Kinetics of electron transfer between the tetrahemic cytochrome and the special pair in isolated reaction centers of Roseobacter denitrificans

We have analyzed the rate of electron transfer between the tetrahemic cytochrome and the primary electron donor in isolated reaction centers of Roseobacter denitrificans as a function of the ambient redox potential. Three different phases are observed: a slow phase (half-time > ms), and two fast phases with half-times of 5 mu s and 380 ns. The slow phase is present at high redox potential, it corresponds to the kinetics of charge recombination between the photo-oxidized primary electron acceptor pf and the reduced primary acceptor (QA). The 5 mu s phase titrates with the reduction of the highest potential heme (HP1). This phase corresponds to the electron transfer between heme HP1 and P+. At redox potentials where the second high potential heme HP2 becomes reduced, the 5 mu s phase disappears and is replaced by the 380 ns phase, which is therefore related to the electron transfer between the high potential heme HP2 and P+. To explain the large difference in the rate of oxidation of HP1 and HP2 we propose a tentative model where the heme HP2 is closest to P.