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 stearothermophilus TRBE14 produces large cyclic glucans from waxy rice amylopectin similar to those obtained from amylose as described elsewhere [H. Takata, T. Takaha, S. Okada, M. Takagi, and T. Imanaka, J. Bacteriol., 178 (1996) 1600–1606]. The structure of the product (P-1) from the late-stage reaction was analyzed in detail. The weight-average degree of polymerization (dpw) of P-1 was 900. Its chain-length distribution was not significantly changed compared with that of amylopectin, although the amount of long chains (dp > 38) was slightly decreased. The cyclic component of P-1, which was isolated by the extensive action of glucoamylase, had dpw of 49. Three point five α-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, the action and new roles of BE are discussed.