ROLE OF SOLVENT REORGANIZATION ENERGIES IN THE CATALYTIC ACTIVITY OF ENZYMES

The catalytic reaction of lactate dehydrogenase (LDH) is examined by microscopic simulations for the general class of hydride-transfer reactions in enzymes. The free energy surfaces for the enzymatic reaction and the corresponding reference reaction in solution is evaluated by the empirical valence bond (EVB) method combined with a free energy perturbation method. The resulting activation barriers (DELTA-g) are then analyzed in terms of the corresponding solvent reorganization energies (lambda-s) by using linear free energy type formulation but with microscopically deduced parameters. It is found that lambda-s is smaller in the enzyme than in solutions and that the reduction of DELTA-g by the enzyme can bc correlated with the corresponding reduction of lambda-s. This result, which can be formulated by a Marcus-type relationship, is not, however, reproduced by macroscopic models that consider active sites as low dielectric regions. In fact, nonpolar sites would reduce lambda-s but at the same time increase rather than decrease DELTA-g double-ended dagger for charge-transfer reactions. Apparently, enzymes accelerate reactions by using very polar sites with preoriented dipoles. This means that, in contrast to thc customary case in homogeneous solutions, where DELTA-g can bc reduced in nonpolar solvents due to the reduction in lambda-s, enzymes can reduce DELTA-g double-ended dagger by having a small lambda-s in a polar environment. This rather complicated situation requires one to evaluate lambda-s by microscopic models rather than to estimate it by macroscopic approaches. However, once lambda-s is known, it can provide a useful tool for correlating enzyme rate with the effect of different mutations.