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The crystal structure of an oligo-1,6-glucosidase from Bacillus cereus ATCC7064 was determined by the X-ray diffraction method at 3.0 A resolution. The structure was solved by the multiple isomorphous replacement method and refined to a crystallographic R-factor of 0.208, using the molecular dynamics refinement program, X-PLOR. The electron density map revealed the folding of a polypeptide chain consisting of 558 amino acid residues. The molecule can be subdivided into three domains (N-terminal domain, subdomain, and C-terminal domain). The N-terminal domain has an (alpha/beta)8-barrel structure called the TIM-barrel structure. The C-terminal domain is characterized by a beta-barrel structure composed of eight antiparallel beta-strands, while the subdomain has a loop-rich structure with a small alpha-helix and a beta-sheet. The enzyme has a large cleft between the N-terminal domain and the subdomain. The cleft leads from the molecular surface to the molecular center. The bottom of the cleft is at the C-terminal end of the parallel beta-strands of the (alpha/beta)8-barrel. These structural features closely resemble those of alpha-amylases from Aspergillus oryzae and pig pancreas.

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