Starch biosynthesis: mechanism for the elongation of starch chains

Starch granules from eight diverse plant sources all had active starch synthases and branching enzymes inside the granules. The enzymes synthesized both amylose and amylopectin from ADPGlc. Pulsing of the granules with ADP-[C-14]Glc gave synthesis of starch that on reduction and glucoamylase hydrolysis gave C-14-labeled D-glucitol. The pulsed label could be chased by nonlabeled ADPGlc to give a significant decrease of C-14-label in D-glucitol. Evidence further indicated that the synthase forms a high-energy covalent complex with D-glucose and the growing starch chain, and that the D-glucopyranosyl group is added to the reducing end of the growing starch chain by a two-site insertion mechanism. (C) 2002 Elsevier Science Ltd. All rights reserved.