Textbook - BIOLOGICAL CHEMISTRY - Gubsky Yu.I. - 2000

Chapter I. BIOMOLECULES AND CELLULAR STRUCTURES

CHAPTER 4. CARBOHYDRATES AND THEIR DERIVATIVES

CARBOHYDRATES (glucids, sugars) are bioorganic compounds whose chemical Structure represents aldehyde and ketone derivatives of polyhydric alcohols, or polyhydroxy aldehydes and polyhydroxy ketones.

Carbohydrates that correspond to the aforementioned chemical structures and cannot undergo Hydrolysis (i.e., splitting by Water into simpler compounds) are called simple carbohydrates, or Monosaccharides. Carbohydrates that hydrolyze to monosaccharides are termed complex carbohydrates—Oligosaccharides and Polysaccharides.

Carbohydrates are formed in nature from carbon dioxide and water through Photosynthesis occurring in plants and certain microorganisms. They constitute the bulk of organic carbon in the biosphere, accounting for about 80% of the mass of plant Tissues. In turn, plant carbohydrates serve as one of the primary sources of carbon for animal organisms.

In animal organisms, the carbohydrate content is relatively low, averaging 1-2% of dry mass, mainly in the form of the reserve polysaccharide Glycogen. Nevertheless, carbohydrates perform vital energy-yielding (the monosaccharides glucose and fructose, and the homopolysaccharide glycogen) and structural (Heteropolysaccharides) Functions.

4.1. MONOSACCHARIDES AND THEIR DERIVATIVES

Depending on the number of carbon atoms, monosaccharides (monoses) are subdivided into trioses, tetroses, pentoses, hexoses, heptoses, etc. The monosaccharides most prevalent in animal organisms are hexoses and pentoses, which exist in Cells as participants in metabolic pathways—metabolites—and perform specific structural functions as constituents of other Biomolecules.

Class="center">Hexoses

Hexoses are classified into:

- aldohexoses, which include D-glucose and its epimers D-galactose, D-manose, D-fucose, etc.;

- ketohexoses, represented by D-fructose.

D-Glucose (grape sugar, dextrose) is a monosaccharide widely distributed in nature: in a free state, glucose is found in plants, human Blood (4.44-6.66 mmol/L), and other biological fluids. Glucose is a structural component of the Disaccharides sucrose and lactose, as well as the Homopolysaccharides starch, Cellulose, and glycogen. Plant starch is the primary dietary source of glucose for The Human Body. Aerobic oxidation of glucose in the human body releases an average of 60-70% of the chemical energy required to sustain vital physiological processes.

D-Galactose (milk sugar) is a constituent of the disaccharide lactose found in milk, as well as of heteropolysaccharides (glycosaminoglycans, or mucopolysaccharides) in animal tissues. The main source of galactose for the human body is lactose, which upon hydrolysis yields free galactose capable of being converted into glucose.

D-Mannose is a structural component of the polysaccharide mannan, which forms part of plant and bacterial Glycoproteins; in a free state, it is found in citrus peel. In animal organisms, it is part of the oligosaccharide moiety of Glycolipids and glycoproteins in membranes and biological fluids. The reduction product of mannose, the hexahydric alcohol mannitol, is used in medicine as an osmotic diuretic and as a sucrose substitute in the diet of Diabetes Mellitus patients.

Pentoses

Biologically important representatives of pentoses include:

- aldopentoses: D-ribose, 2-deoxy-D-ribose, L-arabinose, D-xylose;

- ketopentoses: D-ribulose and D-xylulose.

D-Ribose is an aldopentose that occurs in the β-furanose form as a component of ribonucleic acid NUCLEOTIDES, free ribonucleotides, A number of Coenzymes (NAD, NADP, FAD, FMN), Glycosides, and Antibiotics.

2-Deoxy-D-ribose (deoxyribose) is a pentose that differs from D-ribose by the absence of an oxygen atom at the C-2 position. It is a constituent of deoxyribonucleic acid nucleotides.

L-Arabinose is an aldopentose that forms part of plant polysaccharides such as arabinans and plant Gums (e.g., gum arabic, see below), as well as the polysaccharides of the tuberculosis bacterium. In its free state, it is found in conifers.

D-Xylose occurs as a component of plant xylans. It is used in the confectionery industry as a nutrient for growing fodder Yeast, as well as for the Synthesis of the alcohol xylitol, which is used as a medicinal agent.

D-Ribulose and D-xylulose are ketopentoses that are formed in Human and Animal organisms as phosphate esters, acting as metabolites in the Pentose Phosphate Pathway of glucose METABOLISM.

Amino Derivatives of Monosaccharides (Amino Sugars)

The most common amino sugars include 2-amino derivatives of the hexoses D-glucose and D-galactose—hexosamines:

N-Acetylated derivatives of hexosamines are most commonly found in heteropolysaccharides (see below)—glycosaminoglycans (components of Proteoglycans), as well as the oligosaccharide chains of glycoproteins and glycolipids.

Neuraminic and Sialic Acids

A biologically important amino derivative of monosaccharides is neuraminic acid. Chemically, neuraminic acid is a derivative of the monosaccharide ketononose (nonulose). In biological systems, neuraminic acid exists in the form of N- and O-acyl derivatives, which are known as sialic acids.

Neuraminic and sialic acids are Structural components of biomembrane glycolipids (gangliosides), glycoproteins, and proteoglycans found in biological fluids, mucus, and Connective Tissue, where they perform vital mechanical and immunochemical functions.

Glycaric (Sugar) Acids

The oxidation of monosaccharides yields glycaric (sugar) acids, which are classified into distinct chemical groups depending on which functional group (aldehyde or primary alcohol) undergoes oxidation.

Aldonic acids are the products of aldose oxidation at the aldehyde carbon atom. An example of an aldonic acid is gluconic acid, which is formed as a phosphate ester (6-phosphogluconic acid) in the reactions of The pentose phosphate pathway of glucose oxidation.

Uronic acids are the products of oxidation of the primary hydroxyl group of aldoses. Of great biological significance are the uronic acids formed by the oxidation of glucose and galactose—namely D-glucuronic and D-galacturonic acids. Along with amino sugars, these occur in the body predominantly as Structural elements of heteropolysaccharides. L-Iduronic acid, a derivative of the hexose L-idose, is a structural component of the heteropolysaccharide heparin and connective tissue dermatan sulfates.

Free glucuronic acid plays a crucial role in detoxification reactions in animal organisms by forming conjugation products—glucuronides—with the metabolites of protein and porphyrin Catabolism, as well as foreign chemical compounds.

Ascorbic acid—a sugar acid that is also known as L-ascorbic acid (the γ-lactone of 2-keto-L-gulonic acid). Ascorbic acid is synthesized from glucose in the tissues of plants and most higher animals. The human body, along with that of other primates and guinea pigs, lacks the enzyme systems necessary for the synthesis of ascorbic acid; therefore, this compound is an essential dietary factor—Vitamin C.

Glycosides

Glycosides are compounds formed by the Condensation of monosaccharides (or monosaccharide residues within a more complex sugar) with alcohols or phenols.

Glycosides are formed through the interaction of the hemiacetal OH group of a carbohydrate with the OH group of an alcohol (or phenol). The names of glycosides are derived from the names of their corresponding monosaccharides by changing the suffix "-ose" to "-oside" (for example: glucoside, galactoside, fructoside, riboside, etc.).

Glycosidic bonds linking individual sugar residues form The basis of The structure of complex carbohydrates (see below). The interaction of the acetal hydroxyl group of a monosaccharide with hydroxyl-containing non-carbohydrate molecules gives rise to numerous glycosides, which are physiologically active compounds; the non-carbohydrate component of a glycoside is called an aglycone. Aglycones can be methanol, glycerol, phenol, or sterol residues.

Plant steroid-containing glycosides exhibit cardiotonic activity and have found widespread application in medical practice as cardiac glycosides. Cardiac glycosides, which are used to stimulate myocardial activity in Heart Failure, are extracted from various species of foxglove (purple foxglove — Digitalis purpurea, woolly foxglove — Digitalis lanata), strophanthus (smooth strophanthus — Strophanthus gratus, Kombé strophanthus — Strophanthus kombé), and lily of the valley (Convallaria).

In addition to the glycosides formed through the interaction of carbohydrates with hydroxyl-containing compounds (O-glycosides) discussed above, N-glycosides are also widely prevalent in living nature. N-glycosides are products of the interaction between monosaccharides and aglycones via the nitrogen atom of an NH group, which is part of aliphatic, aromatic, or heterocyclic amines. Biologically significant Examples of N-glycosides include nucleosides, which serve as structural Components of nucleic acid nucleotides and numerous coenzymes.