Kinetics of the lactate dehydrogenase reaction in high-viscosity media

The effect of the medium viscosity on kinetics parameters of lactate dehydrogenase reaction was studied. The viscosity increase results in a sharp decline in the catalytic rate for both the pyruvate reduction and lactate oxidation reactions. It is shown that the catalytic step and its associated conformational motions is the only step which is considerably retarded when the viscosity increases. The reaction is not sensitive to changes in the dielectric properties of the medium. An inverse power function observed between the rate constant and viscosity cannot be explained by the theory of absolute reaction rates. However, it can easily be interpreted on the basis of the Kramers theory dealing with the transition over the activation barrier as a diffusional motion in the field of random forces. The influence of the medium's viscosity on the kinetic parameters indicates the existence of strong coupling between the dynamics of the solvent and the conformational motions of the protein molecule, which are correlated with the catalytic step.