SUBSTRATE-SPECIFICITY OF SMALL-INTESTINAL LACTASE - STUDY OF THE STERIC EFFECTS AND HYDROGEN-BONDS INVOLVED IN ENZYME-SUBSTRATE INTERACTION

Milk lactose is hydrolysed to D-galactose and D-glucose in the small i ntestine of mammals by the lactase-phlorizin hydrolase complex (LPH, E C 3.2.1.23-62). Lactase activity has broad substrate selectivity and s everal glycosides are substrates. Recently, using the monodeoxy deriva tives of methyl beta-lactoside (1), we have shown the importance of ea ch hydroxyl group in the substrate molecule concerning the interaction with the enzyme. Now we have studied the corresponding O-methyl deriv atives, as well as some of the halo derivatives of 1. We have found th at the enzyme presents steric restrictions to the recognition of subst rates modified in the galactose moiety. In contrast, the binding site for the aglycon part of the substrate is looser. On the other hand, we have previously shown that HO-3' and HO-6 were important for the reco gnition of the substrate by the enzyme. Now we have found that the cor responding fluorine derivatives are not, or very poorly, recognized. T his suggests that the HO-3' and HO-6 participate, as donors, in hydrog en bonds in the interaction with the enzyme.