FUNDAMENTALS OF BIOCHEMISTRY. READER - G. A. Sevryukova - 2018

CHAPTER 2. BIOCHEMISTRY OF CARBOHYDRATES: STRUCTURE, CLASSIFICATION, FUNCTIONS

2.1. Structural Features of CARBOHYDRATES

Carbohydrates are the most widespread Organic compounds on the planet. They are constituents of every component of our biosphere. In plants, they account for up to 80% of total biomass. In animal Tissues, carbohydrates are significantly less abundant, at 2%.

Carbohydrates consist of carbon, hydrogen, and oxygen. The ratio of hydrogen to oxygen in carbohydrates is the same as in a Water molecule, i.e., hydrogen is twice as abundant as oxygen. This is THE ORIGIN OF their name—carbohydrates (composed of carbon and water).

There are several classifications of carbohydrates (Table 1):

✔ according to their chemical Structure, carbohydrates can be divided into two large groups: simple carbohydrates (Monosaccharides) and complex carbohydrates (Oligosaccharides, Polysaccharides);

✔ according to their dietary digestibility, carbohydrates can be tentatively divided into digestible carbohydrates (glucose, sucrose, fructose, lactose, starch) and indigestible carbohydrates (Cellulose, dietary fiber, pectic substances, Lignin).

At the same time, it should be noted that some animals can use cellulose as food. For example, elephants—the largest of modern terrestrial animals—casually feed on surrounding vegetation, stripping branches, peeling bark off trunks, and chewing leaves, thus utilizing cellulose as food. The bolus enters The Stomach, which contains Protozoa harboring Bacteria that break down cellulose. In other animals (cows, sheep, goats), the stomach has a four-chambered structure. The first two chambers contain microorganisms that break down cellulose into D-glucose, which is subsequently fermented into short-chain Fatty acids, carbon dioxide, and methane gas. The resulting Fatty acids are absorbed into the bloodstream, penetrate tissues, and are used by the animal as "fuel". In the remaining stomach chambers, Enzymes secreted by the mucosa digest the spent microorganisms into Amino Acids and sugars, which are absorbed and utilized by the animal as nutrients.

Class="center">Table 1. Classification of Carbohydrates

2.2. Main representatives of carbohydrates

Glucose, one of the monosaccharides, was remarkably selected in the course of evolution for the "starting point" in All living organisms.

Glucose is an aldohexose, and the presence of an aldehyde group in its molecule makes glucose conditionally "hazardous" to the Organism, since the aldehyde group is chemically active (Fig. 3).

Fig. 3. STRUCTURE OF THE glucose molecule

The "hazard" lies not so much in the fact that the aldehyde group can be oxidized to a carboxyl or reduced to an alcohol, but rather in the fact that glucose, possessing such an aldehyde group, can bind to some enzyme and inactivate it.

In the Human and Animal body, glucose in small amounts serves as the primary and most universal energy source to support metabolic processes. However, when the glucose concentration in The Cell is high, it is extremely dangerous; therefore, during intravenous administration of glucose for hypoglycemia, carbohydrate malnutrition, or toxicoinfection, instructions must be strictly followed. Some plants accumulate glucose as a storage substance, for example, grapes. In this case, glucose is "hidden" in special vacuoles within the Cells.

In The Human Body, three hexoses are of the greatest importance: glucose, galactose, and fructose. Chemically, glucose and galactose are aldohexoses, whereas fructose is a ketohexose. Glucose and galactose rotate the plane of light polarization to the right (dextroses), and fructose to the left (levulose). The Rotation of the beam's polarization plane is due to the presence of asymmetric carbon atoms in the molecule, all 4 valences of which are saturated with different atoms or groups of atoms.

There are 4 such asymmetric carbon atoms in the glucose molecule, and 3 in fructose. The presence of asymmetric carbon atoms results in isomers that differ from one another in the spatial arrangement of atoms or groups of atoms. The number of stereoisomers depends on the number of asymmetric carbon atoms and is calculated by the formula in which it equals 2ⁿ, where n is the number of asymmetric carbon atoms. Thus, glucose has 2⁴ = 16 stereoisomers, and fructose has 2³ = 8.

In addition to these 16 acyclic glucose isomers—i.e., with carbon atoms arranged in a chain—cyclic forms of glucose were discovered, in which the carbon chain closes to form a ring via an oxygen atom connecting the 1st carbon to the 4th or 5th carbon of the chain. This is how glucopyranose and glucofuranose, each possessing 5 asymmetric carbon atoms, were discovered. An aqueous solution of glucose contains 32 stereoisomers, leading to the Conclusion that they exist predominantly in the cyclic form. Upon oxidation of glucose, its cyclic forms transition into labile, so-called enolic forms of acyclic structure.

More complex carbohydrates are Disaccharides, consisting of two monosaccharides linked together with the release of a water molecule: lactose (milk sugar), maltose (malt sugar), and sucrose (beet or cane sugar). They are of major nutritional importance and break down in the intestine to yield hexoses.

The true preservation of glucose nutritional equivalents is ensured by polysaccharides, which are molecules consisting of dozens, hundreds, or thousands of monomers (monosaccharides).

Polysaccharides are one of the Main sources of energy generated As a result of METABOLISM in the Body. Polysaccharides that possess poly-1,4-bonds are suitable for intracellular Nutrition.

The aldehyde group in the glucose molecule is conditionally "hazardous," and organisms attempt to eliminate it by concealing it within a glycosidic bond.

A glycosidic bond is a type of covalent bond that links a sugar molecule to another molecule, often another sugar. Polysaccharides are formed by the removal of water from successive molecules at the α-1,4 position (Fig. 4). Soluble starch, or amylose, is formed from alpha-glucose monomers via 1,4-linkages.

Fig. 4. Formation of amylose via a glycosidic bond

In addition to 1,4-linkages, 1,6-linkages can also occur, meaning the polysaccharide molecule becomes branched; this substance is known as amylopectin (Fig. 5).

Fig. 5. Formation of amylopectin via an α-1,6 glycosidic bond

Starch (C₆H₁₀O₅)n is a polysaccharide composed of amylose and amylopectin, with alpha-glucose as their monomer. The three-dimensional packing of the polysaccharide

formed by glucose residues connected by α-1,4 glycosidic bonds, with branched regions formed by α-1,6 glycosidic bonds, is called Glycogen (Fig. 6).

Fig. 6. Formation of glycogen

Polysaccharides differ from one another both in The Nature of their constituent monosaccharide residues and in chain length and degree of branching. Two types are distinguished: Homopolysaccharides, consisting of residues of the same monosaccharide (e.g., starch), and Heteropolysaccharides, containing residues of two or more different monosaccharides (e.g., hyaluronic acid).

The most important storage polysaccharide in plant cells is starch, whereas in animal cells it is glycogen. Glycogen is found in the highest amounts in the Liver, where it accounts for up to 7% of the organ's total weight; glycogen is also present in skeletal Muscles. In liver cells, glycogen occurs as granules that are tightly bound to the enzymes responsible for glycogen Synthesis and Breakdown.

In the gastrointestinal tract (saliva, pancreatic juice), glycogen is broken down by α-amylase, which hydrolyzes the α-1,4 bonds in the outer branches of glycogen. This leaves behind an amylase-resistant "core" known as limit dextrin. In limit dextrin, the α-1,6 bonds are hydrolyzed by α-1,6-glucosidase. Thus, through the combined action of α-amylase and α-1,6-glucosidase, glycogen is degraded into glucose and small amounts of maltose.

In animal cells, glycogen is broken down into glucose-1-phosphate, a process catalyzed by the enzyme phosphorylase.

Phosphorolysis is an Enzymatic Cleavage of chemical bonds involving phosphoric acid, accompanied by the incorporation of a phosphate group (-H₂PO₃) into the resulting products (Fig. 7), thereby yielding glucose-1-phosphate.

Fig. 7. Phosphorolysis reaction

Glucose-1-phosphate is converted into glucose-6-phosphate by the enzyme phosphoglucomutase for subsequent entry into metabolic pathways.

The formation of starch and glycogen involves α-1,4 glycosidic bonds of D-glucose residues, but There is a polysaccharide whose monomers are linked by β-1,4 glycosidic bonds. This completely inedible polysaccharide is called cellulose (Fig. 8).

Fig. 8. Fragment of starch and cellulose

Cellulose is a tough, fibrous, water-insoluble substance found in Plant Cell Walls, primarily in the branches, stems, and trunks of woody plants. Cellulose is a linear, unbranched homopolysaccharide consisting of D-glucose residues joined by β-1,4 linkages.

There are very significant differences between starch, glycogen, and cellulose; specifically, due to the geometric features of α-1,4 bonds, the linear segments of polymer chains in glycogen and starch molecules tend to adopt a coiled, helical conformation, which facilitates the formation of dense granules.

Due to the β-configuration of their 1,4-glycosidic bonds, cellulose polymer chains are highly extended and held together by intra- and intermolecular H-bonds. Consequently, the cellulose molecule adopts a chair-like, fibrous Spatial Structure and is insoluble (Fig. 9).

Fig. 9. Formation of the fibrous structure of the cellulose molecule

The most common oligosaccharide, consisting of glucose and fructose, is sucrose. The sucrose molecule conceals both aldehyde and ketone groups; in other words, it hides potentially "hazardous" groups in their most accessible form (Fig. 10).

Fig. 10. Structure of sucrose

To participate in metabolism, sucrose is broken down into glucose and fructose by the enzyme sucrase. Another name for this enzyme is invertase, since the Hydrolysis of sucrose is accompanied by A change in the direction of optical rotation of the plane of polarization. Artificially inverted honey is produced by converting sucrose using acid at 70-80 °C rather than by bees. This yields a monosaccharide mixture consisting of equal amounts of glucose and fructose, closely resembling natural honey in appearance. Natural honey is added to artificial invert sugar for aroma and flavor, making it quite difficult to distinguish such a mixture from natural honey without chemical analysis.

Lactose is found in milk and dairy products (Fig. 11). Hydrolysis of lactose yields D-galactose and D-glucose. During Digestion, lactose undergoes Enzymatic hydrolysis through the action of lactase. In infants, The activity of this enzyme is very high; however, lactase activity decreases with age, which can lead to intolerance to milk and dairy products, manifested by abdominal pain and diarrhea.

Fig. 11. Lactose and its monomers galactose and glucose

Hyaluronic acid is a polysaccharide composed of alternating residues of D-glucuronic acid and N-acetyl-D-glucosamine. Hyaluronic acid interacts with water and, even at low concentrations, forms highly viscous, gel-like solutions. Hyaluronic acid is a component of the Skin, where it plays a role in tissue regeneration.