Mechanism of proton-coupled electron transfer for quinone (Q(B)) reduction in reaction centers of Rb-sphaeroides

The mechanism of the proton-coupled electron transfer reaction, Q(A)(-)Q(B)(-) + H+ --> Q(A)(Q(B)H)(-) (i.e.k(AB)((2))), was studied in reaction centers (RCs) from the photosynthetic bacterium Rb. sphaeroides by substituting quinones with different redox potentials into the Q(A) site. These substitutions change the driving force for electron transfer without affecting proton transfer rates or proton binding equilibria around the Q(B) site, The measured rate constants, k(AB)((2)), increased with increasing electron driving force (by a factor of 10 per 160 meV change in redox free energy), The proton-coupled electron transfer was modeled by (i) four possible two-step mechanisms in which electron transfer can precede or follow proton transfer and can be either the rate determining or fast step in the overall reaction and (ii) a one-step mechanism involving the concerted transfer of an electron and a proton, The free energy dependencies of these possible mechanisms were predicted using Marcus theory and were compared to the observed dependence. The two stepwise mechanisms in which proton transfer is rate limiting predict very different free energy dependencies from that observed. The stepwise mechanism in which rate limiting electron transfer is followed by fast proton transfer predicts a free energy dependence similar to, but significantly larger than, the observed dependence. Additional arguments are presented against this mechanism. Thus, these three two-step mechanisms are excluded by the experimental data. The best agreement with the experimental data is given by a two-step mechanism in which fast reversible proton transfer is followed by rate limiting electron transfer. For this mechanism the observed free energy dependence for k(AB)((2)) can be fitted using reasonable parameters of the Marcus theory. The free energy dependence predicted using a simple model for a concerted reaction also provides a reasonable fit to the data. Although the two-step mechanism (2) fits slightly better to the experimental data than the concerted mechanism, the uncertainty in the assumed parameters precludes a definitive conclusion. Thus, we propose a mechanism for proton-coupled electron transfer in native RCs (called proton-activated electron transfer) in which complete or partial protonation of the semiquinone increases the rate of the reaction by increasing the driving force for electron transfer.