STRUCTURE OF THE CYCLIC GLUCAN PRODUCED FROM AMYLOPECTIN BY BACILLUS-STEAROTHERMOPHILUS BRANCHING ENZYME

The thermostable branching enzyme (BE, EC 2.4.1.18) from Bacillus stea rothermophilus TRBE14 produces large cyclic glucans from waxy rice amy lopectin similar to those obtained from amylose as described elsewhere [H. Takata, T. Takaha, S. Okada, M. Takagi, and T. Imanaka, J. Bacter iol., 178 (1996) 1600-1606]. The structure of the product (P-1) from t he late-stage reaction was analyzed in detail. The weight-average degr ee of polymerization (dpw) of P-1 was 900. Its chain-length distributi on was not significantly changed compared with that of amylopectin, al though the amount of long chains (dp >38) was slightly decreased. The cyclic component of P-l, which was isolated by the extensive action of glucoamylase, had dpw of 49. Three point five alpha-1,6 linkages were directly involved in the formation of the ring structure with several non-cyclic side chains linked to the ring. Based on these results, th e action and new roles of BE are discussed. (C) 1996 Elsevier Science Ltd.