Structural Biochemistry - Study Guide - E. A. Bessolitsyna 2015

In nature, the majority of CARBOHYDRATES exist as high-molecular-weight Polysaccharides. The Biological Significance of various polysaccharides is that some serve as a storage form of Monosaccharides for METABOLISM/26.html">Energy Metabolism in an insoluble and, therefore, osmotically inactive form, while others act as Structure/83.html">Structural elements of Cell walls and Connective Tissue. Upon complete Hydrolysis by acids or specific Enzymes, polysaccharides are cleaved to yield monosaccharides or their derivatives.

Polysaccharides, also known as glycans, differ from one another in The Nature of their constituent monosaccharide residues, as well as in chain length and the degree of branching. They can be divided into two main types: Homopolysaccharides, consisting of residues of a single type of monosaccharide, and Heteropolysaccharides, containing residues of two or more different monosaccharides. An example of a homopolysaccharide is the storage carbohydrate starch, which is composed exclusively of D-glucose residues. An example of a heteropolysaccharide is hyaluronic acid found in connective tissue, which consists of alternating residues of two different monosaccharides.

Unlike Proteins, polysaccharides cannot be characterized by a strictly defined molecular weight; as a rule, they are represented by mixtures of high-molecular-weight compounds. Depending on the metabolic needs of the Cells, monosaccharide residues can be enzymatically added to or cleaved from polysaccharides. Like Disaccharides, polysaccharides are classified into reducing and non-reducing based on the presence of a free aldehyde group that, upon oxidation, reduces certain Metal Ions.

According to their function, polysaccharides are divided into structural and storage types.

Storage Polysaccharides ensure the accumulation of monosaccharides involved in energy metabolism in the form of compact, insoluble structures (inclusions). Their insolubility ensures they have no effect on the osmotic pressure within The Cell.

Structural polysaccharides serve as extracellular supporting elements in the cell walls of unicellular microorganisms, Fungi, and higher plants, and also constitute part of vertebrate connective tissue and the arthropod exoskeleton. Structural polysaccharides protect cells, Tissues, and Organs, giving them shape and maintaining it. Storage and structural polysaccharides vary among different organisms.

Class="center">

Figure 21. Structure of Glycogen

Glycogen is a polysaccharide used by animal organisms to store carbohydrates. It is often referred to as animal starch. Glycogen is most abundant in the Liver, where it accounts for up to 7% of the organ's total weight; it is also present in skeletal Muscles. In liver cells, glycogen occurs as large granules composed of smaller granules, which in turn are formed by single, highly branched glycogen molecules with an average Molecular Weight of several million. The enzymes responsible for Glycogen Synthesis and degradation are tightly bound to these granules. Glycogen is deposited as granules in the Cell Cytoplasm.

In fungi, glycogen is stored in the hyphal cells.

Glycogen is a reducing homopolysaccharide formed by α-D-glucopyranose residues. It is characterized by a more branched structure than amylopectin, with linear chain segments comprising 11–18 α-D-glucopyranose residues [linked by α(1–4) glycosidic bonds], while branching points are connected by α(1–6) glycosidic bonds (Figure 21).

The most widespread bacterial polysaccharide is glycogen, The structure of which was discussed in the previous section. However, other types also occur (Figure 22).

Figure 22. Structure of bacterial storage polysaccharides

Dextran. This is a linear reducing homopolysaccharide of α-D-glucose, with monomers joined by 1–6 O-glycosidic bonds. Dextran is a linear or slightly branched molecule with a molecular weight of approximately 10⁶. For a long time, this polymer attracted the interest of scientists only, but later found Practical Application. Partial hydrolysis products of this polysaccharide are used as additives in plasma expanders to establish natural oncotic pressure, which significantly improves The properties of artificial plasma substitutes in Blood loss replacement.

Levan. A storage polysaccharide of Bacillus subtilis. It is a linear non-reducing homopolysaccharide formed by methyl ester residues of β-D-fructose, with monomers linked by 1–6 glycosidic bonds. The polymer has a relatively short chain length of 10–12 monomers.

Starch. Monosaccharide residues in starch are linked by α-glycosidic bonds (Figure 23). A compound of this structure formed exclusively by glucose residues is a homopolymer, referred to as a glucosan or glucan. It is the most important type of dietary carbohydrate, found in cereals, potatoes, legumes, and other plants. The two Main Components of starch are amylose (15–20%), which has an unbranched helical structure, and amylopectin (80–85%), formed by branched chains where each branch consists of 24–30 glucose residues linked by (1–4) bonds [with branching points connected by (1–6) bonds]. 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 twisted, helical conformation, which facilitates The formation of dense granules found in most plant cells. The α(1–4) bonds of glycogen and starch are readily hydrolyzed by α-amylase in the gastrointestinal tract of vertebrates, and the resulting D-glucose enters the bloodstream to be subsequently utilized in energy metabolism. Like glycogen, starch is a reducing polysaccharide.

Dextrins are substances formed during the partial Hydrolysis of Starch. The products formed at a specific stage of hydrolysis are termed "limit dextrins".

Fructans. These are linear, reducing homopolymers of β-fructose linked by β(2–6) glycosidic bonds (Figure 23). Fructans are found in various plant species: irisin in iris rhizomes, asparagosin in asparagus roots, and secalin in rye. Differences among fructans isolated from various sources are related to the Variability in molecular weight and the number of monomers comprising the polysaccharide.

Figure 23. Plant structural and storage polysaccharides. A - starch; B - schematic Organization of a starch molecule; C - inulin; D - fructans; E - Cellulose; F - xylose; G - Pectins

Inulin is a linear, non-reducing polysaccharide found in the tubers and roots of dahlias, artichokes, and dandelions. Upon hydrolysis, it yields fructose, classifying it as a fructan. It is a heteropolymer consisting of a single glucose molecule linked to a β-fructose via a β(1—1) glycosidic bond, while the remaining β-fructose monomers are joined by β(1—2) glycosidic bonds (Figure 23). Unlike potato starch, this polysaccharide readily dissolves in warm Water and is used in physiological research to determine the Glomerular Filtration rate in Kidneys. Furthermore, this polymer—composed primarily of fructose—attracts the interest of nutritionists and physicians as a starch substitute for patients with Diabetes Mellitus. In such patients, cellular glucose uptake is impaired, whereas fructose is readily absorbed by cells and can thus serve as a glucose substitute. This same property has generated significant interest in plants that store fructans. Both inulin and other fructose polymers can be utilized in diabetic diets as starch substitutes.

Cellulose is a linear, unbranched, non-reducing homopolysaccharide composed of 10,000 or more D-glucose residues linked together by (1—4) glycosidic bonds; in this respect, it resembles amylose and the linear segments of glycogen chains. However, one very important difference exists between these polysaccharides: in cellulose, the (1—4) bonds have the β-configuration, whereas in amylose, amylopectin, and glycogen, they have the α-configuration.

This seemingly minor structural difference between cellulose and amylose leads to profound differences in their properties. Due to the specific configuration of its bonds, cellulose polymer chains are highly extended and align side by side to form long, insoluble fibrils.

Cellulose is a tough, fibrous, water-insoluble substance found in Plant Cell Walls, predominantly in branches, stems, tree trunks, and other woody plant tissues (Figure 23, E). Wood consists mainly of cellulose and other polymeric substances, while cotton is almost pure cellulose. If proteins are the most widespread intracellular Biopolymers, cellulose is undeniably not only the most abundant extracellular structural polysaccharide in the plant kingdom, but also the most widespread biopolymer in nature overall.

Enormous quantities of cellulose are synthesized by plants annually, produced not only by forest trees but also by agricultural crops. Calculations indicate that plants generate approximately 50 kg of cellulose daily for every person living on Earth. Cellulose has widespread industrial Applications. It is insoluble in water and acids, transitioning into a semi-liquid state only in concentrated alkalis upon heating, a property utilized in the manufacture of viscose rayon.

The β(1—4) bonds in the cellulose molecule are not hydrolyzed by α-amylases. Because the vertebrate intestine lacks an enzyme capable of hydrolyzing cellulose, it remains undigested, and its D-glucose residues cannot serve as food for most higher organisms.

Termites digest cellulose efficiently, but only because their gut harbors parasitic microorganisms such as Trichonympha, which secrete cellulase—the enzyme responsible for hydrolyzing cellulose and enabling wood Digestion in termites. Cellulase is also synthesized by certain wood-decaying bacteria and fungi.

Among vertebrates, only cattle and other ruminants (sheep, goats, camels, giraffes, etc.) can utilize cellulose as food, though they do so in a rather unusual manner: the major portion of the digestive tract, accounting for 15% of a cow's total body weight, consists of four interconnected stomachs. The first two of these comprise the rumen. Microorganisms residing within the rumen secrete cellulase and break down cellulose into D-glucose, which is subsequently fermented into short-chain Fatty acids, carbon dioxide, and methane gas (CH₄). The resulting Fatty acids are absorbed into the cow's bloodstream, distributed to tissues, and utilized as fuel. Methane and CO₂, produced at a rate of 2 L/min, are continuously eliminated through an involuntary process resembling a faint burp. In the remaining two compartments of the ruminant Stomach, the microorganisms that have completed their task are themselves digested by enzymes secreted by the gastric mucosa; this process yields Amino Acids, sugars, and other products that are absorbed and utilized as nutrients by the cow's Organism.

Thus, a symbiotic relationship is established between the cow and the ruminal microorganisms, wherein the microbes enjoy a brief but prosperous life in a comfortable, warm environment, while cellulose from clover and other grasses serves as the primary fuel source for both the symbionts and the host organism.

Hemicelluloses

These are diverse plant polysaccharides that make up The Cell wall, with xylose and pectins as Examples.

Xylose. A linear reducing homopolymer of xylose in the β-pyranose form, with monomers linked by β (1–4) glycosidic bonds (Figure 23, E).

Pectins. These polymers are very common in the cell walls of fruits and are responsible for their gelling properties (the higher the pectin content, the firmer the jams and preserves). It is a linear reducing homopolymer whose monomer is galacturonic acid methyl ester (Figure 23, F). The monomers are linked by an α (1–4) glycosidic bond. Pectic substances are abundant in the fruits of quince, certain varieties of pears, and apples; they facilitate the gelling process, which results in a firmer marmalade. Until recently, this property was of interest only to confectioners. However, these molecules have recently attracted considerable attention as studies have shown that pectins are involved in the removal of heavy metal salts and radionuclides. Consequently, pectins and their partial hydrolysis products are used as anti-radiation agents or additives thereof.

The plant cell wall is formed with the participation of The Plasma Membrane and is an extracellular, multi-layered structure that protects the cell surface and acts as an external exoskeleton of the plant cell. The plant cell wall consists of two components: an amorphous, plastic, gel-like matrix (ground substance) with a high water content, and a supportive fibrillar system (Figure 24). This structure is reminiscent of reinforced concrete. The iron reinforcement role is played by fibrils made of cellulose molecule bundles, forming a three-dimensional network. The space between the reinforcement molecules is filled with hemicelluloses, which form a gelatinous matrix.

Additional polymeric substances and salts often incorporated into the cell walls impart rigidity and render them water-repellent. Chemically, the main components of plant cell walls belong to structural polysaccharides. The matrix of plant cell walls contains heterogeneous groups of polysaccharides that dissolve in concentrated alkalis, namely hemicelluloses and pectic substances. Hemicelluloses are branched polymer chains consisting of various hexoses (glucose, mannose, galactose, etc.), pentoses (xylose, arabinose), and uronic acids (glucuronic and galacturonic). These hemicellulose components combine in various quantitative ratios to form diverse combinations.

Hemicellulose chains do not crystallize or form elementary fibrils; instead, they constitute an amorphous, gel-like substance. Due to the presence of polar groups in uronic acids, they are highly hydrated. Pectic substances form a heterogeneous group comprising branched, highly hydrated polymers bearing negative charges due to numerous galacturonic acid residues. Owing to the properties of its components, the matrix represents a soft, plastic mass reinforced by fibrils. The fibrous components of plant cell walls typically consist of cellulose, a linear, unbranched glucose polymer. The molecular weight of cellulose ranges from 5—10⁴ to 5—10⁵, which corresponds to 300–3000 glucose residues. Such linear cellulose molecules can associate into bundles or fibers.

Figure 24. Diagram of the plant cell wall structure; 1 — primary wall (two layers on either side of 2); 2 — middle lamella; 3 — secondary wall layers; 4 — tertiary wall

Within the cell wall, cellulose forms fibrils that consist of submicroscopic microfibrils up to 25 nm thick, which in turn are composed of numerous parallel cellulose molecular chains.

The quantitative ratios of cellulose to matrix substances (hemicelluloses) can vary significantly across different objects. Over 60% of the dry mass of primary walls consists of their matrix, and about 30% is accounted for by the skeletal substance, cellulose. In fresh cell walls, almost all water is bound to hemicelluloses; therefore, the mass of the ground substance in a swollen state reaches 80% of the total fresh mass of the wall, whereas the fibrous content is reduced to just 12%. In cotton fibers, the cellulosic component makes up 90%, while in wood, cellulose accounts for 50% of the cell wall components.

In addition to cellulose, hemicelluloses, and pectins, Cell walls contain supplementary components that impart specific properties. For instance, the encrustation (incorporation) of cell walls with Lignin (a coniferyl alcohol polymer) leads to the lignification of cell walls and increases their mechanical strength. In such walls, lignin replaces the plastic matrix substances and acts as the primary high-strength material. Often, the matrix is reinforced with Mineral Substances (Si0₂, Са₂СО₃, etc.).

Various adcrusted substances, such as cutin and suberin, can accumulate On the surface of the cell wall, leading to cell suberization (Figure 25). In epidermal cells, wax is deposited on The surface of the cell walls, forming a waterproof layer that prevents cellular water loss. Due to its porous, loose structure, the plant cell wall is highly permeable to low-molecular-weight compounds such as water, sugars, and ions. However, macromolecules penetrate cellulose walls poorly: the pore size in the walls that permits free diffusion of substances is only 3–5 nm.

Experiments with labeled compounds have demonstrated that during cell wall growth, the secretion of the substances forming it occurs across the entire surface of the cell. The amorphous matrix substances, hemicelluloses, and pectins are synthesized in the vacuoles of the Golgi apparatus and released through the Plasmalemma via exocytosis. Cellulose fibrils are synthesized by specialized enzymes embedded within the plasmalemma. The walls of differentiated, mature cells are typically multilayered, where cellulose fibrils in different layers are oriented variably, and their quantity can also fluctuate significantly. Primary, secondary, and tertiary cell walls are commonly distinguished. To understand the structure and origin of these walls, it is necessary to examine how they form following Cell Division.

Figure 25. Cell wall encrustation. A — fibrillar framework and interfibrillar matrix; B — a lignin-encrusted wall that has lost its extensibility, retaining matrix remnants; C — subsequent encrustation with phenols and (or) mineral substances, leading to increased wall rigidity

Figure 26. Fungal structural polysaccharides

On the one hand, the cell walls of fungi are poorly understood in terms of structure; on the other hand, they are highly diverse. For this reason, one can speak of the polysaccharides that form The basis of the fungal cell wall, but not of the cell wall structure itself.

In various groups of fungi, the cell wall is formed by several types of polysaccharides. In oomycetes (which include the CAUSATIVE AGENT OF late blight), the cell wall is composed of cellulose. In Yeasts, it is formed by Mannans. Mannans are reducing homopolymers of β-D-mannose, where monosaccharides are linked by β (1—6) glycosidic bonds. The polymer is branched. The "branches" are attached by a β (1—2) bond, and the monomers within the branch are connected by the same type of bond. This results in a complex intersecting framework resembling felt in structure. Additional reinforcement of molecular bonding is provided by cell wall proteins. In Basidiomycetes, the cell wall is formed by Chitin (Figure 26), which is a long, unbranched, reducing homopolysaccharide. The structure of chitin is composed of N N-acetyl-O-glucosamine units linked by β (1—4) glycosidic bonds.

The Introduction/37.html">Bacterial cell wall forms a rigid, porous envelope around the cell, physically protecting the delicate Cell Membrane and cytoplasm. The structural foundation for the cell walls of most bacteria is a cross-linked covalent framework that almost entirely surrounds the cell (Figure 27).

Figure 27. Formulas of bacterial structural polysaccharides

It consists of long, parallel polysaccharide chains interconnected at regular intervals by cross-bridges made of short polypeptide chains. The polysaccharide chains consist of alternating monosaccharide residues of N-acetyl-D-glucosamine and N-acetylmuramic acid (a complex nine-carbon sugar) linked to each other by β (1-4) bonds. Attached to each N-acetylmuramic acid residue is a side tetrapeptide chain formed by sequentially linked L-Alanine, D-glutamic acid, meso-diaminopimelic acid, and D-alanine. As a result, parallel polysaccharide chains are cross-linked by short polypeptide bridges, the structure of which varies among different bacterial species. In the pyogenic bacterium Staphylococcus aureus, which causes boils and wound suppuration, the acetylmuramic acid residues in adjacent polysaccharide chains are linked to one another by peptide chains consisting of five Glycine residues.

This entire cross-linked structure surrounding the cell is called murein or peptidoglycan; the latter name emphasizes the hybrid nature of this structure, which represents a combination of peptide and polysaccharide elements. Extending continuously along the entire surface of the bacterial cell, peptidoglycan can be viewed as a single giant sac-like molecule with a net-like structure. In Gram-positive bacteria (which retain the Gram stain, i.e., Treatment with the dye crystal violet), peptidoglycan forms about 60 concentric layers around the cell, penetrated by other macromolecular components, primarily teichoic acids.

In Gram-negative bacteria, such as E. coli, the peptidoglycan framework is covered by a lipid-rich outer envelope containing hydrophobic proteins. The integrity of cell walls is of vital importance for bacterial defense, growth, and division. The action of penicillin—one of the most valuable Antibiotics used to combat bacterial infections—is based on its ability to inhibit The final stage of enzymatic peptidoglycan synthesis in susceptible microorganisms, leading to the formation of defective cell walls and the suppression of bacterial growth.

Figure 28. Structure of bacterial cell walls. A — Gram-positive; B — Gram-negative. C — Structural formula of glycerol teichoic acid. It contains alternating residues of D-alanine and N-acetylglucosamine

In terms of Structure and Chemical composition, the Prokaryotic Cell wall differs sharply from that of eukaryotic organisms. It contains specific polymer complexes not found in other cellular structures. The chemical Composition and Structure of the cell wall are constant for a given species and serve as an important diagnostic feature. Depending on the cell wall structure, prokaryotes belonging to eubacteria are divided into two major groups (Figure 28).

It was discovered that if fixed eubacterial cells are treated first with crystal violet and then with iodine, a colored complex is formed. Upon subsequent treatment with alcohol, The Fate of this complex depends on the cell wall structure: in so-called Gram-positive species, this complex is retained by the cell, and the cells remain stained; in Gram-negative species, conversely, the colored complex is washed out of the cells, and they are decolorized. In some eubacteria, a positive staining reaction using the method described above is characteristic only of cells in an active growth phase. It has been established that the colored complex is formed on the protoplast, but its retention within the cell or washout during subsequent alcohol treatment is determined by the Structural Features of the cell wall.

The cell walls of Gram-positive and Gram-negative eubacteria differ dramatically both in Chemical Composition and in ultrastructure. The eubacterial cell wall comprises seven distinct groups of chemical substances, with peptidoglycan being exclusively present in the cell wall.

In Gram-positive eubacteria, it constitutes the bulk of the cell wall substance (from 40 to 90%), whereas in Gram-negative bacteria, the peptidoglycan content is significantly lower (1–10%). The cell wall of cyanobacteria, similar to that of Gram-negative eubacteria, contains 20 to 50% of this heteropolymer. Under an Electron microscope, the cell wall of Gram-positive eubacteria appears as a homogeneous, electron-dense layer, the thickness of which varies from 20 to 80 nm depending on the species.

In Gram-negative eubacteria, a multilayered cell wall has been discovered. The inner electron-dense layer, approximately 2–3 nm thick, consists of peptidoglycan. Adjoining it externally is typically a wavy layer (8–10 nm) with a characteristic structure: two electron-dense bands separated by an electron-transparent space. This appearance is typical of unit membranes. Therefore, the triple-layered outer component of the Gram-negative eubacterial cell wall is termed the outer membrane. The cell wall of Gram-positive eubacteria closely adheres to the membrane, in contrast to the cell wall of Gram-negative species, whose components (the peptidoglycan layer and the outer membrane) are separated by an electron-transparent gap and are distinctly separated from the membrane in a similar manner.

The space between the cytoplasmic and outer membranes is termed the periplasmic space. As can be seen from the cell wall structure of both eubacterial groups, it is characteristic exclusively of Gram-negative forms. Cell wall of Gram-positive eubacteria.

The bulk of the Gram-positive eubacterial cell wall consists of a specific heteropolymer, peptidoglycan. More than 100 different chemical types of peptidoglycan have been found in Gram-positive eubacteria. Most variations pertain to the peptide moiety of the molecule. Two Features of the peptide tail deserve attention: the presence of amino acids in the D-form (an unnatural configuration) and a high content of amino acids with two amino groups. This is of fundamental importance for the Spatial Organization of peptidoglycan. Both amino groups of these Amino acids can participate in the formation of peptide bonds, with the second amino groups forming additional peptide bonds between the heteropolymer chains. In most cases, peptide bond formation involves the carboxyl group of D-alanine from one tetrapeptide and the free amino group of diaminopimelic acid from another. Sometimes, the linkage between tetrapeptides of different glycan chains is mediated by Other Amino Acids. It is easy to envision that a multitude of heteropolymer chains can be "stitched" together in this manner. The frequency of cross-linking varies, as not all peptide tails participate in the formation of interchain bonds. Some form covalent bonds with other chemical molecules that make up the cell wall, and finally, a portion of the tetrapeptide tails remains in a free state.

The peptidoglycan surrounding the protoplast of Gram-positive eubacteria is essentially a single giant molecule "cross-linked" via glycosidic and peptide bonds. It is the latter that provide it with a three-dimensional spatial organization. In addition to peptidoglycan, the cell walls of Gram-positive eubacteria contain another unique class of chemical compounds: teichoic acids. These are polymers based on ribitol (a pentahydric alcohol) or glycerol (a trihydric alcohol), whose residues are interconnected by phosphodiester bonds. Some free hydroxyl groups in the alcohol molecules may be substituted with residues of D-alanine, glucose, N-acetylglucosamine, and certain other sugars. Teichoic acids can covalently bind to N-acetylmuramic acid. Because these are long linear molecules, they can span the entire peptidoglycan layer, reaching the outer surface of the cell wall. In this case, they likely serve as the primary Antigens of Gram-positive eubacteria. The remaining free hydroxyl groups of phosphoric acid impart polyanionic properties to teichoic acid. As polyanions, teichoic acids determine the surface charge of the cell. The sugar components of teichoic acids are part of the receptors for certain Bacteriophages and determine the possibility of phage adsorption on the cell surface.

Polysaccharides, proteins, and Lipids are also found in small quantities within the cell wall of Gram-positive eubacteria. Unlike proteins—which, in species that possess them, form a separate layer on the outer surface of the cell wall—polysaccharides and lipids have been shown capable of covalent binding to the macromolecules of the cell wall.

Thus, the main Components of the cell wall in Gram-positive eubacteria are Three types of macromolecules: peptidoglycans, teichoic acids, and polysaccharides. Through covalent bonds, they form a complex structure with a highly ordered spatial organization. The cell wall of bacilli, such as Bacillus subtilis, is approximately equivalent in thickness to 40 peptidoglycan molecules.

Overall, the cell wall of Gram-positive eubacteria can be visualized as a spongy structure with pores approximately 1 — 6 nm in diameter. The passage of molecules through such a cell wall is determined by its charge and pore size.

Cell wall of Gram-negative eubacteria.

In Gram-negative eubacteria, the STRUCTURE OF THE cell wall is much more complex than in Gram-positive ones. It incorporates a significantly larger number of macromolecules of various chemical types. Peptidoglycan forms only the innermost layer of the cell wall, lying loosely against the membrane. The content of this heteropolymer varies widely among different species of Gram-negative eubacteria. In most species, it forms a single- or double-layered structure characterized by very sparse cross-links between the heteropolymer chains.

The Chemical Structure of peptidoglycan in Gram-negative eubacteria is largely similar to that of typical Gram-positive peptidoglycan. Exterior to the peptidoglycan lies an additional cell wall layer known as the outer membrane. It consists of Phospholipids (typical of elementary membranes), proteins, lipoprotein, and lipopolysaccharide. A specific component of the outer membrane is a lipopolysaccharide of complex molecular structure, which occupies about 30 — 40% of its surface and is localized in the outer layer.

Outer Membrane Proteins can be divided into Major and minor proteins. The major proteins are represented by a small number of distinct types yet account for nearly 80% of all outer membrane proteins. One of the Functions of these proteins is the formation of hydrophilic Pores in the membrane with a diameter of approximately 1 nm, through which non-specific diffusion of molecules with masses up to 600 — 900 Da occurs. This means that sugars, amino acids, small Oligosaccharides, and Peptides can pass through such pores.

Chitin is an important structural polysaccharide of invertebrates. In particular, the external Skeleton of crustaceans and insects is built from it. The structure of chitin consists of N,N-acetyl-D-glucosamine units linked by β(1—4)-glycosidic bonds.

Glycosaminoglycans (mucopolysaccharides). In vertebrates, these are components of the Extracellular matrix as part of Proteoglycans (compounds of carbohydrates and proteins where carbohydrates make up 90%).

Figure 29. Formulas of structural polysaccharides in vertebrates. A — hyaluronic acid, B — heparin, C — chondroitin-4-sulfate

Hyaluronic acid forms highly viscous, gel-like solutions and is a component of loose and dense Connective Tissues as well as Cartilage. It consists of repeatedly alternating residues of D-glucuronic acid and N-acetyl-D-glucosamine linked by a β(1—3)-glycosidic bond. This is a linear reducing heteropolysaccharide.

Figure 30. Structure of proteoglycans

Chondroitin, the main polysaccharide of cartilage proteoglycans, is a linear reducing heteropolysaccharide containing alternating residues of D-glucuronic acid and sulfated N-acetyl-D-galactosamine linked by a β(1—3)-glycosidic bond. Depending on the degree of sulfation, there is 4- or 6-chondroitin sulfate. Additionally, keratan sulfate and other polysaccharides are part of the extracellular matrix of connective tissue (Figure 30). There are many other polysaccharides that are part of proteoglycans, forming the amorphous ground substance of connective tissues. Depending on the type of connective tissue, one or another type of polysaccharide predominates in the proteoglycans. All of them are poorly soluble, with one exception: the Blood Plasma polysaccharide synthesized by the cells lining the Blood Vessels. This polysaccharide is heparin.

Heparin is a short, linear, reducing heteropolysaccharide dissolved in blood plasma. Heparin is composed of repeating units of six sugar residues, each representing a sequence of alternating sulfated derivatives of N-acetyl-D-glucosamine and D-iduronate residues. Heparin prevents blood clotting, meaning it acts as an anticoagulant. It is secreted by endothelial cells lining the capillaries. Heparin extracted from lung tissue is used in medicine to prevent the clotting of donor blood, as well as to prevent blood clotting in blood vessels during various pathological conditions, such as after angina attacks.

Structural polysaccharides that make up the extracellular matrix form proteoglycans—protein-carbohydrate complexes where protein accounts for about 10%. A typical cartilage proteoglycan contains about 150 polysaccharide chains, each with a molecular weight of 20,000; these are covalently attached (as side chains) to core Polypeptides. Such proteoglycans are heavily hydrated structures, 20,000 each; these are covalently attached (as side chains) to core polypeptides. Such proteoglycans are heavily hydrated structures.

Glycoproteins are protein molecules to which short polysaccharide molecules are attached. Unlike proteoglycans, in glycoproteins, protein constitutes 95% of the molecule. Furthermore, the carbohydrate component is shorter—consisting of several dozen monomeric units—and highly diverse; one might say that each protein possesses a unique polysaccharide both in composition and shape. Polysaccharides are attached to secreted cellular proteins as well as membrane proteins, with the carbohydrate portion of membrane proteins facing the extracellular environment of the cell and forming the glycocalyx.

In this case, the polysaccharide performs a signaling function; cells recognize each other via these branched heteropolysaccharides. In particular, the ABO Blood Groups are determined by the polysaccharides located on the erythrocyte surface. Here, we can speak not only of The Diversity of the polysaccharide component in each protein, but also of the variability of this molecule for a single protein within a population.