Effects of medium polarization and pre-existing field on activation energyof enzymatic charge-transfer reactions

The highly organized spatial structure of proteins' polar groups results in the existence of a permanent intraprotein electric field and in protein's weak dielectric response, i.e. its low dielectric constant. The first facto r affects equilibrium free energy gap of a charge-transfer reaction, the se cond (medium polarization effect) influences both equilibrium and non-equil ibrium (reorganization) energies, decreasing the latter substantially. In t he framework of the rigorous 'fixed-charge-density' formalism, the medium p olarization component of the reaction activation energy has been calculated , both for the activation energy of the elementary act proper, and the effe ctive activation energy accounting for the charges' transfer from water int o a low-dielectric structureless medium. In all typical cases of reactions, the energy spent for charge transfer from water into structureless 'protei n' is larger than the gain in activation energy due to the protein's low re organization energy. Therefore, the low dielectric constant of proteins is not sufficient to ensure their high catalytic activity, and an additional e ffect of the pre-existing intraprotein electric field, compensating for an excessive charging energy, is necessary. Only a combined action of low reor ganization energy and pre-existing electric field provides proteins with th eir high catalytic activity. The dependence of activation energy on the glo bule geometry has been analyzed. It is shown that, for each reaction, an op timum set of geometric parameters exists. For five hydrolytic enzymes, the optimum globule radii have been calculated using the experimental geometry of their active sites. The calculated radii agree satisfactorily with the r eal sizes of these macromolecules, both by absolute and by relative values. (C) 2000 Elsevier Science B.V. All rights reserved.