Recent Progress toward Understanding Biosynthesis of the Amylopectin Crystal

Abstract

Plant starch granules provide the largest percentage of calories in the human diet. Starch consists almost entirely of the Glc homopolymers amylopectin and amylose. Amy- lopectin is the major contributor to both mass and granule structure. Because of the very basic role that starch plays in our society, increased understanding of the biosynthetic mechanisms that produce amylopectin is important. Amy- lopectin is an important industrial raw material, and much remains to be determined about the relations between its biosynthesis and functionality. It is also a fascinating mol- ecule in its own right, because it exhibits a high degree of architectural specificity despite containing only one type of monomeric unit (i.e. the glucosyl group) connected via just two linkage types. Thus, significant insights into basic biochemical mechanisms may be obtained by studying amylopectin biosynthesis. Amylopectin is chemically similar to glycogen in that both are a(134)-linked, a(136)-branched Glc homopoly- mers, although a major difference between them is the organization of the latter into large, insoluble, semicrystal- line granules. Polysaccharides of this type synthesized in vitro do not readily crystallize, which is indicative of the relation between amylopectin biosynthesis and granule formation. In this Update we address the biological mech- anisms underlying amylopectin crystallization, in particu- lar those steps that could distinguish the amylopectin path- way in plants from the glycogen pathway in a presumptive cyanobacterial ancestor. The enzymes to be discussed in- clude starch synthases (SSs) and starch branching enzymes (BEs), both of which are involved directly in amylopectin biosynthesis. In addition, we discuss two enzymes for which potential roles in amylopectin biosynthesis are sug- gested by genetic data, namely starch debranching en- zymes (DBEs) and disproportionating enzyme (D-enzyme). Other important aspects of amylopectin biosynthesis that are outside the scope of this Update include production of the glucosyl unit donor ADPGlc, amylose biosynthesis (Ball et al., 1998), polymer chain initiation, and granule initiation.