A Note on the Metabolic System for the Synthesis of the Tandem-Cluster Structure of Amylopectin in Rice Endosperm

Abstract

The tandem-cluster structure of amylopectin must be a refined product that higher plants have evolved from the cluster-free random branched structure of glycogen. During the evolutionary process, plants have increased the number of isozymes of three classes of enzymes, namely soluble starch synthase (SS), starch branching enzyme (BE) and starch debranching enzymes (DBE), by giving each individual enzyme a distinct function in constructing the amylopectin cluster structure. On the basis of biochemical and molecular analyses of starch mutants and transgenic plants of rice, the present paper presents a model for amylopectin cluster biosynthesis in rice endosperm to understand how each enzyme is involved in this process. The author also proposes a notion called “cluster-world” where by the structure of amylopectin varies through changes in several distinct patterns of the cluster structure, and alteration of activities of key enzymes such as SSIIa, BEIIb and isoamylase1 results in the structural variation within the fixed pattern depending on which enzyme activity is altered.