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Abstract
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 overproduced rice branching enzyme I (BEI) in Escherichia coli cells, and the resulting enzyme (rBEI) was characterized with respect to biochemical and crystallographic properties. Specific activities were calculated to be 20.8 units/mg and 2.5 units/mg respectively when amylose and amylopectin were used as substrates. Site-directed mutations of Tyr235, Asp270, His275, Arg342, Asp344, Glu399, and His467 conserved in the α-amylase family enzymes drastically reduced catalytic activity of rBEI. This result suggests that the structures of BEI and the other α-amylase family enzymes are similar and that they share common catalytic mechanisms. Crystals of rBEI were grown under appropriate conditions and the crystals diffracted to a resolution of 3.0 Å on a synchrotron X-ray source.
