate consisting of a large number of glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants as energy storage. It is the most common carbohydrate in human diets and is contained in large amounts in staple foods like potatoes, wheat, maize (corn), rice, and cassava.

Pure starch is a white, tasteless and odorless powder that is insoluble in cold water or alcohol. It consists of two types of molecules: the linear and helical amylose and the branched amylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin by weight. Glycogen, the glucose store of animals, is a more highly branched version of amylopectin.

In industry, starch is converted into sugars, for example by malting, and fermented to produce ethanol in the manufacture of beer, whisky and biofuel. It is processed to produce many of the sugars used in processed foods. Mixing most starches in warm water produces a paste, such as wheatpaste, which can be used as a thickening, stiffening or gluing agent. The biggest industrial non-food use of starch is as an adhesive in the papermaking process. Starch can be applied to parts of some garments before ironing, to stiffen them.

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