Structure of raw starch-digesting Bacillus cereus β-amylase complexed with maltose

The crystals of beta-amylase from Bacillus cereus belong to space group P2(1) with the following cell dimensions: a = 57.70 Angstrom, b = 92.87 Angstrom, c = 65.93 Angstrom, and beta =101.95 degrees. The structures of free and maltose-bound beta-amylases were determined by X-ray crystallography at 2.1 and 2.5 Angstrom with R-factors of 0.170 and 0.164, respectively, The final model of the maltose-bound form comprises 516 amino acid residues, four maltose molecules, 275 water molecules, one Ca2+, one acetate, and one sulfate ion. The enzyme consists of a core (beta/alpha)(8)-barrel domain (residues 5-434) and a C-terminal starch-binding domain (residues 435-613). Besides the active site in the core where two maltose molecules are bound in tandem, two novel maltose-binding sites were found in the core L4 region and in the C-terminal domain. The structure of the core domain is similar to that of soybean beta-amylase except for the L4 maltose-binding site, whereas the C-terminal domain has the same secondary structure as domain E of cyclodextrin glucosyltransferase. These two maltose-binding: sites are 32-36 Angstrom apart from the active site. These results indicate that the ability of B, cereus beta-amylase to digest raw starch can be attributed to the additional two maltose-binding sites.