Lactase - Wikipedia
The overall reaction that lactase catalyzes is C 12 H 22 O 11 + H 2 O → C 6 H 12 O 6 + C 6 H 12 O 6 + heat. The catalytic mechanism of D-lactose hydrolysis retains the substrate anomeric configuration in the products. While the details of the mechanism are uncertain, the stereochemical retention is achieved off a double displacement reaction.