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. Studies of "E. coli" lactase have proposed that hydrolysis is initiated when a glutamate nucleophile on the enzyme attacks from the axial side of the galactosyl carbon in the β-glycosidic bond. The removal of the D-glucose leaving group may be facilitated by Mg-dependent acid catalysis. The enzyme is liberated from the α-galactosyl moiety upon equatorial nucleophilic attack by water, which produces D-galactose. The 3′-hydroxy group is involved in initial binding to the substrate while the 2′- group is not necessary for recognition but needed in subsequent steps. This is demonstrated by the fact that a 2-deoxy analog is an effective competitive inhibitor (Ki = 10mM). Elimination of specific hydroxyl groups on the glucopyranose moiety does not completely eliminate catalysis.