Role of protein dynamics in guiding electron-transfer pathways in reaction centers from rhodobacter sphaeroides

The role of protein dynamics in guiding multistep electron transfer is explored in the photosynthetic reaction center of Rhodobacter sphaeroides. The energetics of the charge-separated intermediates, P⁺B _A^- and P⁺H_A^- (P is the initial electron donor bacteriochlorophyll pair and B_A and H _A are early bacteriochlorophyll and bacteriopheophytin acceptors, respectively), were systematically varied in a series of mutants. A fast phase of P⁺H_A^- recombination was resolved that is very sensitive to driving force. Either increasing or decreasing the relative free energy of P⁺H_A^- resulted in a more prominent fast recombination component, and thus a decreased yield forward electron transfer. The fast phase apparently represents P⁺H _A^- charge recombination via an activated state, probably P⁺B_A^- (B_A is situated between P and H_A). In wild type, this activated state is largely inaccessible, presumably due to dynamic stabilization of P⁺H_A ^- within the first 100 ps. In mutants that change the energetics, the rate of decay via the activated state accelerates and that pathway becomes significant. The dynamic stabilization of the protein makes it possible to achieve a nearly optimum environment of H_A in wild type on two different time scales and for two rather different reactions. On the picosecond time scale, the energetics is nearly, though not perfectly, optimized for transfer between the excited state of P and H_A. After dynamic stabilization of the state P⁺H_A^-, the environment is optimized to avoid rapid recombination of the charge-separated state and instead carry out forward electron transfer to the quinone with very high yield on the hundreds of picosecond time scale. Thus, by employing protein dynamics, the reaction center is able to optimize multiple reactions, on very different time scales involving the same reaction intermediate.