STRUCTURE AND PROPERTIES OF BIOMOLECULES - A. E. Zemlyakov - 2017

11. CARBOHYDRATES: OLIGO- AND POLYSACCHARIDES

Oligo- and Polysaccharides can be viewed as polycondensation products of Monosaccharides. The primary type of bond linking monosaccharide residues is the O-glycosidic bond. Oligosaccharides typically refer to Carbohydrates containing from 2 to 10 sugar residues. The boundary between oligo- and polysaccharides is rather conventional. The properties of higher oligosaccharides are similar to those of polysaccharides. An important feature of polysaccharides is their heterogeneity. Natural polysaccharides represent a set of structurally similar polymers with varying degrees of polymerization.

Classification and Nomenclature

❖ Classification of oligosaccharides.

✵ By the number of monosaccharide residues:

♦ Disaccharides - composed of two monosaccharide residues;

♦ trisaccharides - composed of three monosaccharide residues, etc.

✵ By the presence of a free hemiacetal hydroxyl group:

♦ reducing oligosaccharides - one of the monosaccharide residues has a free hemiacetal hydroxyl group, allowing the sugar to convert into an open-chain aldehyde tautomeric form and act as a reducing agent in the Tollens' (silver mirror) test, for example, cellobiose;

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♦ non-reducing oligosaccharides - lack free hemiacetal hydroxyl groups, since the glycosidic bond is formed through the interaction of the glycosidic hydroxyl groups of two monosaccharides, for example, trehalose.

✵ By chain architecture:

♦ linear oligosaccharides - monosaccharide residues are connected sequentially to one another, for example, maltotriose (trisaccharide A);

♦ branched oligosaccharides - contain monosaccharide residues in which two or more alcoholic hydroxyls are linked to adjacent monosaccharides, for example, trisaccharide B.

❖ Nomenclature of oligosaccharides. The most general approach for naming oligosaccharides involves listing the O-glycosidic residues starting from the non-reducing end. Parentheses are placed between the names of the glycosyl radicals, enclosing the carbon locants that form the glycosidic bond, connected by an arrow. The name of the reducing monosaccharide residue written at the end ends with the suffix -ose. Names of non-reducing oligosaccharides are constructed similarly, with the monosaccharide residue at the end terminating in the suffix -oside.

To describe The Structure of oligosaccharides, alongside structural formulas, abbreviated three-letter designations of monosaccharides are frequently used. If necessary, the size of the hemiacetal ring is indicated at the end of the combination by the italicized letter f (for furanoses) and p (for pyranoses). To designate uronic acids, a fourth capital letter A (acid) is added, and in the case of 2-amino-2-deoxy sugars, the letter N.

❖ Classification of polysaccharides.

✵ By source of isolation:

♦ phytopolysaccharides - polysaccharides derived from plant Materials;

♦ zoopolysaccharides - polysaccharides of animal origin;

♦ microbial polysaccharides.

✵ By composition:

♦ Homopolysaccharides — composed of a single type of monosaccharide;

♦ Heteropolysaccharides — composed of several different monosaccharides.

✵ Based on chain architecture:

♦ linear — monosaccharide residues are connected to no more than two neighboring monosaccharides;

♦ branched — monosaccharides with three or more bonds to adjacent monosaccharides are present.

✵ Based on structural regularity of the chain:

♦ regular — monosaccharides and linkage types repeat regularly;

♦ block — specific segments of the chain exhibit a regular structure;

♦ random — lacking any structural regularity.

❖ Polysaccharide nomenclature. Many polysaccharides have trivial names: Cellulose, Glycogen, Chitin, heparin, etc. Systematic names of polysaccharides are derived from the names of their constituent monosaccharides by replacing the -ose suffix with -an, for example, D-glucomannan, α-D-glucan, β1 —> 4-Xylan.

Natural oligosaccharides

❖ Sucrose group oligosaccharides. Sucrose ("beet sugar", "cane sugar") is a white crystalline substance, highly soluble in Water and sweet to the taste. It is classified as a non-reducing disaccharide.

Sucrose is found in virtually all plants. In sugar beets and sugarcane, its content reaches up to 20%, making them the primary sources for sugar production.

Related sugars are also widespread: raffinose (α-D-Gal-(1 —> 6)-α-D-Glc-(1 —> 2)-β-D-Fruf) and stachyose (α-D-Gal-(1 —> 6)-α-D-Gal-(1 —> 6)-α-D-Glc-(1 ⇄ 2)-β-D-Fruf), in which the carbohydrate chain is extended by galactose residues attached to the glucose moiety.

Melezitose (α-D-Glc-(1 —> 3)-β-D-Fruf-(2 ⇄ 1)-α-D-Glc) also belongs to this group of oligosaccharides and is a component of the sweet secretions of lindens and poplars. In this trisaccharide, an additional glucose residue is attached to the fructose component of sucrose. Furthermore, this oligosaccharide has been detected in bee honey.

❖ Milk oligosaccharides.

The main component of milk oligosaccharides is lactose ("milk sugar"), the concentration of which reaches 50 g/L in cow's milk and 70 g/L in human breast milk.

Milk also contains 12–14 g/L of other oligosaccharides composed of D-glucose, D-galactose, L-fucose, N-acetylglucosamine, sialic acids (such as N-acetylneuraminic acid), and other sugars. Until recently, it was believed that there were about 100 milk oligosaccharides, but specialized studies have shown that their number approaches 900.

The groups identified are neutral oligosaccharides (accounting for about 90% of the total mass, including approximately 70% fucosylated oligosaccharides) and sialylated oligosaccharides (about 10%).

Nitrogen-containing oligosaccharides, which incorporate 2-aminosugars and neuraminic acid, play a crucial role. Their concentration in human breast milk is ~3 g/L, which is 100 times higher than in cow's milk. Specifically, the presence of lacto-N-tetraose in the infant gut promotes the proliferation of Lactobacillus bifidus Bacteria, which inhibit the growth of pathogenic and putrefactive bacteria.

✵ Hypolactasia is a pathological condition caused by a deficiency of lactase, the enzyme responsible for lactose Hydrolysis. This condition is prevalent among a significant portion of the global population, particularly indigenous peoples of the Americas, as well as residents of Southeast Asia and South America. Low lactase levels are observed in ~15% of Russians and Ukrainians, which can lead to adverse reactions to medications containing this disaccharide as an excipient. Lactose intolerance tends to increase with age.

❖ Cyclodextrins. Under the catalytic action of cyclodextrin glucanotransferase from certain bacteria, amylose is converted into cyclic hexa-, hepta-, and octasaccharides composed of α1 —> 4 linked glucose residues. These oligosaccharides are designated as α-, β-, and γ-cyclodextrins.

In three-dimensional space, these molecules form a torus 0.78 nm in height, featuring a hydrophilic outer surface and a hydrophobic inner cavity with diameters of 0.52, 0.66, and 0.84 nm, respectively. Due to this cavity, cyclodextrins readily form inclusion complexes (clathrates).

This property enables The Use of such oligosaccharides as drug delivery carriers. Cyclodextrins are also utilized as chiral stationary phases in Chromatography and as matrices for Enzyme Immobilization. Global production of cyclodextrins exceeds 10,000 tons.

Natural Polysaccharides

❖ Cellulose (“plant fiber”) is a white solid substance that is insoluble in water and most Solvents. In nature, it serves as the structural framework of Plant Cell Walls. Wood is roughly half cellulose, while cotton consists of nearly 100%.

Cellulose is composed of D-glucose residues joined by β1 —> 4 linkages. The number of monosaccharide units can reach 10,000, with a molecular weight exceeding one million Da. Due to intramolecular hydrogen bonding, the molecule adopts a linear, extended conformation.

These linear chains can pack closely together, stabilized by intermolecular bonds, to form thread-like structures known as microfibrils.

Cellulose is widely used in industry for the manufacture of paper, gunpowder, and synthetic fibers.

❖ Hemicelluloses are plant Cell wall polysaccharides. Softwood timber contains an average of 30% hemicelluloses, whereas hardwood timber contains about 1.5 times more. The polysaccharide chains of hemicelluloses are relatively short, consisting of 50–300 monosaccharide residues. Softwoods are dominated by hexose-based polysaccharides (glucomannans and galactoglucomannans), while hardwoods and cereal grasses predominantly contain pentosans (xylans and arabinoxylans).

Glucomannan consists of D-glucose and D-mannose residues linked by β1 —> 4 bonds. In many plants, this polysaccharide additionally contains D-galactose residues attached via α1 —> 6 linkages to the D-mannose units. Partial Acetylation of the polysaccharide has also been observed, predominantly at the C2 and C3 positions of mannose.

Along with D-xylose, the xylan polysaccharide incorporates 4-O-methyl-D-glucuronic acid residues attached via α1 —> 2 bonds (at a ratio of 10:1), which imparts acidic properties to the molecule.

In arabinoxylans, L-arabinofuranosyl groups are randomly attached to the xylan backbone via α1 —> 2 and/or α1 —> 3 linkages.

Pentosan-containing raw materials are industrially used to produce xylose, xylitol, furfural and its derivatives, as well as ethanol.

❖ Starch — a white, odorless, and tasteless solid substance, insoluble in cold water, that swells in hot water to form a paste. In nature, it serves as an energy reserve. Excess energy allows for the extension of polysaccharide chains, while their hydrolysis and subsequent biochemical degradation are accompanied by energy release in the form of ATP molecules. Starch is particularly abundant in plant grains and tubers. Traditionally, starch is obtained from potatoes, rice, and corn.

Starch is a mixture of two polysaccharides: amylose (15-35%) and amylopectin (65-85%). Both consist of D-glucose residues.

✵ Amylose. Glucose fragments are linked together by α1 —> 4 bonds. This polysaccharide has an unbranched structure consisting of 200–1000 monosaccharide units, with a molecular weight reaching 150–600 kDa.

In solutions, the amylose polysaccharide chain can coil into a helix, with each turn consisting of 6 glucose units. Molecules of appropriate size can enter the inner space of this helix, forming inclusion compounds. For example, the well-known blue color reaction of starch in the presence of iodine involves The formation of such an amylose-iodine complex.

✵ Amylopectin has a branched structure. The polymer chain with α1 —> 4 bonds is combined with branch points formed by α1 —> 6 bonds.

Branch points occur approximately every 20th to 25th glucose residue. As a result, a highly branched structure is formed. The Molecular Weight of amylopectin reaches several tens of millions of Da.

✵ Glycogen — animal starch, accounting for up to 10% of Liver cell mass and up to 2% of Skeletal Muscle mass. In liver Cells, it forms granules up to 0.1 µm in diameter, which is significantly smaller than starch granules in METABOLISM/14.html">Chloroplasts (~1 µm). Structurally, it is similar to amylopectin, but differs by having a higher molecular weight (over ~10⁸ Da) and a more compact packing, with chain branching occurring every 8–12 glucose residues.

❖ Inulin. In the roots of A number of plants, such as dandelion, dahlia, Jerusalem artichoke, or earth pear, the function of a reserve polysaccharide, along with starch, is performed by Fructans, the most prominent of which is inulin. This linear polysaccharide consists of 15–40 β-D-fructofuranose residues linked by (2 —> 1) bonds, terminating in a glucose residue. The polysaccharide has a relatively low molecular weight (~8 kDa). Inulin is used as a starch and sugar substitute for Diabetes Mellitus.

❖ Pectic substances. Pectins are found in the highest amounts in the peel of sugar beets, apples (~15%), and citrus fruits (up to 30%).

The core of the polysaccharide is poly-D-galacturonic (pectic) acid, containing from 400 to 1000 monosaccharide residues. Some of the polymer's carboxyl groups are methylated. Small amounts of α-L-arabinofuranan and β1 —> 4-D-galactan chains may be randomly attached to this backbone. Due to their gelling properties, pectins are widely used in the food industry.

❖ Agar (“agar-agar”) — a gelling polysaccharide extracted from red Algae (such as Gracilaria and Gelidium) and, to a lesser extent, from brown seaweeds. It is used in the food industry, in microbiology for preparing bacterial culture media, and as a support matrix in Electrophoresis.

Similar to starch, agar consists of two components: agarose and agaropectin.

✵ Agarose (50-80% of the total amount) is a polymer composed of D-galactose and 3,6-anhydro-L-galactose. Some of the latter monosaccharide units are sulfated at the C2 position. The total number of monosaccharide residues reaches 600–750, and the molecular weight is 120 kDa.

In solution, agarose forms Double helices.

✵ Agaropeptin is a mixture of irregular polysaccharides that differ from agarose by the presence of D-galactose residues that form an acetal with pyruvic acid ("pyruvated" D-galactose, Pyr-D-Gal), randomly attached sulfuric acid residues, and the replacement of some 3,6-anhydro-L-galactose residues with L-galactose 6-sulfate residues.

❖ Alginic acids. Marine brown algae, including kelp (Laminaria), contain up to 40% polysaccharides by dry weight. The primary polysaccharide in these plants is alginic acid, which consists of blocks of poly-β1 —> 4-D-mannuronic acid and poly-α1 —> 4-L-guluronic acid, with a total number of monosaccharide residues ranging from 200 to 1000.

Sodium alginate is widely used in industry and Pharmaceuticals as a stabilizer and binding agent.

❖ Chitin (from the Greek chiton – tunic, Skin, husk) is a structural polysaccharide found in crustaceans and insects. For instance, the shells of crabs and lobsters contain up to 25% chitin. It is also a component of fungal cell walls. Chitin is the second most abundant polysaccharide in nature, second only to cellulose.

The structure of this polymer is similar to that of cellulose, differing by the replacement of D-glucose residues with N-acetyl-D-glucosamine. It has a molecular weight on the order of several million Da and exhibits high mechanical and chemical stability. Approximately one-fifth of the amino groups in chitin remain free and unacylated, allowing it to form strong complexes with Proteins.

Upon prolonged heating with strong mineral acids, it hydrolyzes to N-acetyl-D-glucosamine. Treatment with concentrated alkali deacetylates chitin to form the polysaccharide chitosan, which is widely used as a sorbent, a mild cation exchanger, and a polymer matrix for the immobilization of various biological agents.