tailieunhanh - Báo cáo Y học: Granule-bound starch synthase I A major enzyme involved in the biogenesis of B-crystallites in starch granules

Starch defines a semicrystalline polymer made of two different polysaccharide fractions. The A- and B-type crystalline lattices define the distinct structures reported in cereal and tuber starches, respectively. Amylopectin, the major fraction of starch, is thought to be chiefly responsible for this semicrystalline organization while amylose is generally considered as an amorphous polymer with little or no impact on the overall crystalline organization. STA2 represents a Chlamydomonas reinhardtii gene required for both amylose biosynthesis and the presence of significant granule-bound starch synthase I (GBSSI) activity. We show that this locus encodes a 69 kDa starch synthase and report the organization. | Eur. J. Biochem. 269 3810-3820 2002 FEBS 2002 doi Granule-bound starch synthase I A major enzyme involved in the biogenesis of B-crystallites in starch granules Fabrice Wattebled1 Alain Buleon2 Brigitte Bouchet2 Jean-Philippe Ral1 Luc Lienard1 David Delvalle1 Kim Binderup1 David Dauvillee1 Steven Ball1 and Christophe D Hulst1 1Unite de Glycobiologie Structurale et Fonctionnelle Unite Mixte de Recherche CNRS USTL n 8576 Unite Sous Contrat de 1 INRA Universite des Sciences et Technologies de Lille Villeneuve d Ascq France 2Institut National de la Recherche Agronomique Centre de Recherches Agroalimentaires Nantes France Starch defines a semicrystalline polymer made of two different polysaccharide fractions. The A- and B-type crystalline lattices define the distinct structures reported in cereal and tuber starches respectively. Amylopectin the major fraction of starch is thought to be chiefly responsible for this semicrystalline organization while amylose is generally considered as an amorphous polymer with little or no impact on the overall crystalline organization. STA2 represents a Chlamydomonas reinhardtii gene required for both amylose biosynthesis and the presence of significant granule-bound starch synthase I GBSSI activity. We show that this locus encodes a 69 kDa starch synthase and report the organization of the corresponding STA2 locus. This enzyme displays a specific activity an order of magnitude higher than those reported for most vascular plants. This property enables us to report a detailed characterization of amylose synthesis both in vivo and in vitro. We show that GBSSI is capable of synthesizing a significant number of crystalline structures within starch. Quantifications of amount and type of crystals synthesized under these conditions show that GBSSI induces the formation of B-type crystals either in close association with pre-existing amorphous amylopectin or by crystallization of entirely de novo synthesized .