Chapter 9 - Disaccharides, Oligosaccharides and Polysaccharides

Abstract

This chapter discusses common cellulose and glycogen and oligomers and polymers derived from single carbohydrate monomers. Cellulose is a b-1 4-linked polymer of D-glucose, and is one of the most abundant biopolymers on earth. It is the major structural component of green plants in which it is found in primary and secondary cell walls, and it is estimated that 1015 kg of cellulose are synthesized annually. Cellulose is the major component of plant secondary walls and thus the main component of plant fibers. In the cell wall, the cellulose of plant fibers is embedded in a matrix of other polysaccharides, forming a biocomposite. Glycogen particles contain at their core a dimeric protein termed glycogenin. Glycogenin is a glycosyltransferase that initiates glycogen biosynthesis and acts as a primer for the glycogen molecule. Glycogenin catalyzes the transfer of D-glucose from UDP-glucose to a tyrosine residue on glycogenin to form an α-glucosyl tyrosine. Additional catalytic cycles lead to elongation of the α-glucosyl tyrosine, building an α-1 4-linked D-glucose chain of about 10 residues. There is evidence suggesting that one molecule of glycogenin within the dimer glucosylates the other partner, although an analysis of an X-ray crystal structure of glycogenin suggested that self-glucosylation might occur for the earliest glucosylation events. Following the initiation of glycogen biosynthesis by glycogenin, an α-glucosyltransferase, glycogen synthase, elongates the short chains formed on glycogenin, leading to extended α-1,4-linked chains.