Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Chemistry of Carbohydrates
Monosaccharides
Main reactions of monosaccharides, reaction products, and their properties

Reactions of the hemiacetal hydroxyl group. As noted earlier, Monosaccharides exist predominantly in hemiacetal forms, both in the crystalline state and in solution. The hemiacetal hydroxyl group exhibits high reactivity and can be replaced by other groups in reactions with alcohols, carboxylic acids, phenols, etc.

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Fig. 5.1. a-D-glucose.

a — linear formula of glucose (aldohexose); b — Haworth structural formula; c — conformational formula (chair conformation).

The reaction product is called a glycoside. Corresponding to the a- and ß-isomers of monosaccharides are a- and ß-glucosides. For example, the reaction of methyl alcohol with glucose (suppose in the ß-pyranose form) in the presence of inorganic acids yields the alkylation product methyl ß-D-glucopyranoside:

The action of acetic acid on ß-D-glucopyranose yields the acylation product acetyl-ß-glucopyranoside:

The remaining groups of monosaccharides can also undergo acylation and methylation, but under much more drastic conditions. When alcohols, phenols, or carboxylic acids enter into the reaction, the resulting products are called O-Glycosides. Consequently, methyl ß-D-glucopyranoside and acetyl-ß-glucopyranoside are O-glycosides (the linkage is formed via oxygen). Naturally occurring O-glycosides, most of which are produced by plants, exist predominantly in the ß-form.

An important class of glycosides is N-glycosides, in which the glycosidic bond is formed through nitrogen rather than oxygen*. N-glycosides are regarded as derivatives of monosaccharides in which the glycosidic moiety of the molecule is linked via a nitrogen atom to an organic radical R that is not a carbohydrate. Like O-glycosides, N-glycosides can be structured as pyranosides or furanosides and can occur in both a- and ß-forms:

N-glycosides include crucially important metabolic products of nucleic acid and nucleoprotein degradation (NUCLEOTIDES and nucleosides), ATP, NAD, NADP, certain Antibiotics, etc. (see Chapter 3).

Reactions Involving the carbonyl group. Although the linear form is present in small amounts in crystalline monosaccharide preparations and their solutions, its Participation in the tautomeric equilibrium provides monosaccharides with all the properties typical of aldehydes (in aldoses) or ketones (in ketoses). The ability of aldoses and ketoses to add alcohols was discussed earlier.

Let us examine some of their other properties.

Oxidation of monosaccharides. Treating aldoses with mild oxidizing agents converts the aldehyde group at the C-1 position into a carboxyl group, forming so-called aldonic acids. An example of an aldonic acid is D-gluconic acid, which is formed by The oxidation of the aldehyde group of D-glucose. The phosphorylated form of D-gluconic acid plays a vital role as an intermediate in Carbohydrate METABOLISM. Another example is D-galactonic acid, the product of oxidation of the aldehyde group of D-galactose.

* There are also S-glycosides, which are derivatives of cyclic forms of thiosugars in which the hydrogen atom of the mercapto group (—SH) at C-1 is replaced by a radical. S-glycosides are found in A number of plants (mustard, pheasant's eye, hawthorn, etc.).

In alduronic, or uronic, acids, the primary alcohol group is oxidized (to form a carboxyl group), while the aldehyde group remains unoxidized. The uronic acid derived from D-glucose is called D-glucuronic acid, and that derived from D-galactose is called D-galacturonic acid.

Uronic acids are of great biological importance, and many of them serve as components of Polysaccharides.

Reduction of monosaccharides. Monosaccharides are easily hydrogenated at the C—O bond and thereby converted into polyhydric alcohols (sugar alcohols). D-glucose, for instance, yields the alcohol sorbitol, and D-manose yields mannitol. The reduction of D-fructose yields an equimolar mixture of epimers — D-mannitol and D-sorbitol — because hydrogenation renders the second carbon atom asymmetric. This type of reduction can also proceed enzymatically.

Sugar phosphate esters. Monosaccharides esterified with phosphoric acid play an exceptionally significant role in metabolism. The first naturally occurring sugar phosphate ester discovered was fructose-1,6-bisphosphate, identified during Fermentation by L.A. Ivanov, as well as A. Harden and W. Young in 1905. In subsequent years, many new mono- and bisphosphates of monosaccharides were isolated from natural sources, notably A large number of ketose phosphates, such as ribulose and xylulose phosphates. Currently, alongside hexose and pentose phosphates, the important role of heptose phosphates (primarily sedoheptulose-7-phosphate) and tetrose phosphates (erythrose-4-phosphate, etc.) in many biochemical processes has been established. In 1980, a group of Belgian researchers (G. Hers et al.) discovered fructose-2,6-bisphosphate, an important regulator of carbohydrate metabolism.

Of great interest are sugar pyrophosphate esters, such as 5-phosphoribosyl-1-pyrophosphate (PRPP), which participates in the Synthesis of purine and pyrimidine nucleotides.

Formulas for some sugar phosphates that play a crucial role in metabolism are given below:

Deoxysugars. In deoxysugars, one of the hydroxyl groups attached to the ring Structure is replaced by a hydrogen atom. They are formed during the Hydrolysis of several compounds that play a vital role in biological processes. An example is deoxyribose, which is a component of Nucleic Acids (DNA):

Aminosugars. These are Monosaccharide Derivatives in which a hydroxyl group (—OH) is replaced by an amino group (—NH₂). Depending on THE POSITION OF the amino group (at specific carbon atoms) in the aminosugar molecule, 2-amino, 3-amino, and 4-amino sugars are distinguished, and so on. Based on the number of amino groups, monoaminosugars and diaminosugars are identified.

Aminosugars exhibit all The properties of amines and conventional monosaccharides, as well as specific properties arising from the spatial proximity of the hydroxyl and amino groups.

In humans and animals, the most important aminosugars are D-glucosamine and D-galactosamine:

Aminosugars are constituents of mucopolysaccharides of animal, plant, and bacterial origin, and serve as carbohydrate components of various Glycoproteins and Glycolipids. Within these macromolecular compounds, the amino group of the aminosugar is most frequently acylated and occasionally sulfated (see Chapter 21).