Solvent deuterium isotope effect on the binding of β-D-galactopyranosyl derivatives to β-galactosidase (Escherichia coli, lac Z)

A value of 1.8 has been determined for (K₁)(HOH)/(K₁)(DOD), the ratio of the values of K₁ for competitive inhibition of β-galactosidase by isopropyl β-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 determined for the β-galactosidase-catalyzed hydrolysis of 4- nitrophenyl β-D-galactopyranoside (Gal-OPNP) in H₂O and D₂O. The similarity of these solvent deuterium isotope effects suggests that the observed isotope effect on K(m) coresponds, mainly, to the isotope effect on the dissociation constant K(d) for Gal-OPNP. The implications of these results for the interpretation of the solvent deuterium isotope effects on K(cat) and k(cat)/K(m) for β-galactosidase-catalyzed hydrolysis of Gal-OPNP is discussed.