MECHANISM OF PROTON-COUPLED ELECTRON-TRANSFER FOR QUINONE (Q(B)) REDUCTION IN REACTION CENTERS OF RHODOBACTER-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 reac tion centers (RCs) from the photosynthetic bacterium Rb. sphaeroides b y substituting quinones with different redox potentials into the Q(A) site. These substitutions change the driving force for electron transf er without affecting proton transfer rates or proton binding equilibri a around the Q(B) site, The measured rate constants, k(AB)((2)), incre ased with increasing electron driving force (by a factor of 10 per 160 meV change in redox free energy), The proton-coupled electron transfe r was modeled by (i) four possible two-step mechanisms in which electr on transfer can precede or follow proton transfer and can be either th e 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 depen dence. The two stepwise mechanisms in which proton transfer is rate li miting predict very different free energy dependencies from that obser ved. The stepwise mechanism in which rate limiting electron transfer i s followed by fast proton transfer predicts a free energy dependence s imilar to, but significantly larger than, the observed dependence. Add itional arguments are presented against this mechanism. Thus, these th ree two-step mechanisms are excluded by the experimental data. The bes t agreement with the experimental data is given by a two-step mechanis m in which fast reversible proton transfer is followed by rate limitin g electron transfer. For this mechanism the observed free energy depen dence for k(AB)((2)) can be fitted using reasonable parameters of the Marcus theory. The free energy dependence predicted using a simple mod el for a concerted reaction also provides a reasonable fit to the data . Although the two-step mechanism (2) fits slightly better to the expe rimental data than the concerted mechanism, the uncertainty in the ass umed parameters precludes a definitive conclusion. Thus, we propose a mechanism for proton-coupled electron transfer in native RCs (called p roton-activated electron transfer) in which complete or partial proton ation of the semiquinone increases the rate of the reaction by increas ing the driving force for electron transfer.