Electron Transfer from Cytochrome c2 to the Reaction Center: A Transition State Model for Ionic Strength Effects Due to Neutral Mutations

摘要

Interprotein electron transfer plays an important role in biological energy conversion. In thisnwork, the electron transfer reaction between cytochrome c2 (cyt) and the reaction center (RC)was studied tondetermine the mechanisms coupling association and electron transfer. Previous studies have shown thatnmutation of hydrophobic residues in the reaction interface, particularly Tyr L162, changes the bindingnaffinity and rates of electron transfer at low ionic strengths. In this study, the effect of ionic strength on thensecond-order electron transfer rate constant, k2, between cyt c2 and native or mutant RCs was examined.nMutations of hydrophobic and hydrogen bonding residues caused k2 to decrease more rapidly with annincrease in ionic strength. This change is explained with a transition state model by a switch from andiffusion-limited reaction in native RCs, where electron transfer occurs upon each binding event, to a fastnexchange reaction in the Tyr L162 mutant, where dissociation occurs before electron transfer and k2ndepends upon the equilibrium between bound and free protein complexes. The difference in ionic strengthndependence is attributed to a smaller effect of ionic strength on the energy of the transition state comparednto the bound state due to larger distances between charged residues in the transition state. This modelnexplains the faster dissociation rate at higher ionic strengths thatmay assist rapid turnover that is importantnfor biological function. These results provide a quantitative model for coupling protein association withnelectron transfer and elucidate the role of short-range interactions in determining the rate of electronntransfer.