Virtually all of the starch used by humans is storage starch, that which accumulates in seeds and tubers, and our knowledge of starch structure is mainly based on studies of this. Starch consists of two components, amylose and amylopectin, both of which are largely a-1,4-1 inked glucan polymers. Starch structure has been subject to many general reviews (2-4). Generally, amylose constitutes 20-30% of the total and contains rare, a-1,6 branches, whereas amylopectin, which has frequent a-1,6 branches, makes up the rest.
Amylopectin consists of linear, a-l,4-linked glucan chains frequently branched by a-1,6 bonds. The average chain length (degree of polymerization, DP) in amylopectin is on average 21-25 glucoses which, because of the frequent branching, yields a weight-average molecular weight (Mw) for amylopectin about 300 times larger than that of amylose. The branch points are not randomly distributed within amylopectin but rather are clustered. The chains of amylopectin are classified as the C-chain, the "core" chain containing the only reducing glucose in the molecule; the B-chains, which branch from the C-chain; and the A-chains, which are defined as the outermost branches of the molecule. Progressive amylolytic digestion followed by chromatography has shown that the B-chains are distributed in several size classes. Chains of DP 15-20 are found in the linear portions of clusters, whereas chains of DP 45-60 extend between clusters. A third form of starch, the highly branched "phytoglycogen," appears in certain mutants and is now thought to be an intermediate in the biosynthesis of amylopectin (5,6). The general properties of these components are presented in Table 1.
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