Kinetics and energetics of electron transfer in reaction centers of the photosynthetic bacterium Roseiflexus castenholzii

Abstract

The kinetics and thermodynamics of the photochemical reactions of the purified reaction center (RC)-cytochrome (Cyt) complex from the chlorosome-lacking, filamentous anoxygenic phototroph, Roseiflexus castenholzii are presented. The RC consists of L- and M-polypeptides containing three bacteriochlorophyll (BChl), three bacteriopheophytin (BPh) and two quinones (Q A and Q B), and the Cyt is a tetraheme subunit. Two of the BChls form a dimer P that is the primary electron donor. At 285 K, the lifetimes of the excited singlet state, P*, and the charge-separated state P +H A − (where H A is the photoactive BPh) were found to be 3.2 ± 0.3 ps and 200 ± 20 ps, respectively. Overall charge separation P* → → P +Q A − occurred with ≥ 90% yield at 285 K. At 77 K, the P* lifetime was somewhat shorter and the P +H A − lifetime was essentially unchanged. Poteniometric titrations gave a P 865/ P 865 + midpoint potential of + 390 mV vs. SHE. For the tetraheme Cyt two distinct midpoint potentials of + 85 and + 265 mV were measured, likely reflecting a pair of low-potential hemes and a pair of high-potential hemes, respectively. The time course of electron transfer from reduced Cyt to P + suggests an arrangement where the highest potential heme is not located immediately adjacent to P. Comparisons of these and other properties of isolated Roseiflexus castenholzii RCs to those from its close relative Chloroflexus aurantiacus and to RCs from the purple bacteria are made.