Fundamentals of Biochemistry - A. A. Anisimov 1986

Carbohydrates
Secondary Polysaccharides (Glycans)

The bulk of naturally occurring CARBOHYDRATES are high-molecular-weight compounds known as secondary Polysaccharides, or glycans. Upon Hydrolysis, they yield A large number of monosaccharide residues (up to several tens of thousands).

6.4.1. Classification and Nomenclature. Polysaccharides are divided into Homopolysaccharides (homoglycans) and Heteropolysaccharides (heteroglycans). The former consist of monosaccharide units of a single type, whereas the latter contain units of two or more types. Depending on the biological function of glycans, they are classified as either storage or structural polysaccharides. According to the nature of their polyglycoside chain, secondary polysaccharides can be linear or branched. Based on their source of origin, they are categorized as zooglycans, phytoglycans, and microbial polysaccharides.

There is no systematic nomenclature for polysaccharides. However, a rational naming principle is frequently applied: the name of the main monosaccharide (or Monosaccharides) is used as the base, with the suffix -ose replaced by -an. Examples include D-glucan (composed of glucose residues) and L-arabino-D-galactans (composed of L-arabinose and D-galactose residues). Alongside these, other names are also in use. For instance, polymers of uronic acids are termed polyuronides, and polysaccharides associated with Cellulose are called hemicelluloses. For many representatives, traditional names established long ago are still employed, such as starch, Glycogen, heparin, chondroitin, etc.

Unlike mono- and Oligosaccharides, glycans are either insoluble in Water or form highly viscous colloidal solutions; they lack a sweet taste, are extremely difficult to isolate from Tissues, and often undergo various changes during isolation (depolymerization, oxidation), which greatly complicates The Study of their Structure.

6.4.2. Selected Representatives of Homo- and Heteropolysaccharides.

Starch. Starch is not a chemically uniform substance: it consists of 96.1–97.6% polysaccharides (amylose and amylopectin), while Mineral Substances, primarily phosphates, account for 0.2 to 0.7%. Fatty acids such as palmitic, stearic, and others have been found in starch in amounts up to 0.6%. They are adsorbed onto the polysaccharide fraction of starch and can be removed by extraction with neutral organic Solvents (e.g., methanol).

In plants, starch serves as a storage nutrient and occurs in the form of starch granules, which vary in shape, size, chemical composition, and properties across different species and even among different Organs of the same plant.

α-Amylose typically consists of long, unbranched chains in which D-glucopyranose residues are linked by α(1→4) bonds. Amylose readily dissolves in warm water to yield solutions of relatively low viscosity. Amylose solutions are unstable and may precipitate upon standing. The Molecular Weight of potato amylose is approximately 400,000, whereas that of corn and rice seed amylose is ∼100,000–200,000. Amylose turns blue when treated with an iodine solution due to The formation of a coordination complex, in which iodine molecules are accommodated inside the helically coiled amylose chains.

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Fig. 6.3. Structure of amylose (A), amylopectin (B), and glycogen (C) molecules

Amylopectin consists of highly branched chains built from α-D-glucopyranose. In addition to α(1→4) linkages, it contains α(1→6) branch-point bonds located approximately every 20 glucose residues (Fig. 6.3). The molecular weight of amylopectin can reach 20∙10⁶. The blue-violet color reaction with iodine appears to result from the formation of both complex and adsorption compounds. Because the bonds between the carbohydrate components of starch and iodine are relatively weak, they easily dissociate upon heating or alkali addition, causing the blue color to disappear.

The ratio of amylose to amylopectin varies among different plant species (Table 6.1).

Table 6.1. Ratio of amylose to amylopectin in starch

During the Hydrolysis of Starch by heating in the presence of mineral acids, polysaccharides of varying molecular weight—dextrins—are formed as intermediates,

1. Amylodextrins — give a violet-blue color with iodine; they are white powders soluble in 25% ethanol but precipitated by 40% ethanol.

2. Erythrodextrins — turn red-brown with iodine, dissolve in 55% ethanol, and precipitate at a 65% concentration in the form of spherocrystals.

3. Achrodextrins — do not react with iodine and dissolve in 70% ethanol.

4. Maltodextrins — do not give a color reaction with iodine and are not precipitated by ethanol.

Starch has found widespread Practical Application in medicine and numerous industrial sectors, including food, textile, paper, leather, and pharmaceutical industries. On an industrial scale, starch is obtained in our country from potato tubers and corn grain. Potato tubers contain on average 15–25% starch by wet weight, while the seeds of certain cereal crops contain 40–60%.

Glycogen. Glycogen serves as the primary Energy and Carbohydrate reserve in humans and animals. Its concentration is particularly high in the Liver (up to 10%) and Muscles (up to 4%). It is also found in Fungi, certain higher plants (such as sweet corn), and microorganisms. Glycogen is a branched polymer formed by D-glucopyranose residues linked by α-1,4 bonds in the linear segments of the molecule and by α-1,6 bonds at the branch points. Unlike amylopectin, which has an average of one branch point per 20 glucose residues, glycogen features one α-1,6 bond per 8–12 D-glucopyranose residues. The higher degree of branching in glycogen molecules compared to amylopectin makes them more compact (see Fig. 6.3). The molecular weight of glycogen ranges from 3∙10⁵ to 1∙10⁸, and its molecular shape approaches spherical. Acid hydrolysis yields primarily α-glucose, α-maltose, and α-isomaltose.

Dextrans act as Storage Polysaccharides in Yeast and Bacteria. They are typically branched polysaccharides composed of α-D-glucopyranose residues joined by (1→6) linkages. Different dextrans vary in The Nature of their branch-point bonds, which may be of the 1→2, 1→3, or 1→4 type. The molecular weights of dextrans are very high (M ≈ up to 5∙10⁸). Dextrans and their derivatives exhibit antigenic Specificity and have found wide application as plasma expanders and anticoagulants.

Products of chemical modification of dextrans—Sephadex and Sepharose—are widely used in biochemical laboratories for the Separation of substance mixtures.

Polyfructosans. These are storage polysaccharides particularly characteristic of the Asteraceae family, where they replace starch. Among them, inulin is the most thoroughly studied. It is found in the tubers of Jerusalem artichoke, dahlia, and artichoke, where its content reaches up to 50%. It has also been detected in the roots of dandelion, kok-saghyz, and chicory.

Cellulose. This is the most widespread structural polysaccharide in the plant kingdom, accounting for more than 50% of all organic carbon in the biosphere. It is also present in bacteria and certain lower animals.

Fig. 6.4. Hydrogen bonding scheme in dry (A) and hydrated (B) cellulose

Wood consists of approximately 50% cellulose, while cotton is nearly 100%. Cellulose is a linear polysaccharide composed of ß-D-glucopyranose units linked by 1→4 bonds. These linear molecules typically run parallel to each other, with Hydrogen Bonds forming between them to produce microfibrils (Fig. 6.4).

Acid hydrolysis yields β-D-glucopyranose, whereas milder conditions produce the disaccharide cellobiose. The cellulose molecule exhibits a bent ("Hermans") conformation, which constitutes its Secondary structure. In this conformation, a Hydrogen bond forms between the hydroxyl group at C-3 and the ring oxygen of the adjacent residue, imparting even greater rigidity to the structure (Fig. 6.5).

Cellulose is insoluble in water, but soluble in ammoniacal copper salt solutions.

The molecular weight of cellulose has not been precisely established, though it is generally accepted to range from 3∙10⁵ to 2∙10⁶, with the number of glucose residues per molecule varying from 2∙10³ to 1,1∙10⁴. Cellulose microfibrils, together with associated substances such as hemicelluloses, Lignin, and pectic substances, form The plant Cell wall, which features a highly complex, multilayered structure. Within each layer, microfibrils are oriented differently relative to The Cell axis (mutually perpendicular, at an acute angle, or spiraled). As a result, the tensile strength of The Cell wall is reinforced in all directions. Given that the microfibrils themselves possess a rigid structure, the intricate architecture of the cell wall as a whole exhibits exceptional strength, a property that is also imparted to wood, which consists primarily of cell walls.

Fig. 6.5. "Hermans" conformation of cellulose (hydrogen bonds between O and H are shown by dashed lines)

X-Ray Diffraction studies have demonstrated that cellulose microfibrils contain crystalline regions where linear molecules are arranged in a strictly ordered, parallel manner along the fiber axis, forming micelles. In the intervals between crystalline micelles, cellulose molecules are less ordered, forming amorphous regions. These regions have lower tensile strength and undergo hydrolysis and oxidation reactions more rapidly.

Cellulose is of immense practical importance. It forms the bulk of cotton fabrics, paper, rayon, certain plastics and explosives, emulsifiers, protective colloids, etc. Carboxymethylcellulose and DEAE-cellulose are widely utilized in biochemical laboratory practice.

Chitin. A structural polysaccharide widely distributed in nature. It is a major component of the cuticle or exoskeleton of Arthropods and certain other invertebrates, as well as the cell walls of fungi. Chitin is never found in a free state; it is typically bound to Proteins, inorganic salts (such as CaCO₃), Lipids, and pigments. Structurally, chitin is a linear polymer composed of N-acetylglucosamine residues linked by ß(1→4)-glycosidic bonds. Chitin performs mechanical, supportive, and protective Functions in various organisms.

Hemicelluloses. This term encompasses a large group of polysaccharides that are insoluble in water but soluble in alkaline solutions. They serve as the principal Components of the matrix cementing cellulose fibers within Plant Cell Walls. They are found in significant quantities in lignified plant tissues: wood, straw, nuts, seeds, bran, and corn cobs.

They are hydrolyzed by acids more readily than cellulose, yielding mannose, galactose, arabinose, xylose, and occasionally glucose. Based on their hydrolysis products, hemicelluloses are divided into several groups: Mannans, galactans, xylans, etc.

Pectic substances. In plants, they occur as insoluble protopectin in the intercellular substance and cell wall matrix, as well as soluble pectin in fruit and vegetable juices. Insoluble protopectin is a methyl ester of polygalacturonic acid bound to the galactan and araban of the cell wall. L-arabinose, D-galactose, and L-rhamnose residues participate alongside D-galacturonic acid in forming the main chain. Cellulose, Ca and Mg ions, and H₃PO₄ take part in the formation of protopectin along with pectic substances. Protopectin is converted into soluble pectin upon Treatment with dilute acids or the enzyme protopectinase.

The breakdown of protopectin proceeds as follows;

The conversion of protopectin into soluble pectin is observed during fruit ripening, which leads to a decrease in fruit firmness and an improvement in flavor. Pectic substances play a crucial role in flax Processing, a retting process based on the hydrolysis of pectic substances by Enzymes secreted by specific microorganisms. This results in the maceration of flax stems and the separation of fibers from one another. A characteristic and important property of soluble pectin is its ability to form gels in the presence of sugar (65–70% solution) and acid (pH 3.1–3.5). The resulting gel contains 0.2–1.5% pectin. This property is widely exploited in the confectionery industry for The production of jellies, marshmallows, marmalades, pastilles, and fruit caramel fillings.

Pectic substances are highly beneficial for humans. They regulate bowel function and possess detoxifying properties (for instance, in cases of mercury poisoning).

Lichenin. A polysaccharide found in Lichens, particularly abundant in Iceland moss (Cetraria islandica), where lichenin content reaches 45–50% of dry weight. Approximately 73% of its a-D-glucose residues are linked by 1→4-glycosidic bonds, and 27% by 1→3 bonds. Human beings cannot digest lichenin. However, reindeer, for whom it serves as a primary food source, digest it thanks to specialized bacteria present in their digestive tract.

Agar-agar. A high-molecular-weight polysaccharide found in certain marine Algae. In the former USSR, it is extracted from the red alga Ahnfeltia, which grows in the White, Barents, and Baltic Seas, as well as in Far Eastern water bodies.

Agar-agar dissolves in water upon heating, and its aqueous solutions solidify into gels; therefore, it is widely used in bacteriology for preparing solid nutrient media, and in the confectionery industry for manufacturing jellies, pastilles, and marmalades. It is a mixture of agarose and agaropectin. Agarose consists of alternating D-galactose and 3,6-anhydro-L-galactose residues linked alternately by β(1→4) and a(1→3) bonds. Agaropectin contains chains formed by D-galactopyranose residues, some of which are sulfated.

6.4.3. Glycosaminoglycans. Formerly known as mucopolysaccharides. They contain alternating paired units consisting of amino sugar and hexuronic acid residues, less frequently monosaccharides, and possess a high molecular weight. In the body, they are invariably bound to proteins and form the ground substance of the Extracellular matrix in Connective Tissue.

Hyaluronic acid. Found in many types of connective tissue, with its highest concentrations in the umbilical cord, vitreous humor of the eye, synovial (joint) fluid, and Skin. In tissues and fluids, hyaluronic acid exists in a free state or associated with proteins, forming highly viscous solutions that confer tissue resistance to infection penetration. Hyaluronic acid also acts as a joint lubricant.

Many bacteria, particularly Gram-positive ones, produce a protective capsule composed of hyaluronic acid, which is directly related to virulence (removal of the capsule in group C streptococci reduces their virulence by a factor of 100,000). The repeating unit of hyaluronic acid is a disaccharide consisting of ß-D-glucuronic acid and ß-N-acetyl-D-glucosamine residues linked by a (1→5) bond. The disaccharide units are linked linearly by a (1→4) bond (see the presented fragment of the hyaluronic acid molecule).

Chondroitin sulfates. They serve as major Structural components of Cartilage, tendons, and the Cytology/practical/76.html">Cornea of the eye, and are also found in Bone tissue and skin. Several types are distinguished: chondroitin-4-sulfate (chondroitin sulfate A), chondroitin-6-sulfate (chondroitin sulfate C), and dermatan sulfate (chondroitin sulfate B). The polysaccharide chains of chondroitin sulfates A and C consist of repeating disaccharide units of ß-D-glucuronosyl-(1→3)-ß-D-N-acetylgalactosamine linked by a ß(1→4)-glycosidic bond. Chondroitin sulfate A contains a sulfo group at the C-4 position of N-acetylgalactosamine, whereas chondroitin sulfate C is sulfated at C-6.

In dermatan sulfate, D-glucuronic acid residues are replaced by L-iduronic acid residues linked via an a(1→3)-glycosidic bond to sulfated N-acetylgalactosamine. Little information is available on the Biological Role of dermatan sulfate; it exhibits anticoagulant activity and stabilizes Collagen fibers.

Keratan sulfates. Their chains consist of alternating disaccharide fragments composed of D-galactose and N-acetylglucosamine-6-sulfate residues joined by a ß(1→4) bond. The disaccharide units are linked by a ß(1→3) bond. Galactose residues may also be sulfated. Occasionally, the molecule contains sialic acid, fucose, and mannose. Keratan sulfates are present in the ground substance of cartilage and the cornea of the eye.

Heparin and heparan sulfate. They share a very similar structure with Other types of glycosaminoglycans, yet differ from them in their localization and function in animal tissues. Heparin is typically present On the surface of many Cells but functions as an intracellular substance of mast cells, where it is synthesized. It was first discovered in the liver, as reflected in its name. It is also found in the skin, Lungs, and gastric mucosa.

Heparin is a vital natural anticoagulant; it participates in Lipid METABOLISM by inducing the release of lipase into the bloodstream and influences Cholesterol Metabolism. In medical practice, it is used in the treatment of thrombosis, burn disease, and cardiovascular disorders, as well as a Blood stabilizer during transfusions.

The carbohydrate structure of this polysaccharide can be represented as a repeating tetrasaccharide fragment consisting of two Disaccharides linked by an a(1→4) bond. One of these contains L-iduronic acid and N-acetylglucosamine sulfated at the C-2 position, while the other contains ß-D-glucuronic acid and N-acetylglucosamine sulfated at the C-6 position.

Heparan sulfate apparently consists of similar fragments with a larger number of N-acetyl groups, fewer N-sulfate groups, and a low degree of O-sulfation. It is present on The surface of platelets and endothelial cells, which is associated with its anticoagulant function.

6.4.4. Bacterial cell walls and their polysaccharides. Bacterial Cell walls contain mixed Biopolymers that yield not only monosaccharides but also other substances upon hydrolysis. The primary polymer of this type is peptidoglycan (murein). It forms the framework of cell walls in both Gram-positive and Gram-negative bacteria. This framework constitutes a single entity because the murein forming it is a single giant molecule (M≈5∙10¹⁰).

The murein of the Staphylococcus aureus cell wall has been studied the most. The main repeating unit of its polysaccharide chains is a muropeptide—a disaccharide in which N-acetyl-D-glucosamine is linked by a ß(1→4) bond to N-acetylmuramic acid. Attached to the OH group of lactic acid is a tetrapeptide side chain consisting of D- and L-Alanine, L-Lysine, and D-isoglutamine residues. The terminal D-alanine residue in the side peptide chain of one polysaccharide is covalently linked to the L-lysine of the side chain of another polysaccharide via a pentaglycine bridge. In E. coli, L-lysine is replaced by meso-diaminopimelic acid, and the peptide linkage is direct.

The presence of a network of parallel polysaccharide chains linked by numerous peptide cross-links creates a closed meshwork. In St. aureus and other Gram-positive bacteria, this network is dense and relatively thick (up to 12 nm), whereas in Gram-negative bacteria it is loose and thin (2 nm).

In addition to murein, bacterial cell walls contain other components that vary among different bacterial species. For instance, Gram-positive bacteria are characterized by Teichoic Acids, polysaccharides, and Polypeptides (or proteins) that are intricately interwoven into the murein network. Teichoic acids are long chains consisting of glycerol or ribit residues linked together by phosphodiester bonds. The free OH groups of the teichoic acid backbone can be occupied by residues of D-alanine, D-glucose, D-galactose, L-rhamnose, N-acetyl-D-glucosamine, and N-acetylgalactosamine.

The polysaccharides accompanying murein are represented by rhamnose, glucose, and galactose residues (or their amines), as well as mannose residues. Both teichoic acids and the cell wall polysaccharides of Gram-positive bacteria exhibit antigenic activity.

The accompanying components interwoven into the murein network of Gram-negative bacteria are more numerous and are represented by polypeptides, Lipoproteins, and complex lipopolysaccharides. All these components impart complex antigenic specificity and acceptor specificity toward Viruses and bacteriocins to Gram-negative bacterial cells. While the cell wall of Gram-positive bacteria is a rigid, brittle cellular shell (akin to the exoskeleton of crustaceans), the cell walls of Gram-negative bacteria feature a smooth, pliable coating rich in lipids that shields the underlying murein Skeleton.

6.4.5. Polysaccharide structure and phylogeny. Comparing the structures of various polysaccharides across different Representatives of the living world allows for A number of interesting Conclusions. Almost all D-glucans discovered in either plant or animal cells have also been isolated from one or another unicellular Organism. Lower unicellular organisms (bacteria, mycoplasma, blue-green algae) are characterized by a greater diversity of a-D-glucans than higher unicellular organisms (myxomycetes, fungi, algae, and Protozoa), in which ß-D-glucans predominate. A multitude of ß-D-glucans with a predominance of ß(1→3) bonds has been found in various fungi. Typically, ß-D-glucans are classified as structural or extracellular polysaccharides, whereas a-D-glucans (glycogen, starch) represent intracellular reserve compounds. However, fungal ß-D-glucans and marine algal ß(1→3)-laminaran, being extracellular polymers, can also serve as sources of carbon and energy.

Cellulose, the most widespread organic substance in nature and quantitatively dominant in plants, is found in representatives of all major life forms as a structural or extracellular polysaccharide. Glycogen and galactans are universally distributed in the living world. However, the stereoisomeric configuration of the latter varies among different taxa. For example, in Penicillium species, it is a polymer of D-galactofuranose with (1→5) bonds, whereas in higher plants (lupine) it is a polymer of D-galactopyranose with ß(1→5) bonds. Similarly, other homoglycans (mannans, xylans, ketoglycans) and heteroglycans from different "kingdoms" of the living world differ in monomer stereoisomerism and the nature of their linkages. Only cellulose and glycogen are universal in this regard across All living organisms. Certain polysaccharides are found only in higher unicellular organisms (galactosaminoglycans, polyglucuronic acid), while others are found in lower unicellular organisms (polymannuronic acid, V_i antigen or poly-N-acetylgalactosaminuronic acid, colominic acid or poly-N-acetylneuraminic acid).

Since chondroitin sulfate B and hyaluronic acid are present in both lower unicellular organisms and animals, some authors suggest that these animal polysaccharides originated from analogous polymers of unicellular organisms.