Fundamentals of Molecular Biology. Part 1: Cell Molecular Biology - A. N. Ogurtsov 2011

Components of Biomolecular Complexes
Carbohydrates

CARBOHYDRATES account for up to 80% of the dry matter in certain Plant Tissues and up to 20% in some animal tissues. The simplest carbohydrates found in Living organisms are Monosaccharides with the general formula C_n(Н₂O)_n, where n = 3-7 (which explains their general name, "carbohydrates"). Almost all natural monosaccharides, much like Amino Acids, can exist in both D- and L-forms (Figure 46). Unlike amino acids, nearly all monosaccharides in living organisms belong to the D-series.

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Figure 46 - Enantiomers of glyceraldehyde CH₂(OH)CH(OH)CHO: a - L-glyceraldehyde; b - D-glyceraldehyde

Monosaccharides containing five or more carbon atoms exist in solutions in both a linear form and as closed cyclic structures (Figure 47).

Figure 47 - Formation of the cyclic form of monosaccharides

Furthermore, these closed structures can exist as two stereoisomers, a and ß (Figure 47).

In the Cytosol, a monosaccharide molecule constantly "switches" between its linear form and two cyclic stereoisomers. For example, the monosaccharide glucose switches between the cyclic isomers a-glucose and ß-glucose via a linear intermediate (Figure 48).

Figure 48 - Interconversion of glucose isomers

It should be noted that six- and five-membered rings are actually non-planar. The six-membered ring of most sugars adopts a chair conformation (Figure 49). The furanose form is also non-planar.

Figure 49 - Two chair Conformations of ß-D-glucopyranose. The letter "C" stands for chair conformation; the numbers indicate which atom is displaced upward (superscript) and downward (subscript) in the canonical projection

Typically, the equilibrium mixture is dominated by the conformer in which the largest number of bulky substituents occupy equatorial positions. ß-D-Glucopyranose exists in solution almost entirely as the ⁴C₁ conformer (with all bulky substituents located on equatorial bonds).

The main monosaccharides in living organisms are glucose, fructose, ribose (a component of RNA), and 2-deoxyribose (a component of DNA), along with galactose, mannose, and ribulose (Figure 50).

Figure 50 - Structures of major monosaccharides

Monosaccharides can join together to form a glycosidic bond, which is established between the hydroxyl group of one monosaccharide and the aldehyde or keto group of another. This process releases a Water molecule and yields a disaccharide (Figure 51).

The sequential addition of further monosaccharides in a similar manner leads to The formation of increasingly longer Oligosaccharides (trisaccharides, tetrasaccharides, etc.) up to massive polysaccharide molecules containing hundreds or thousands of monosaccharide residues.

Polysaccharides, also referred to as glycans, differ from one another in The Nature of their constituent monosaccharide residues, as well as in chain length and degree of branching.

Because each monosaccharide possesses multiple free hydroxyl groups capable of forming bonds with another monosaccharide or a different compound, the number of potential polysaccharide structures is exceptionally large.

Figure 51 - Schematic formation of Disaccharides

Since each hydroxyl group can lie on either side of the ring plane (a- and ß-positions), any specific glycosidic bond is designated as an a/β-m, n-bond, where m and n represent the numbers of the carbon atoms bearing the hydroxyl groups involved in forming the glycosidic linkage. For example, Figure 51 illustrates ß-1,4- and a-1,2-glycosidic bonds.

Even the simplest disaccharide composed of two glucose residues can be formed through eleven different glycosidic bonds, while three different hexoses (e.g., glucose, fructose, and galactose) combined together can produce several thousand distinct trisaccharides.

The structural function of polysaccharides. In living organisms, carbohydrates perform structural, energetic, and specialized Functions. The primary structural polysaccharides are Cellulose and Pectins in plants, and Chitin in animals and Fungi.

Cellulose is the most abundant organic compound on Earth, as it forms The basis of Plant Cell Walls. Specifically, wood and cotton consist almost entirely of cellulose. Every day, plants synthesize approximately 50 kg of cellulose for every human living on Earth.

Cellulose is a strong, fibrous, water-insoluble substance whose molecules each consist of 10⁴ or more D-glucose residues joined into an unbranched chain via β-1,4-glycosidic bonds (Figure 52). The polymer chains of cellulose are highly extended and linked to one another by Hydrogen Bonds.

Figure 52 - Diagram of the cellulose polysaccharide chain

Like cellulose, chitin is a linear, unbranched polysaccharide; however, its structural units are not D-glucose, but N-acetyl-D-glucosamine (Figure 53).

Figure 53 - Diagram of the chitin polysaccharide chain

Chitin forms the tough, insoluble outer coverings of crustaceans and insects, as well as The Cell walls of fungi. The chitinous framework of many crustaceans is strengthened by calcium carbonate deposits.

The energy storage function of polysaccharides. Carbohydrates serve as the primary operational source of energy in Cells. Through oxidation, glucose and other monosaccharides are broken down into СО₂ and Н₂O, and the chemical energy released in the process is utilized by the cell.

Glycolysis is a nearly universal process in which a six-carbon glucose molecule (С₆Н₁₂O₆) is anaerobically converted into two three-carbon Pyruvate molecules (С₃Н₅О₃) through a series of enzymatic reactions in the cytosol. This process consumes two ATP molecules while synthesizing four, meaning the glycolysis of a single glucose molecule yields a net gain of two ATP molecules. In addition, two molecules of the coenzyme nicotinamide adenine dinucleotide (NAD) are reduced:

Глюкоза + 2НАД⁺ +2АДФ + 2Р → 2 пирувата + 2АТФ + 2НАДН + Н⁺.

To store energy for future use, organisms utilize polysaccharides built from repeating glucose residues: starch (in plants) and Glycogen (in animals).

When Energy is required, glucose molecules are cleaved from starch or glycogen; conversely, when glucose is abundant, its molecules are added to the polymer chains of starch or glycogen, lengthening them. Thus, Reserve Polysaccharides constantly adjust their size depending on the Organism's energy demands.

Starch is a mixture of two D-glucose polymers — α-amylose and amylopectin (Figure 54).

α-Amylose consists of long, unbranched chains with a molecular weight ranging from several thousand to 5x10⁵, formed by D-glucose residues linked by α-1,4-glycosidic bonds.

Amylopectin also has a high molecular weight, but unlike α-amylose, its molecules are highly branched. In the unbranched regions of amylopectin, D-glucose residues are connected by α-1,4-glycosidic bonds, whereas at the branching points, they are linked by α-1,6-glycosidic bonds.

Glycogen is a highly branched polysaccharide which, much like amylopectin, consists of D-glucose residues linked by α-1,4-glycosidic bonds, with α-1,6-glycosidic bonds at the branch points.

Glycogen is most abundant in Liver cells, where it accounts for up to 7% of the organ's total weight. Within liver cells, glycogen occurs in the form of large granules, which in turn consist of smaller granules, each formed by a single highly branched molecule with an average Molecular Weight of several million. The Enzymes responsible for glycogen Synthesis and Breakdown are tightly bound to these granules.

Figure 54 - Diagram of starch polysaccharides: a - amylose; b - amylopectin

Although cellulose, starch, and glycogen are all composed of glucose residues, they differ significantly in their properties due to variations in the glycosidic bonds linking the glucose units within these molecules.

Due to the geometric constraints of α-1,4-glycosidic bonds, the linear segments of polymer chains in glycogen and starch tend to adopt a twisted, helical conformation, which facilitates the formation of the dense starch and glycogen granules found in cells.

In contrast, owing to the Structural Features of β-1,4-glycosidic bonds, cellulose molecules maintain a highly extended, linear conformation.

α-1,4-Bonds are readily hydrolyzed by the enzyme amylase, making starch easily digestible by animals and humans as it is broken down into D-glucose. However, the animal gut lacks the enzyme required to hydrolyze β-1,4-glycosidic bonds; consequently, cellulose is indigestible, and its D-glucose residues cannot serve as food for most organisms.

Nevertheless, ruminants and termites utilize cellulose as a food source thanks to the microorganisms (Bacteria and Protozoa) inhabiting their gut, which synthesize cellulase, an enzyme that hydrolyzes ß-1,4-glycosidic bonds. Cellulase is also synthesized by certain wood-rotting fungi.

Special functions of polysaccharides. Polysaccharides with special functions include highly complex compounds whose biochemical roles are not always fully understood, such as Gums and mucilages.

Carbohydrates can covalently bind to Proteins and Lipids, forming Glycoproteins and Glycolipids.

Typically, such hybrid molecules are components of cell membranes, and their oligosaccharide moiety participates in cell-to-Cell Recognition and signal molecule reception.

The rigidity of oligosaccharide structures, their water solubility, and the enormous variety of possible configurations make them exceptionally well-suited to perform these functions.

Proteoglycans (or peptidoglycans) are glycoproteins in which polysaccharides account for the major part of the molecule—over 95%. They are Components of the ground substance that fills the extracellular space in most tissues, whereas in bacteria (prokaryotes), peptidoglycans (rather than chitin and cellulose) serve as the structural basis of cell walls. In Eukaryotic cells, The Cell wall, when present, may contain chitin or cellulose (but not peptidoglycan).

A large number of hydroxyl groups in polysaccharides form hydrogen bonds with other Donors or acceptors, giving rise to two vital structural types in living organisms.

In one case, individual polysaccharide chains associate with a large number of water molecules to form a viscous gel. In this form, carbohydrates coat most of our cells, creating a sticky protective envelope. Mucus glycoproteins provide a good example of the appearance and properties of such structures.

In another case, carbohydrate chains tightly associate with one another via a network of hydrogen bonds, forming sturdy fibers that are virtually devoid of water. In this form, polysaccharides are used to build the macroscopic infrastructure of cells and extracellular matrices, as well as for energy storage. Some of the most remarkable biological structures created by nature—including the sturdy trunks of giant trees and the rigid, waterproof exoskeletons of Arthropods—owe their strength and durability to polysaccharides.