BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 11. STRUCTURE AND PROPERTIES OF CARBOHYDRATES

11.3. Polysaccharides

Polysaccharides (polyoses) are natural polymers composed of A large number of monosaccharide residues and their derivatives. Based on their monosaccharide composition, polysaccharides are subdivided into Homopolysaccharides (or homoglycans), which consist of residues of a single type of monose, and Heteropolysaccharides (or heteroglycans), whose molecules contain residues of different Monosaccharides. Polysaccharides also vary in molecular weight and molecular Structure. Depending on The Nature of their polyglycoside chain, they can be linear or branched.

Due to the lack of a systematic nomenclature for polysaccharides, a rational principle is applied, whereby polysaccharide names are derived from the name of the monose monomer by replacing the suffix -ose with -an. For example, polysaccharides consisting exclusively of D-glucose or D-fructose residues are called D-glucans or D-Fructans, respectively. If a polyose contains both types of these monoses, it is referred to as a D-gluco-D-fructan. The Nature of the glycoside bond is indicated by the letters a- or β- preceding the group name of the polysaccharide: e.g., a-D-glucans or β-D-fructans.

Alongside the systematic names of the most important homoglycans, their traditional common names are often used—such as starch, Cellulose, and Glycogen—or common names for heteroglycans like heparin, pectin, and hyaluronic acid.

The most abundant monomer in natural polysaccharides is D-glucose; other frequently occurring monoses include D-manose, D- and L-galactose, L-arabinose, D-xylose, D-glucuronic acid, D-galacturonic acid, D-manuronic acid, D-glucosamine, D-galactosamine, and sialic acids.

The glycosidic bonds linking monose residues in polysaccharide molecules are formed predominantly via the glycosido-glycosidic type, As a result of which polyose molecules are virtually devoid of free hemiacetal hydroxyl groups.

Polysaccharides constitute the bulk of Plant Tissues and are also found in animal Cells and microorganisms, where they perform a variety of essential functions. Primarily, these include mechanical and structural functions, such as supporting roles played by cellulose and its derivatives in Higher Plants and Algae, as well as Chitin in crustaceans, Mollusks, and insects. Polysaccharides like starch, Pectins, and fructans in plants, and glycogen in animals, serve a reserve function—meaning they store metabolic energy that can be mobilized for vital cellular processes. The Protective Functions of polysaccharides in animal tissues are related to their role in Immunity, The formation of the protective mucous layer in the gastrointestinal tract and bronchopulmonary Airways, and joint synovial fluid. Among the many diverse functions of polyoses, one should also highlight their complex specific roles in Blood Coagulation, nervous activity, Cell-to-cell interactions in Multicellular Organisms, and interactions between Introduction/7.html">Cells and Viruses.

According to their Chemical Composition and molecular structure, polysaccharides are classified into the following groups:

✵ glucans (starch, glycogen, cellulose, and other glucans);

✵ Mannans (mannans, glucomannans, galactomannans);

✵ fructans (fructans, glucofructans);

✵ xylans (xylans, arabinoxylans, glucuronoxylans);

✵ arabinans;

✵ galactans (galactans, arabinogalactans);

✵ polyuronides (pectic substances, alginic acid);

✵ chitin;

✵ glycosaminoglycans.

The most important natural glucans are cellulose, starch, and glycogen.

Cellulose (or fiber) is the most widespread organic compound in nature, forming The basis of The Cell walls of plants, Bacteria, and certain lower animals (Tunicata). Its presence imparts mechanical strength, rigidity, and elasticity to the Cells and Tissues of these organisms. It is a high-molecular-weight linear carbohydrate composed of glucosyl residues linked together by β-1,4-glycosidic bonds, as well as Hydrogen Bonds between the hydroxyls of the third carbon atom of the monose and the oxygen heteroatom of the adjacent glucosyl residue:

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Cellulose is a white amorphous substance with a characteristic ordered structure. Its linear macromolecular chains, ranging from 1500 nm in length, form microfibrils via intermolecular hydrogen bonds, with each microfibril containing up to several dozen linear macromolecules. The microfibrils, in turn, associate into fibers in which the axes of the microfibrils are positioned at an angle to the fiber axis. In certain Regions of the fibers, the linear cellulose molecules assemble into ordered bundles parallel to the fiber axis, forming crystalline micelles ~60 nm in length and 5 nm in diameter, which possess high mechanical strength and chemical stability. The intermediate intermicellar amorphous regions of the fiber are characterized by significantly lower strength, and reactions such as Hydrolysis, oxidation, and substitution proceed rapidly within them.

The complex STRUCTURE OF THE plant Cell wall, with its multidirectional orientation of microfibrils, gives wood—which consists of these cell walls—the mechanical properties that have made it an exceptionally important construction material utilized by humanity since ancient times.

Cellulose is insoluble in Water, ether, acetone, alcohol, and Hydrocarbons. It is resistant to dilute solutions of acids and alkalis, but dissolves in concentrated solutions of mineral acids and organic bases. In the presence of concentrated acids, cellulose undergoes

undergoes hydrolysis to form β-D-glucose, which serves as the basis for The production of hydrolyzed ethyl alcohol. The Enzymatic hydrolysis of cellulose takes place through the action of symbiotic bacterial Enzymes found in the digestive tracts of ruminants and insects (such as termites).

Bacterial enzymes, specifically cellulase and cellobiase, sequentially break down the cellulose molecule into cellobiose and D-glucose According to the following scheme:

The chemical properties of cellulose are determined by the presence of free hydroxyl groups within its molecular structure, which facilitate the formation of alcoholates, cellulose ethers, and cellulose esters.

Industrially, alkali celluloses, methyl-, ethyl-, and benzylcelluloses, cellulose nitrates, as well as cellulose xanthate and acetate, are produced in large quantities. These derivatives are widely used in the food, cosmetic, and textile industries, as well as in the manufacture of paper, varnishes, films, plastics, explosives, and other products.

In biochemical research, diethylaminoethyl- (DEAE) and carboxymethylcellulose (CMC) are widely utilized as ion-exchangers in Ion-exchange Chromatography for the Separation of Amino Acids, Peptides, Proteins, Hormones, NUCLEOTIDES, and Nucleic Acids.

Starch, the primary reserve carbohydrate in plants and an essential component of Human Nutrition, is a product of glucose polymerization formed via Photosynthesis in green leaves. Starch is synthesized in cereal grains and vegetable tubers, where it accumulates in significant quantities (for example, up to 80% in rice grains, up to 85% in wheat, and 25% in potato tubers) in the form of microscopic oval granules ranging from 2 to 150 µm in diameter. In addition to glucose, natural starch contains proteins (up to 0.8%), Higher Fatty acids (0.6%), and phosphoric acid (up to 0.7%).

Starch is a white, odorless, and tasteless powder that is insoluble in water at room Temperature. When heated in water at 65–70 °C, gelatinization occurs—the starch granules swell and rupture, forming a colloidal solution. Starch solutions are optically active; the specific rotation angle of potato starch, for instance, is [α]_D²⁰ = 204.3^o.

Prolonged heating of starch in water in the presence of a 10% H₂SO₄ solution initially leads to dextrinization—the formation of low-molecular-weight dextrins—and subsequently results in stepwise hydrolysis, producing Oligosaccharides and D-glucose according to the scheme:

When treated with an iodine solution, starch and dextrin solutions develop a blue, violet, orange, or red coloration, which is used for the identification of these compounds.

In terms of chemical structure, starch is a structurally heterogeneous homopolysaccharide composed of two components: amylose and amylopectin.

Amylose features a linear chain in which glucose residues are linked together by α-1,4-glycosidic bonds:

Amylose is characterized by a relatively low molecular weight (20–60 kDa), high viscosity, instability In aqueous solutions, and the formation of a dark blue complex with iodine. It is readily hydrolyzed by amylolytic enzymes—α- and β-amylases—into molecules of the disaccharide maltose:

Amylopectin is a branched glucan whose molecules contain branch points formed by α-1,6-glycosidic bonds occurring every 20–25 glucose units linked by α-1,4-glycosidic bonds. On average, 16% of the glucose residues in amylopectin are phosphorylated:

Compared to amylose, amylopectin has a more complex architecture consisting of Three types of polyglucosyl chains: main (C), side (B), and intermediate (A). These chains can form three structural patterns: layered, herringbone, and branched cluster:

Amylopectin is characterized by a high molecular weight (exceeding 10³ kDa), poor solubility in water, and an inability to crystallize. In water, it forms highly stable colloidal solutions that stain red-violet with iodine. The enzymatic hydrolysis of amylopectin proceeds more slowly than that of amylose: β-amylase sequentially cleaves maltose disaccharide units from the non-reducing ends of the side polyglucosyl chains up to the branch point, halting hydrolysis just before the terminal α-1,4-glycosidic bond. Subsequently, the high-molecular-weight dextrin products are cleaved by α-amylase (into low-molecular-weight dextrins) and glucoamylase, which also hydrolyzes α-1,3- and α-1,6-glycosidic bonds, releasing D-glucose molecules from the non-reducing ends of the dextrin molecules:

Maltose, in turn, is cleaved by maltase (α-glucosidase) into two molecules of D-glucose. Thus, through the action of amylolytic enzymes in the gastrointestinal tract of animals, starch is almost completely broken down into D-glucose molecules.

Modified starches are widely used in the food and textile industries; these are starches whose natural properties have been altered through physical, chemical, biological, or combined treatments involving ultrasound, ionizing radiation, oxidizing agents, acids, other chemical Reagents, or enzymes. The food industry utilizes oxidized, pregelatinized, phosphated, acetylated, and other modified starches in the production of

bread, puddings, ice cream and dairy concentrates, mayonnaises, sauces, semi-finished meat products, dietary and baby food, etc. In the textile sector, substituted starches are employed in the manufacture of natural and synthetic fibers.

Glycogen is a reserve carbohydrate in animal and human tissues, where it functions as animal starch. It is synthesized in virtually all Cells of the body, but is found in especially high amounts in the Liver (up to 7%) and Muscles (up to 0.9% of wet tissue mass). Glycogen is also present in Fungi, Yeasts, microorganisms, and some higher plants (such as sweet corn).

Pure glycogen is a white amorphous powder that dissolves readily in water. Glycogen solutions are opalescent and optically active, with a specific rotation angle of [α]_D²⁰ = 196°.

Glycogen is a branched polysaccharide composed of a-D-glucopyranose residues linked in the linear portions of the molecule by a-1,4-bonds, and at branching points by a-1,6-glycosidic bonds, and occasionally a-1,3-bonds.

Compared to amylopectin, the glycogen molecule has a higher degree of branching: an a-1,6-bond occurs every 8-12 residues of α-1,4-glycosidic bonds. This makes glycogen more compact in structure than amylopectin. Its macromolecular shape approaches spherical, and its molecular mass reaches 1 · 10⁶ kDa (Fig. 11.1).

Fig. 11.1. Cytology/cytology/92.html">SCHEMATIC STRUCTURE OF the glycogen macromolecule according to W. Whelan:

A - outer straight side chains linked to other chains by a-1,6-bonds; B - intermediate chains connected to chains A;

C - the single chain in the molecule with a free reducing end

The macromolecular structure of glycogen is heterogeneous, consisting of three types of particles. The largest of these, with a diameter of 60-200 nm and a molecular mass of 10⁵-10⁶ kDa, are called a-particles; the smallest, with a diameter of 20-40 nm and a molecular mass of 2-5 · 10³ kDa, are γ-particles; and intermediate particles, consisting of several γ-particles, are called β-particles (Fig. 11.2):

Fig. 11.2. Schematic structure of particulate glycogen according to L. Leloir

Glycogen readily undergoes acid or enzymatic hydrolysis, breaking down to form oligosaccharides—maltotriose, isomaltotriose, isomaltose, maltose—and the final product, α-D-glucose. This provides further evidence that glucosyl residues in the glycogen molecule are linked by a-1,4- and a-1,6-glycosidic bonds.

Chitin is a widely distributed natural homopolysaccharide that makes up the cuticles of insects, the exoskeletons of Arthropods (Arthropoda), as well as the cell coverings of fungi (Eumycophyta), the cell walls of many microorganisms (Aspergillus niger, Neurospora crassa, etc.), and some lower plants. Chitin is a linear polysaccharide with a molecular mass of 150-200 kDa, composed of N-acetylglucosamine residues linked together by β-1,4-glycosidic bonds:

Chitin can be viewed as a 2-N-acetyl derivative of cellulose. Much like cellulose in plants, it fulfills a mechanical and protective function in the organisms that synthesize it.

Heteroglycans. Alongside cellulose, other polysaccharides also participate in the Formation of Plant cell walls: hemicelluloses and pectic substances. Hemicelluloses are a group of heteropolysaccharides that vary in their monosaccharide composition and The structure of their main and branched polyglycoside chain segments. This macromolecular group, diverse in Composition and Structure, includes mannans, galactans, fructans, xylans, and arabinans.

Mannans are found in softwood and algae, and consist of D-mannopyranose residues that may be linked by β-1,3-, β-1,4-, and β-1,6-glycosidic bonds.

Galactans are built from D-galactopyranose residues via β-1,4-glycosidic bonds. They are found in algae (Agar), seeds, and the ROOT crops of higher plants.

Fructans include inulin, the most widespread natural polyfructose, which accumulates as a reserve carbohydrate in significant amounts in the tubers of Asteraceae family plants (Jerusalem artichoke, artichokes, chicory, etc.). In addition to fructofuranosyl residues linked by β-2,1-glycosidic bonds, the inulin molecule contains α-D-glucopyranose residues (3-6% of the total monose content). The molecular mass of inulin is 5-6 · 10³ Da. It dissolves well in water, and its solutions are optically active ([α]_D²⁰ = -39°); it crystallizes from solutions in the form of spherocrystals. Among the three molecular forms of inulin (α-, β-, and γ-), γ-inulin is the most biologically active, exhibiting significant hypoglycemic, hypocholesterolemic, and fibrinolytic activity.

Pectic substances, which along with hemicellulose are permanent companions of cellulose, are found in the cell walls and intercellular substance of all higher plants and algae. In some of them, they accumulate in considerable quantities—up to 15% in apples and up to 30% of dry matter in citrus peels. In most plants, pectic substances exist for a certain period in the water-insoluble form of protopectin, which later, as fruits ripen, converts into the soluble form—pectin, a component of fruit and vegetable juices.

The structural backbone of pectic substance molecules is a polymer chain of D-galacturonic acid residues linked together by a-1,4-glycosidic bonds.

Polyosides containing methyl esters of D-galacturonic acid are called pectic acids, and their salts are called pectinates:

Unmethoxylated polygalacturonides are called pectic acids (or pectic substances lacking methoxyl groups), and their salts are called pectates.

Pectin molecules may also contain from 5% to 25% of neutral sugars, including L-arabinose, L-rhamnose, and D-galactose, with smaller amounts of D-xylose, L-fructose, and their methyl esters.

A specific property of pectic substances is their ability to form gels, which are widely used in the food industry for manufacturing jellies, jams, marmalades, etc., as well as in medicine as effective sorbents and chelating agents for detoxifying endo- and exotoxins, and for eliminating radionuclides and heavy metals from The Human Body.

Agar-agar is a complex polysaccharide found in certain seaweeds of northern seas, representing a mixture of polyozagarose and agaropectin. Agarose is composed of D-galactopyranose and 3,6-anhydro-L-galactopyranose residues linked alternately by β-1,4 and α-1,3 glycosidic bonds:

Agaropectin consists of D-galactosyl residues, some of which are sulfated.

When heated in water, agar-agar forms solutions that turn into gels upon cooling. These properties of agar-agar are widely utilized in the food industry and biotechnology for preparing culture media.

CARBOHYDRATES of Connective Tissue. Connective tissue, which accounts for up to 50% of the mass of all animal tissues, consists of the intercellular (ground) substance, cellular elements, and fibrous (Collagen) structures.

The ground substance is a heavily hydrated gel formed by high-molecular-weight compounds—proteins and carbohydrates—which make up about 30% of the intercellular matrix mass. The carbohydrate component of the ground substance is represented by heteropolysaccharides known as glycosaminoglycans (formerly mucopolysaccharides), an important structural feature of which is the presence of disaccharide units composed of hexuronic acid and amino sugar residues.

Glycosaminoglycans are hydrophilic compounds featuring numerous polar OH groups and a significant negative charge due to a high content of carboxyl and sulfo groups. This promotes the binding of positively charged cations such as K⁺, Na⁺, Cu²⁺, and Mg²⁺, which substantially enhances the water-retaining capacity of glycosaminoglycans.

The monomeric Disaccharides typically contain hexuronic acids (β-D-glucuronic acid and its epimer β-L-iduronic acid) and amino sugars, most commonly D-glucosamine and D-galactosamine along with their acetyl derivatives—N-acetyl-D-glucosamine and N-acetyl-D-galactosamine—linked together by a β-1,3 glycosidic bond, with the exception of heparin monomers, which feature an α-1,4 bond.

Based on monomer composition, quantity, and linkage patterns, glycosaminoglycans are classified into hyaluronic acid, chondroitin sulfates, heparan sulfate, and heparin.

Hyaluronic acid is a white, solid, amorphous substance that is soluble in water and insoluble in organic Solvents. It is optically active, with a specific optical rotation in water of [α]_D²⁰ = -70° to -80^o.

Hyaluronic acid is the only non-sulfated representative of glycosaminoglycans. Its molecules exhibit a disordered conformation as randomly coiled chains with a molecular weight ranging from 5·10⁴ to 8·10⁶ Da. They are built from disaccharide monomers consisting of β-D-glucuronic acid and N-acetyl-β-D-glucosamine residues linked by a β-1,3 bond. In turn, these monomers are joined into a linear polymer molecule via a β-1,4 glycosidic bond:

Due to The ionization of carboxyl groups in the hexuronic acid residues, hyaluronic acid has The ability to form complexes with proteins.

In tissues and fluids, hyaluronic acid exists either in a free state or associated with proteins, forming highly viscous solutions. It is most abundant in the vitreous body, tendons, Skin, and synovial fluid of joints. In mesenchymal and other tissues, it provides resistance to mechanical compression, helps prevent infection penetration, etc.

Chondroitin sulfates differ from one another in the positioning of their sulfuric acid residues. Four variants of chondroitin sulfates, designated as letters A, B, C, and D, are present in various animal tissues and Organs.

Chondroitin sulfates are found in Cartilage, skin, tendons, Heart cells, the aorta and Arteries, the cornea, bones, the sclera, the umbilical cord, and other tissues.

The disaccharide fragments of chondroitin sulfate A (chondroitin-4-sulfate) contain β-D-glucuronic acid and N-acetylgalactosamine-4-sulfate residues linked by a β-1,3 glycosidic bond:

In the molecule of chondroitin sulfate C (or chondroitin-6-sulfate), the disaccharide fragments consist of β-D-glucuronic acid and N-acetylgalactosamine-6-sulfate residues, also connected via a β-1,3 bond:

The main difference between chondroitin sulfate B (or dermatan sulfate) and the aforementioned chondroitin sulfates is the presence of an L-iduronic acid residue instead of D-glucuronic acid within its disaccharide unit:

Chondroitin sulfate D, isolated from invertebrate cartilage tissue, differs from other chondroitin sulfates by its high content of sulfuric acid residues. In the disaccharide fragments of this chondroitin sulfate, the sulfated residues are D-glucuronic acid (at C₂ or C₃) and N-acetylgalactosamine at C₆.

Heparan sulfate and heparin are also highly sulfated glycosaminoglycans with a Molecular Weight of 15–20 kDa. Their monomer units include glucuronate-2-sulfate and N-acetylglucosamine-6-sulfate linked together by an α-1,4-glycosidic bond:

These glycosaminoglycans are readily soluble in water; their solutions are optically active ([α]_D²⁰ = 45 - 70^o) and exhibit the lowest viscosity among all glycosaminoglycans.

Heparan sulfate is present On the surface of platelets and endothelial cells. Heparin was first discovered in the liver, but in addition to the liver, high concentrations of heparin are found on the surface and inside cells in the Lungs and muscles, and in smaller amounts in The Heart, Thymus, Spleen, Kidneys, and blood. Because heparin and heparan sulfate possess anticoagulant properties, they are widely used in medical practice as anticoagulants.

Typically, in their Native State, glycosaminoglycans are linked to a protein via a neutral trisaccharide, galactosyl-galactosyl-xylose, attached to a Serine residue of the polypeptide chain:

The polysaccharide chains of sulfated glycosaminoglycans adopt a helical conformation. Long polysaccharide chains of glycosaminoglycans form globules that occupy a fairly large volume.

Proteoglycan molecules—complex proteins of the Extracellular matrix—consist of a specific protein called the core protein (or 'core protein'), to which glycosaminoglycan molecules are attached via trisaccharides. Up to 100 polyglycoside chains of glycosaminoglycans can attach to a single polypeptide chain of the core protein (Fig. 11.3).

The helical chains of glycosaminoglycans within proteoglycan molecules form elastic macromolecular meshwork structures with defined pore sizes, which function as a molecular sieve during The transport of nutrients and metabolic products. Pore sizes are determined by the shape of the dominant glycosaminoglycan in a given tissue. Together with hyaluronic acid and specialized linking proteins, Proteoglycans participate in the formation of complex supramolecular structures (Fig. 11.4):

Fig. 11.3. Schematic structure of proteoglycan macromolecules

Fig. 11.4. Structure of the supramolecular Organization OF THE extracellular matrix ground substance

In addition to proteoglycans, the protein component of the extracellular matrix ground substance includes Glycoproteins, whose carbohydrate moiety is highly variable. Glycoproteins are complex proteins in which the carbohydrate component is represented by linear or branched oligosaccharides typically composed of glucose, galactose, mannose, methylmannose, xylose, arabinose, rhamnose, fucose, glucosamine, and galactosamine residues. Sialic acid residues are usually located at the ends of the oligosaccharide chains in glycoprotein molecules. The carbohydrate component can account for 1 to 60 % of the mass of these proteins. Glycoproteins include the majority of extracellular matrix and Blood Plasma Proteins, as well as proteins located on the outer surface of animal cell membranes.

Extracellular matrix glycoproteins include Fibronectin (with a molecular weight of 440 kDa). Its molecules consist of two polypeptide chains connected by a disulfide bond, along with a carbohydrate component that contains binding sites for proteoglycans, Collagen and Elastin fibrous structures of the matrix, and cell membrane Glycolipids. Thus, fibronectin functions as a "molecular glue," facilitating Cell Adhesion and intercellular binding.

Membrane glycoproteins also include Glycophorin, which is present in the erythrocyte cell membrane (Fig. 11.5). Glycophorin ("sugar carrier") is an integral membrane protein (molecular weight 30 kDa) containing 130 amino acid residues and a carbohydrate component that makes up approximately 60 % of the entire molecule's mass. Glycophorin is embedded in the membrane such that the hydrophilic C-terminus of its polypeptide chain—composed of glutamic and aspartic acid residues and negatively charged at pH 7.0—resides in the Cytoplasm, the central hydrophobic region spans the membrane, and the N-terminus projects onto the outer surface of The cell membrane. Attached to it via N- and O-glycosidic bonds is a polar carbohydrate component comprising up to 16 oligosaccharide chains that contain antigenic determinants defining specific Blood Groups (A, B, O) and also participate in Influenza virus reception.

Fig. 11.5. Schematic arrangement of glycophorin in the erythrocyte Plasma Membrane

A number of Blood Plasma glycoproteins form the antiproteinase system of blood, notably α1-antitrypsin, inter-α-Trypsin inhibitor, thermoacid-stable trypsin inhibitor, α1-antichymotrypsin, etc.; carbohydrates account for 20–35 % of their molecular weight.

All IMMUNOGLOBULINS, or Antibodies, of blood serum—specifically IgA, IgG, IgM, IgD, and IgE—are glycoproteins containing covalently linked oligosaccharide chains that make up 3 to 13 % of the mass of these macromolecules.

Thus, oligosaccharide chains within glycoprotein molecules directly participate in the Formation of the extracellular matrix ground substance, cell-cell interactions, antigen and virus reception, as well as the immunological and antiproteinase properties of blood, among other functions.