The Structure of Glucose Can Be Represented in Three Ways

The straight-chain structural formula (aldohexose; Figure 1A) can account for some of the properties of glucose, but a cyclic structure (a hemiacetal formed by reaction between the aldehyde group and a hydroxyl group) is thermodynamically favored and accounts for other properties. The cyclic structure is normally drawn as shown in Figure 1B, the Haworth projection, in which the molecule is viewed from the side and above the plane of the ring; the bonds nearest to the viewer are bold and thickened, and the hydroxyl groups are above or below the plane of the ring. The hydrogen atoms attached to each carbon are not shown in this figure. The ring is actually in the form of a chair (Figure 1C).

Fig1. d-Glucose.(A) Straight-chain form. (B) α-d-glucose; Haworth projection. (C) α-d-glucose; chair form.

Monosaccharides Exhibit Various Forms of Isomerism

 Glucose, with four asymmetric carbon atoms, can form 16 isomers. The more important types of isomerism found with glucose are as follows:

1. d- and l-isomerism: The designation of a sugar isomer as the d form or its mirror image as the l form is determined by its spatial relationship to the parent compound of the carbo hydrates, the three-carbon sugar glycerose (glyceraldehyde). The l and d forms of this sugar, and of glucose, are shown in Figure 2. The orientation of the —H and —OH groups around the carbon atom adjacent to the terminal alcohol carbon (carbon 5 in glucose) determines whether the sugar belongs to the d or l series. When the —OH group on this carbon is on the right (as seen in Figure 2), the sugar is the d-isomer; when it is on the left, it is the l-isomer. Most of the naturally occurring monosaccharides are d sugars, and the enzymes responsible for their metabolism are specific for this configuration.

Fig2. d- andl-isomerism of glycerose and glucose.

 2. The presence of asymmetric carbon atoms also confers optical activity on the compound. When a beam of plane polarized light is passed through a solution of an optical isomer, it rotates either to the right, dextrorotatory (+), or to the left, levorotatory (−). The direction of rotation of polarized light is independent of the stereochemistry of the sugar, so it may be designated d(−), d(+), l(−), or l(+). For example, the naturally occurring form of fructose is the d(−) isomer. Confusingly, dextrorotatory (+) was at one time called d-, and levorotatory (−) l-. This nomenclature is obsolete, but may sometimes be found; it is unrelated to d- and l-isomerism. In solution, glucose is dextrorotatory, and glucose solutions are sometimes known as dextrose.

3. Pyranose and furanose ring structures: The ring structures of monosaccharides are similar to the ring structures of either pyran (a six-membered ring) or furan (a five membered ring) (Figures 3 and 4). For glucose in solution, more than 99% is in the pyranose form.

Fig3. Pyranose and furanose forms of glucose.

Fig4. Pyranose and furanose forms of fructose.

4. Alpha- and beta-anomers: The ring structure of an aldose is a hemiacetal, since it is formed by reaction between an aldehyde and an alcohol group. Similarly, the ring structure of a ketose is a hemiketal. Crystalline glucose is α-d-glucopyranose. The cyclic structure is retained in the solution, but isomer ism occurs about position 1, the carbonyl or anomeric car bon atom, to give a mixture of α-glucopyranose (38%) and β-glucopyranose (62%). Less than 0.3% is represented by α- and β-anomers of glucofuranose.

 5. Epimers: Isomers differing as a result of variations in configuration of the —OH and —H on carbon atoms 2, 3, and 4 of glucose are known as epimers. Biologically, the most important epimers of glucose are mannose (epimerized at carbon 2) and galactose (epimerized at carbon 4) (Figure 5).

Fig5. Epimers of glucose.

 6. Aldose-ketose isomerism: Fructose has the same molecular formula as glucose but differs in that there is a potential keto group in position 2, the anomeric carbon of fructose, whereas in glucose there is a potential aldehyde group in position 1, the anomeric carbon. Examples of aldose and ketose sugars are shown in Figures 6 and 7, respectively. Chemically, aldoses are reducing compounds, and are sometimes known as reducing sugars. This provides the basis for a simple chemical test for glucose in urine in poorly controlled diabetes mellitus by reduction of an alkaline copper solution .

Fig6. Examples of aldoses of physiological significance.

Fig7. Examples of ketoses of physiological significance.

Many Monosaccharides Are Physiologically Important

 Derivatives of trioses, tetroses, and pentoses and of the seven-carbon sugar sedoheptulose are formed as metabolic intermediates in glycolysis and the pentose phosphate pathway . Pentoses are important in nucleotides, nucleic acids, and several coenzymes (Table 1). Glucose, galactose, fructose, and mannose are the physiologically important hexoses (Table 2). The biochemically important aldoses and ketoses are shown in Figures 6 and 7, respectively.

Table1. Pentoses of Physiological Importance

Table2. Hexoses of Physiological Importance

In addition, carboxylic acid derivatives of glucose are important, including d-glucuronate (for glucuronide formation and in glycosaminoglycans), its metabolic derivative, l-iduronate (in glycosaminoglycans, Figure 8) and l-gulonate (an intermediate in the uronic acid pathway).

Fig8. α-d-Glucuronate (left) and β-l-iduronate (right).

Saccharides Form Glycosides With Other Compounds & With Each Other

Glycosidesare formed by condensation between the hydroxyl group of the anomeric carbon of a monosaccharide, and a second compound that may be another monosaccharide (denoted glycone) or, a nonsaccharide group (denotedaglycone). If the second group is also a hydroxyl, theO-glycosidic bond is anacetal link because it results from a reaction between a hemiacetal group (formed from an aldehyde and an —OH group) and another —OH group. If the hemiacetal portion is glucose, the resulting compound is aglucoside; if galactose, agalactoside; and so on. If the second group is an amine, an N-glycosidic bond is formed, for example, between adenine and ribose in nucleotides such as ATP.

Glycosides are widely distributed in nature; the aglycone may be methanol, glycerol, a sterol, a phenol, or a base such as adenine. The glycosides that are important in medicine because of their action on the heart(cardiac glycosides), all contain steroids as the aglycone. These include derivatives of digitalis and strophanthus such asouabain, an inhibitor of the Na+–K+-ATPase of cell membranes. Other glycosides include antibiotics such as streptomycin.

Deoxy Sugars Lack an Oxygen Atom

Deoxy sugars are those in which one hydroxyl group has been replaced by hydrogen. An example isdeoxyribose(Figure 9) in DNA. The deoxy sugar l-fucose  occurs in glycoproteins. In clinical medicine the tissue accumulation of a tracer quantity of 2-deoxyglucose (¹⁸F 2-fluoro-2-deoxyglucose) is used to detect metabolically active tumors, which have very high rates of glucose uptake. 2-deoxyglucose after being phosphorylated by hexokinase cannot be further metabolized, so it accumulates. This accumulation is detected using positron emission tomography.

Fig9. . 2-Deoxy-d-ribofuranose (β-form).

Amino Sugars (Hexosamines) Are Components of Glycoproteins, Gangliosides, & Glycosaminoglycans

The amino sugars include d-glucosamine, a constituent of hyaluronic acid (Figure 10), d-galactosamine (also known as chondrosamine), a constituent of chondroitin, and d-mannosamine. Several antibiotics (eg, erythromycin) contain amino sugars, which are important for their antibiotic activity.

Fig10. Glucosamine (2-amino-d-glucopyranose) (α-form). Galactosamine is 2-amino-d-galactopyranose. Both glucosamine and galactosamine occur as N-acetyl derivatives in complex carbohydrates, for example, glycoproteins.

 Maltose, Sucrose, & Lactose Are Important Disaccharides

The disaccharides are sugars composed of two monosaccharide residues linked by a glycoside bond (Figure 11).

The physiologically important disaccharides are maltose, sucrose, and lactose (Table 3). Hydrolysis of sucrose yields a mixture of glucose and fructose called “invert sugar” because fructose is strongly levorotatory and changes (inverts) the weaker dextrorotatory action of sucrose.

Fig11. Structures of nutritionally important disaccharides.

Table3. Disaccharides of Physiological Importance

مواضيع ذات صلة

Saccharides are Aldehyde or Ketone Derivatives of Polyhydric Alcohols

A Supply of Oxidizable Fuel is Provided in Both The Fed & Fasting States

Many Metabolic Fuels are Interconvertible

The Flux Through Metabolic Pathways Must be Regulated In A Concerted Manner

Metabolic Pathways at The Organ & Cellular Level

Pathways To Process The Major Products Of our Diet

Overview of Substrate Metabolism in Fasting & Feasting

The selective permeability of the inner mitochondrial membrane necessitates exchange transporters

Many Poisons Inhibit the Respiratory Chain

The Respiratory Chain Provides Most Of The Energy Captured During Catabolism

Electron Transport Via The Respiratory Chain Creates A Proton Gradient Which Drives The Synthesis of ATP

The Respiratory Chain Oxidizes Reducing Equivalents & Acts as A Proton Pump