Abstract
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 ( dp w) 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 dp w 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.
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