The discoidin domain of Bacillus circulans β-galactosidase plays an essential role in repressing galactooligosaccharide production

The recently cloned β-galactosidase from Bacillus circulans ATCC 31382, designated BgaD, contains a multiple domain architecture including a F5/8 type C domain or a discoidin (DS) domain in the C-terminal peptide region. Here we report that the DS domain plays an essential role in repressing the production of galactooligosaccharides (GOSs). We prepared deletion mutants and point-mutated forms of rBgaD-A (deletion of the BgaD signal peptide) to compare their reaction behaviors. The yields of GOSs for all of the pointmutated forms as well as the deletion mutants of rBgaDAs increased as compared to rBgaD-A. In particular, W1540A mutant BgaD-A (rBgaD-A W1540A) produced much more GOSs than rBgaD-A. Surface plasmon resonance experiments indicated that both the wildtype and the W1540A mutant DS domains showed high affinity for galactosyllactose. rBgaD-A, which has a wild-type DS domain, showed high hydrolytic activity toward galactosyllactose, while the hydrolytic activities of rBgaD-D, without a DS domain, and rBgaDA W1540A, with a mutant DS domain were extremely low. The findings obtained in this study indicate that the wild-type DS domain of rBgaD-A has a function that aids galactosyllactose molecules to be properly oriented within the active site, so that they can be hydrolyzed efficiently to produce galactose/glucose by inhibiting the accumulation of GOSs.