Journal Article

Towards a Better Understanding of the Metabolic System for Amylopectin Biosynthesis in Plants: Rice Endosperm as a Model Tissue

Yasunori Nakamura

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 43, issue 7, pages 718-725
Published in print July 2002 | ISSN: 0032-0781
Published online July 2002 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pcf091
Towards a Better Understanding of the Metabolic System for Amylopectin Biosynthesis in Plants: Rice Endosperm as a Model Tissue

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  • Biochemistry
  • Molecular and Cell Biology
  • Plant Sciences and Forestry

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Starch is made up of amylose (linear α-1,4-polyglucans) and amylopectin (α-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 pyrophosphorylase, 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 amylopectin is synthesized. The model emphasizes that two sets of reactions, α-1,6-branch formation and the subsequent α-1,4-chain elongation, are catalyzed by distinct BE and SS isoforms, respectively, are fundamental to the construction of the cluster structure. The model also assesses the role of DBE, namely isoamylase or in addition pullulanase, to remove unnecessary α-1,6-glucosidic linkages that are occasionally formed at improper positions apart from two densely branched regions of the cluster.

Keywords: Keywords: Amylopectin — Endosperm — Rice — Starch.; Abbreviations: BE, starch branching enzyme; DBE, starch debranching enzyme; DP, degree of polymerization; GBE, glycogen branching enzyme; GS, glycogen synthase; SS, soluble starch synthase.

Journal Article.  4810 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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