Starch branching enzyme (SBE) catalyzes the cleavage of α-1,4-linkages and the subsequent transfer of α-1,4 glucan to form an α-1,6 branch point in amylopectin. We determined the crystal structure of the rice branching enzyme I (BEI) in complex with maltopentaose at a resolution of 2.2Å. Maltopentaose bound to a hydrophobic pocket formed by the N-terminal helix, carbohydrate-binding module 48 (CBM48), and α-amylase domain. In addition, glucose moieties could be observed at molecular surfaces on the N-terminal helix (α2) and CBM48. Amino acid residues involved in the carbohydrate bindings are highly conserved in other SBEs, suggesting their generally conserved role in substrate binding for SBEs.
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