Towards a better understanding of the metabolic system for amylopectin biosynthesis in plants: Rice [Oryza sativa] endosperm as a model tissue
2002
Nakamura, Y. (Akita Prefectural Univ. (Japan))
Starch is made up of amylose (linear alpha-1,4-polyglu-cans) and amylopectin (alpha-1,6-branched polyglucans). Amylopectin has a distinct fine structure called multiple cluster structure and is synthesized by multiple subunits or isoforms of four classes of enzymes: ADPglucose pyrophos-phorylase, soluble starch synthase (SS), starch branching enzyme (BE), and starch debranching enzyme (DBE). In the present paper, based on analyses of mutants and transgenic lines of rice in which each enzyme activity is affected, the contribution of the individual isoform to the fine structure of amylopectin in rice endosperm is evaluated, and a new model referred to as the ""two-step branching and improper branch clearing model"" is proposed to explain how anrylopectin is synthesized. The Inodel emphasizes that two sets of reactions, alpha-1,6-branch formation and the subsequent alpha-1,4-chain elongation, are catalyzed by distinct BE and SS isoforms, respectively, are fundamental to the construction of the cluster structure. The Inodel also assesses the role of DBE, namely isoamylase or in addition pullulanase, to remove unnecessary alpha-1,6-glucosidic linkages that are occasionally formed at improper positions apart from two densely branched regions of the cluster.
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