BOTANY VOLUME 1 - CELL BIOLOGY. ANATOMY. MORPHOLOGY - 2007

1. MOLECULAR BASIS — THE BUILDING BLOCKS OF CELLS

1.4. Polysaccharides

Alongside Nucleic Acids and Proteins, Polysaccharides constitute the third major group of essential Biopolymers. Polysaccharides (glycans) are formed by the linkage of Monosaccharides—specifically hexoses and/or pentoses—into unbranched or branched macromolecular chains. Polysaccharides consisting of only a single type of monomeric building block are referred to as homoglycans, whereas those composed of two or more types of monomers are called heteroglycans. Structural polysaccharides are located on the exterior of Cells, where they contribute to the construction of Plant Cell Walls. Both inside and outside The Cell, Reserve Polysaccharides are deposited, serving as reservoirs for energy and matter.

1.4.1. Monosaccharides: The Building Blocks (Chain Units) of Polysaccharides

Monosaccharides possess several hydroxyl groups as well as a single carbonyl functional group, which is part of either an aldehyde group (aldoses) or a keto group (ketoses). Based on the number of carbon atoms (n), they are classified as trioses (n = 3, e.g., glyceraldehyde), tetroses (n = 4, e.g., erythrose), pentoses (n = 5, e.g., ribose, ribulose, xylulose), hexoses (n = 6, e.g., glucose, fructose, galactose), or heptoses (n = 7, e.g., sedoheptulose) (Fig. 1.18). The assignment of a sugar to the D- or L-series is determined by THE POSITION OF the hydroxyl group at the ഏറ്റവും асимметрично substituted carbon atom with the highest number (recall the Fischer projection!) (see 1.3.1). Plants predominantly contain D-series monosaccharides, while L-sugars occur only occasionally in polysaccharides.

Class="center">Fig. 1.18. Structural formulas and intramolecular hemiacetal formation in monosaccharides: A—linear Fischer projections of aldose and ketose series with n = 4 (tetrose), n = 5 (pentose), and n = 6 (hexose) carbon atoms. The position of the OH group at the "lowest" symmetrically substituted C atom serves as the criterion for classifying sugars into the D- or L-series. All depicted monosaccharides belong to the D-series; B—aldehydes react with hydroxyl groups to form hemiacetals. This process occurs in an acidic environment and, as illustrated by the Examples of D-fructose (C) and D-glucose (D), can proceed intramolecularly in monosaccharides, particularly in hexoses and aldopentoses. This results in closed pyranose or furanose ring forms of sugars, named after pyran and furan. In aqueous solution, two isomeric forms—differing in the position of the hydroxyl group within the hemiacetal—predominate over the linear form and exist in equilibrium with each other. These isomers are called anomers, categorized into α- and β-anomers; E—pyranose-series monosaccharides commonly found as monomers in polysaccharides (with standard Abbreviations). Galacturonic acid occurs, for example, in Pectins, N-acetyl-β-D-glucosamine in Chitin, and N-acetyl-β-D-muramic acid in bacterial peptidoglycan. The Haworth projections used in C, D, and E are intuitive, but they do not reflect the actual conformation of the molecule. For instance, the chair conformation predominates in β-D-glucopyranose in solution (F)

The carbonyl group dictates the characteristic reactions of monosaccharides. In an acidic medium, carbonyl groups can react with hydroxyl groups to form hemiacetals. In pentoses and hexoses, this process occurs intramolecularly, resulting in ring structures of two types—furanose and pyranose rings—which are depicted in Fig. 1.18 according to N. Haworth (the so-called Haworth projection). This representation is chosen for its clarity, although it does not convey the actual Spatial Structure of the sugar molecule. As a rule, pyranose rings exist in a chair conformation (Fig. 1.18, F). Hydroxyl groups that appear below the plane of the rings in Haworth formulas correspond to those pointing to the right in Fischer projections. Due to the two possible orientations of the carbonyl group during hemiacetal formation, each furanose or pyranose form has two structural isomers that differ in the position of the OH group. These are termed anomers (α- or β-anomers). In solution, they predominate over the linear form and remain in equilibrium with one another.

1.4.2. Glycoside Formation

Hemiacetals can react with aliphatic or aromatic hydroxyl groups, amines, and acid hydroxyl groups (such as carboxyl groups or phosphoric acid) with the elimination of Water to form full acetals (Fig. 1.19). The resulting linkage is referred to as a glycosidic bond. A distinction is made between O-glycosidic and N-glycosidic bonds accordingly. If the reaction partner is not a carbohydrate (aglycone), the resulting compounds are called Glycosides. When two CARBOHYDRATES are linked by a glycosidic bond, the product is termed a saccharide to emphasize that the compound consists exclusively of monosaccharides. These reactions are reversible. In an acidic environment, glycosidic bonds can be cleaved hydrolytically. Enzymes that catalyze the Hydrolysis of glycosidic bonds are known as glycosidases.

Many naturally occurring low-molecular-weight plant substances (see 6.16) that accumulate in vacuoles are glycosides. As a rule, glycosides are more water-soluble than their parent aglycones. Glycolipids are O-glycosides and represent vital membrane components (see 1.5.2). Galactolipids, which are typical of plastid membranes, are particularly common in plants. Many integral Membrane Proteins, proteins secreted to the exterior of the cell, and certain intracellular proteins are Glycoproteins, containing both O-glycosidic linkages (to the radicals of specific Amino Acids such as Serine, Threonine, and Tyrosine) and N-glycosidic linkages (to the radical of asparagine).

When two monosaccharides combine to form a disaccharide, either both hemiacetal groups may participate, or the hemiacetal group of one sugar may react with an aliphatic hydroxyl group of the other. The former pathway yields trehalose-type Disaccharides lacking a free hemiacetal group, whereas the latter produces maltose-type disaccharides that retain a free hemiacetal group (Fig. 1.19). The hemiacetal group acts as a mild reducing agent. It reduces Cu²⁺ in an alkaline tartrate solution to Cu⁺, which precipitates as Cu₂O (Fehling's test). Consequently, the free hemiacetal group of a saccharide is also referred to as its reducing end. Therefore, trehalose-type sugars, such as trehalose itself or sucrose, lack a reducing end. Depending on the number of linked monosaccharides, di-, tri-, tetrasaccharides, etc. (n = 2, 3, 4...), are formed; these are designated as Oligosaccharides when n < 30 and polysaccharides when n ≥ 30. Because numerous isomers are possible during sugar linkage, the specific type of glycosidic bond must be precisely defined by indicating the participating C atoms, their numbering, and the ring structure (furanose or pyranose), given that certain sugars—such as ribose—exist in both furanose and pyranose forms. Examples of saccharide nomenclature are provided in Figs. 1.18 and 1.19.

Fig. 1.19. Formation of glycosidic bonds.

Glycosidic bonds are formed through the reaction of a sugar's hemiacetal group with the nucleophilic groups of a second molecule, which may be either an aglycone or another sugar. The latter can participate via its own hemiacetal group or through one of its other hydroxyl groups. In the first case, trehalose-type disaccharides are formed; In the second, maltose-type disaccharides. The abbreviated notation of the monosaccharides and the designation of the glycosidic linkage (in parentheses) clearly reflect the structural features. Because α- and β-anomeric forms in solution are in equilibrium with open-chain forms at the reducing end of maltose-type saccharides and monosaccharides, the exact position of the OH group remains flexible

1.4.3. Reserve and Structural Polysaccharides

Polysaccharides (see 6.17.1) serve plants either as structural substances (structural polysaccharides) located on the exterior of cells or as reserve Materials (reserve polysaccharides) functioning as storage forms of reduced carbon. Reserve polysaccharides accumulate predominantly within cells, although extracellular reserve carbohydrates in the form of mucilages are sometimes found in fruits and seeds. Polysaccharides are categorized into groups based on the type of monomer units they contain. Glucans consist entirely (homoglycans) or primarily (heteroglycans) of glucose, Fructans of fructose, galactans of galactose, and so forth. When two monosaccharides are present in roughly comparable proportions, this is reflected in the name as well; for instance, glucomannans contain substantial proportions of both glucose and mannose, whereas arabinogalactans are composed chiefly of arabinose and galactose. A survey of various polysaccharides is presented in Table 1.3, and their principal structures are illustrated in Fig. 1.20. Alongside unbranched polysaccharides, branched ones—such as amylopectin and Glycogen—are also of great importance.

Fig. 1.20. Examples of polysaccharides (for Functions, see Table 1.3).

The molecules vary significantly in size. For example, the number of monomer units (n) is 200–1000 for amylose, 2,000–10,000 for amylopectin and Cellulose, up to 200 for galacturonan, and only 30–40 for inulin or phlein. The number of monomer units (m) between two branch points in amylopectin is 23–25. Glycogen has a similar structure, but with m equal to only 12–14, making glycogen more highly branched than amylopectin

Table 1.3 Common reserve and structural polysaccharides