Hydrogen Tunneling and Protein Motion in Enzyme Reactions - Sharon Hammes-Schiffer

Theoretical studies of proton, hydride, and proton-coupled electron transfer reactions in enzymes will be presented. We have developed theoretical approaches that include the quantum mechanical effects of the active electrons and transferring proton(s), as well as the motions of the entire solvated enzyme. The proton-coupled electron transfer reaction catalyzed by the enzyme lipoxygenase will be discussed. The experimentally measured deuterium kinetic isotope effect of 80 at room temperature is found to arise from the small overlap of the reactant and product proton vibrational wavefunctions in this nonadiabatic reaction. The calculations illustrate that the proton donor-acceptor vibrational motion and the reorganization of the protein, substrate, and cofactor play vital roles in this enzyme reaction. The hydride transfer reaction catalyzed by the enzyme dihydrofolate reductase will also be discussed. An analysis of the simulations leads to the identification and characterization of a network of coupled motions that extends throughout the enzyme and represents conformational changes that facilitate the charge transfer process. Mutations distal to the active site are shown to significantly impact the catalytic rate by altering the conformational motions of the entire enzyme and thereby changing the probability of sampling conformations conducive to the catalyzed reaction. In addition, a computational approach for ranking mutant enzymes according to the catalytic reaction rates and the application to a series of dihydrofolate reductase mutants will be presented. These approaches and concepts have important implications for protein engineering and drug design.