Viscosity-dependent structural fluctuations in enzyme catalysis

Abstract

The effect of viscosity on the rate of catalysis of carboxypeptidase A has been tested. By use of the tripeptide carbobenzoxy-l-alanyl-l-alanyl-l-alanine [Z(L-Ala)3] as substrate, it was shown that most of the effect on the hydrolysis rate caused by the presence of 30 or 40% methanol or glycerol in aqueous solution can be ascribed to a contribution of viscosity to the catalytic rate constant, kcat. Arrhenius plots of kcat in 30 and 40% glycerol or methanol are linear and almost parallel. When the rate constants are "corrected" for the viscosity of various media, the difference between the various Arrhenius plots is considerably reduced; it vanishes, within experimental error, when the effect of the dielectric constant of the solutions is taken into account as well. It is proposed that the viscosity of the medium can influence the rate-limiting step of the enzymic reaction, which is the rate of transitions over the energy barrier preceding product formation. According to the suggested mechanism, the enzyme--substrate complex can overcome this energy barrier by viscosity-dependent structural fluctuations. The quantitative agreement between the theory and the experimental results suggests that (a) due to the temperature dependence of the viscosity of the solution, the potential energy barrier of the reaction is about 5 kcal/mol lower than the observed activation energy and (b) information about the structural flexibility of the complex can be obtained by kinetic measurements.