Solvent deuterium isotope effect on the binding of beta-D-galactopyranosylderivatives to beta-galactosidase (Escherichia coli, lac Z)

A value of 1.8 has been determined for (K-I)(HOH)/(K-I)(DOD), the ratio of the values of K-I for competitive inhibition of P-galactosidase by isopropy l beta-D-thiogalactopyranoside in H2O and D2O. This is similar to the value of 1.7 for (K-m)(HOH)/(K-m)(DOD), the-ratio of the Michaelis constants det ermined for the beta-galactosidase-catalyzed hydrolysis of 4-nitrophenyl be ta-D-galactopyranoside (Gal-OPNP) in H2O and D2O. The similarity of these s olvent deuterium isotope effects suggests that the observed isotope effect on K-m corresponds, mainly, to the isotope effect on the dissociation const ant K-d for Gal-OPNP. The implications of these results for the interpretat ion of the solvent deuterium isotope effects on k(cat) and k(cat)/K-m for b eta-galactosidase-catalyzed hydrolysis of Gal-OPNP is discussed. (C) 2000 A cademic Press.