The 2.0-A resolution structure of soybean beta-amylase complexed with alpha-cyclodextrin.

New crystallographic findings are presented which offer a deeper understanding of the structure and functioning of β-amylase, the first known exo-type starch-hydrolyzing enzyme. A refined three-dimensional structure of soybean β-amylase, complexed with the inhibitor β-cyclodextrin, has been determined at 2.0-A resolution with a conventional R-value of 17.5%. The model contains 491 amino acid residues, 319 water molecules, 1 sulfate ion, and 1 α-cyclodextrin molecule. The protein consists of a core with an (α/β) s supersecondary structure, plus a smaller globular region formed by long loops (L3, L4, and L5) extending from β-strands β3, β4, and β5