National Center for
3TTY: Crystal Structure Of Beta-galactosidase From Bacillus Circulans Sp. Alkalophilus In Complex With Galactose
Structural analysis, enzymatic characterization, and catalytic mechanisms of beta-galactosidase from Bacillus circulans sp. alkalophilus
FEBS J. (2012) 279 p.1788-1798
Crystal structures of native and alpha-D-galactose-bound Bacillus circulans sp. alkalophilus beta-galactosidase (Bca-beta-gal) were determined at 2.40 and 2.25 A resolutions, respectively. Bca-beta-gal is a member of family 42 of glycoside hydrolases, and forms a 460 kDa hexameric structure in crystal. The protein consists of three domains, of which the catalytic domain has an (alpha/beta)(8) barrel structure with a cluster of sulfur-rich residues inside the beta-barrel. The shape of the active site is clearly more open compared to the only homologous structure available in the Protein Data Bank. This is due to the number of large differences in the loops that connect the C-terminal ends of the beta-strands to the N-terminal ends of the alpha-helices within the (alpha/beta)(8) barrel. The complex structure shows that galactose binds to the active site as an alpha-anomer and induces clear conformational changes in the active site. The implications of alpha-D-galactose binding with respect to the catalytic mechanism are discussed. In addition, we suggest that beta-galactosidases mainly utilize a reverse hydrolysis mechanism for synthesis of galacto-oligosaccharides.