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.