Crystal Structure of β-Amylase from Bacillus cereus var. mycoides at 2.2 Å Resolution

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The crystal structure of β-amylase from Bacillus cereus var. mycoides was determined by the multiple isomorphous replacement method. The structure was refined to a final R-factor of 0.186 for 102, 807 independent reflections with F/σ(F)_??_2.0 at 2.2 Å resolution with root-mean-square deviations from ideality in bond lengths, and bond angles of 0.014 Å and 3.00°, respectively. The asymmetric unit comprises four molecules exhibiting a dimer-of-dimers structure. The enzyme, however, acts as a monomer in solution. The β-amylase molecule folds into three domains; the first one is the N-terminal catalytic domain with a (β/α)₈ barrel, the second one is the excursion part from the first one, and the third one is the C-terminal domain with two almost anti-parallel β-sheets. The active site cleft, including two putative catalytic residues (Glu172 and Glu367), is located on the carboxyl side of the central β-sheet in the (β/α) barrel, as in most amylases. The active site structure of the enzyme resembles that of soybean β-amylase with slight differences. One calcium ion is bound per molecule far from the active site. The C-terminal domain has a fold similar to the raw starch binding domains of cyclodextrin glycosyltransferase and glucoamylase.