Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

The Molecules We Are Made Of
Sugars and Polysaccharides
Polysaccharides

Sugar polymers are present in all Cells and perform a multitude of Functions. For instance, Cellulose provides structural rigidity to green plants, while Chitin imparts strength to the exoskeletons of Arthropods. Hyaluronic acid and other mucopolysaccharides form a protective layer between animal cells, and Pectins and related Polysaccharides play an analogous role in plants. Cell surfaces are typically coated with a layer of polysaccharides exhibiting diverse structures. Variations in The Structure of the polysaccharides that make up this outer layer are critically important because they determine the immunological individuality of organisms. Starch, Glycogen, and other Storage Polysaccharides serve as readily mobilizable nutritional reserves for cells [35 a].

One of the components of plant starch, amylose, is a linear polymer composed of numerous a-D-glucopyranosyl units linked together via 1,4-bonds. Starch granules also invariably contain a second component, amylopectin². Both amylopectin and glycogen (animal starch) consist of highly branched molecules. The branches are attached to the a-1,4-chains through a-1,6-bonds (Figs. 2-14 and 2-15). Certain Bacteria, such as Leuconostoc mesenteroides, synthesize linear (1—>6)-poly-D-glucose, or dextran; some species attach branches via a-1,4- or a-1,3-bonds. Dextrans synthesized by bacteria inhabiting tooth surfaces are components of dental plaque. Dextrans are produced on an industrial scale; following chemical Treatment that introduces numerous cross-links, they form gels (commercial name "Sephadex") widely used in biochemical laboratories for separating mixtures of various molecules (Sec. 3.1.g).

Cellulose is arguably the most abundant carbohydrate on Earth (plants produce it at a rate of ~10¹⁴ kg per year); it is a polyglucose formed by means of ß-1,4-bonds. Chitin, another widespread polysaccharide, is found in fungal cell walls and the exoskeletons of arthropods. It is a linear ß-1,4-polymer of N-acetylglucosamine, structurally similar to cellulose. Pectins are ß-1,4-polygalacturonic acids in which A number of carboxyl groups are methylated.

Yeast Cell walls contain polymers of mannose (Mannans); in these, short branches (consisting of 1—3 mannose units) linked by a-1,2- and a-1,3-bonds extend from the main a-1,6-chain. Aside from cellulose, all plants contain xylans, which consist predominantly of ß-1,4-xylopyranosyl chains. It is worth noting that xylose, a five-carbon sugar, adopts the form of a six-membered pyranose ring in this polymer. On the other hand, fructose, a six-carbon sugar, is present in inulin—the storage polysaccharide of the Jerusalem artichoke—in the form of five-membered furanose rings. These differences stem from variations in biosynthetic pathways. Pyranose rings are thermodynamically more stable than furanose rings. The latter are formed in inulin and sucrose because their Biosynthesis proceeds via the 6-phosphate ester of fructose.

² Some types of starch, notably "waxy" corn starch, contain exclusively amylopectin and no amylose.

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FIG. 2-15. Schematic representation of a glycogen molecule. A. Structure of a portion of the glycogen molecule according to Meyer (Meyer K. H., Adv. Enzymol., 3, 109—136, 1943). Circles denote glucose residues joined by a-1,4-bonds or (at branch points) by a-1,6-bonds. The crossed-out circle represents the reducing group. [French D._r in: Symposium on Foods: CARBOHYDRATES and Their Roles (H. W. Schulz ed.), pp. 26—54. Avi Publ., Westport, Connecticut, 1969.] B. Two-dimensional diagram of a glycogen molecule with a Molecular Weight of 760,000, containing ~4,700 glucose residues (the dashed line outlines the outer contours of the molecule). Treatment with ß-amylase (Chap. 7, Sec. B.6) yields a "limit dextrin" with a molecular weight of ~410,000, represented in the figure by circles and the lines connecting them. The letter M denotes highly branched regions that are connected to each other by segments with a low frequency of branch points. Upon treatment with a-amylase (Chap. 7, Sec. B.6), the M-regions are partially degraded to form limit dextrins containing an average of 34 glucose residues each. (Brammer G. A., Rougvie M. A., French D., Carbohyd. Res., 24, 343—354, 1972.)

A number of polysaccharides, including various types of starch, contain a single reducing end, which allows for ring opening and The formation of a free aldehyde group exhibiting reducing properties. In other cases, the reducing end is "blocked." One mode of such blockage (presumably occurring in inulin) is that the chain terminus has a sucrose-like structure. In other polysaccharides, a terminal trehalose may be present. Cyclic polysaccharides also exist, in which the sugar chains are closed into a ring and lack a free reducing end altogether. These polysaccharides are typically attached to protein or lipid molecules.

In many polysaccharides, the repeating units are monomers of different types. Some of these Heteropolysaccharides feature a simple alternation of two sugars [36, 37]. Examples include hyaluronic acid, chondroitin sulfates, and dermatan sulfate—important Components of the ground substance present in the Extracellular matrix of Connective Tissue, where they function as cementing Materials. Hyaluronic acid is a polymer in which glucuronic acid and N-acetylglucosamine alternate; its structure is illustrated in Fig. 2-16. Chondroitin sulfates and dermatan sulfate have a similar architecture, except that the former contains N-acetylgalactosamine and the latter incorporates a-L-iduronic acid; furthermore, sulfate ester groups are present at the positions indicated in Fig. 2-16.

FIG. 2-16. A. Repeating disaccharide units of hyaluronic acid and related mucopolysaccharides. B. Proposed structural model of proteoglycan aggregates in Cartilage Tissues, indicating the positions of the hyaluronic acid molecule, as well as chondroitin sulfate and keratan sulfate. The latter are covalently linked to the core protein forming the central portion of the proteoglycan subunit; a unified proteoglycan aggregate is formed by the attachment of these subunits to the hyaluronic acid molecule. The drawing is approximately to scale, except that the lengths of the chondroitin sulfate chains are depicted as nearly halved. [Rosenberg L., in: Dynamics of Connective Tissue Macromolecules (P. M. C. Burleigh and A. R. Poole, eds.), p. 107, North-Holland, Amsterdam, 1975.] Such mucopolysaccharide chains can electrostatically interact with the Collagen found in cartilage. C. Transmission electron micrograph (negative staining) of a proteoglycan aggregate from bovine articular cartilage (located on load-bearing joint surfaces). The fibrous backbone consists of hyaluronic acid (see Fig. B), with proteoglycan subunits radiating from it (individual polysaccharide chains are not visible) [386]. (Courtesy of L. Rosenberg.)

Heparin is a mucopolysaccharide with anticoagulant properties, secreted into the bloodstream by mast cells present in the Lungs, Liver, and other tissues. This linear polysaccharide most likely contains disaccharide units of the following type: [—uronic acid-(1—>4)-GlcN-2,6-disulfate-(1—>4)—]_n.

Both the amino groups and the 6-hydroxyls of the glucosamine residues carry sulfate groups. In some units, D-glucuronic acid linked via an a-1,4-bond is found, but (L-iduronic acid)-2-sulfate most frequently acts as the first disaccharide component [38].

Skin keratans contain either [—ßGal-(1—>4)-ßGlcNAc-6-sulfate-(1—>3)—]_n as a repeating unit or an identical structure containing galactosamine sulfate. Due to the presence of carboxylate and sulfate groups, all these polymers carry a high net negative charge.