Polysaccharide Assembly in Eukaryotes

Abstract

Polysaccharides, in the broadest sense of that term, differ from the glycoproteins and glycolipids, considered in Chapter 4, in two important respects. Their saccharide chains are generally much larger than those of glycoproteins and glycolipids and, unlike these two classes of glycoconjugate, the chains of polysaccharides have a substantial content of repeating structure. However, these generalisations must be qualified at once. There is a degree of overlap in size between the largest carbohydrate chains of glycoproteins and many smaller polysaccharides, so that distinction by size is not absolute, though it is usually found. Moreover, a small degree of structural repetition is sometimes found in oligosaccharides of glycolipids or glycoproteins, while some hetero-polysaccharides show only limited stretches of repeated structure. In this latter case it is often found that there is either an apparently random co-polymerisation of sugars, or the polymer has a ‘block’ structure, with short sequences (or ‘blocks’) of one repetitive kind alternating with those of another in an irregular fashion. It is now evident that both of these forms of sequence can arise from a more regular polymer by subsequent modification, or can be ‘built’ in at initial assembly. Molecules such as heparan sulphate well illustrate the high degree of irregularity that can be generated by modification of both sugars and their substituents after assembly.