Acceptor Molecule of Granular‐bound Starch Synthase from Sweet‐potato Roots

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Abstract
In order to elucidate acceptor molecule of starch synthase bound to starch granules, the enzyme from sweet‐potato roots was reacted with ADP‐ and UDP‐[14C]‐glucose, and the reaction products were structurally characterized. The radioactivity of the products was released as [14C]‐maltose by β‐amylase. The [14C]‐glucose incorporation was found to be mostly in amylopectin component. However, approximately 30 and 20% of total radioactivity transferred from ADP‐ and UDP‐[14C]‐glucose, respectively, were also present in a molecule with a lower molecular weight than that of amylose component. The radioactivity was incorporated into the much longer unit‐chains of amylopectin than into the usual chains. These results indicate that starch synthase bound to sweet‐potato starch granules transfers glucose from ADP‐ and UDP‐glucose into specific outer chains of amylopectin component, and that the incorporation of the glucose into a short‐chain amylose molecule is simultaneously catalyzed by the starch synthetase.