Crystal Structure of ^-Amylase from Bacillus cereus var. mycoides at 2.2 A Resolution

The crystal structure of /9-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/a(F)^2.Q at 2.2 A resolution with root-mean-square deviations from ideality in bond lengths, and bond angles of 0.014 A 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 y9-amylase molecule folds into three domains; the first one is the N-terminal catalytic domain with a (ff/a)s 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 ^-sheets. The active site cleft, including two putative catalytic residues (Glul72 and Glu367), is located on the carboxyl side of the central jS-sheet in the (/3/a)s barrel, as in most amylases. The active site structure of the enzyme resembles that of soybean /9-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. Key words: active site cleft, Bacillus cereus, /9-amylase, crystal structure, raw starc