Engineering the optimum pH of β-galactosidase from Aspergillus oryzae for efficient hydrolysis of lactose.

β-Galactosidase (lacA) from Aspergillus oryzae is widely used in the dairy industry. Its acidic pH optimum and severe product inhibition limit its application for lactose hydrolysis in milk. In the present study, structure-based sequence alignment was conducted to determine the candidate mutations to shift the pH optimum of lacA to the neutral range. The Y138F and Y364F mutants shifted the pH optimum of lacA from 4.5 to 5.5 and 6.0, respectively. The acid dissociation constant (pKa) values of catalytic acid/base residues with upwards shift were consistent with the increased pH optimum. All variants in the present study also alleviated galactose inhibition to various extents. Molecular dynamics demonstrated that the less rigid tertiary structures and lower galactose-binding free energy of Y138F and Y364F might facilitate the release of the end product. Both Y138F and Y364F mutants exhibited better hydrolytic ability than lacA in milk lactose hydrolysis. The higher pH optimum and lower galactose inhibition of Y138F and Y364F may explain their superiority over lacA. The Y138F and Y364F mutants in the present study showed potential in producing low-lactose milk, and our studies provide a novel strategy for engineering the pH optimum of glycoside hydrolase.