Activities and Kinetics of Electron Transfer in a Reaction Center Complex from the Green Sulfur Bacterium Chlorobium Tepidum

Abstract

The reaction center (RC) of green sulfur bacteria (PS-C) belongs to a Type 1 photosystem as PSI and RC of heliobacteria (PS-H). These RCs have Fe-S center(s) as electron acceptors, and their core polypeptides show amino acid sequence similarities among each other. However, the RC core polypeptides of PS-C and PS-H are assumed to be homodimeric in contrast with PSI. The sequence and kinetics of electron transfer in PSC as well as to and from PS-C are only partially elucidated (1,2). PS-C can directly photoreduce ferredoxin (Fd) in contrast with purple bacterial RC and we report here some of the results of kinetic studies. Kjwr and Scheller (3) found that purified PS-C from Chlorobium vibriofome photoreduced NADP⁺ in the presence of Fd from Clostridium and FNR from spinach. We purified several Fds from Chlorobium tepidum and studied their activities as electron mediators in NADP⁺ photoreduction. In PSI, phylloquinone functions as an electron mediator (A₁) between A₀ and Fx (A₂). In heliobacterial membranes, procedures which were assumed to extract quinones did not result in significant changes in the yield of stable charge separation, suggesting that menaquinone is not an essential participant in the electron acceptor chain (4). More recently, Brettel et al. (5) concluded from time-resolved flash spectroscopy in ns time range and from photovoltage measurements of heliobacterial membranes that quinone is not functioning as an electron acceptor in PS-H. In PS-C, there have been controversies over the functioning of quinone as an electron acceptor (reviewed in (1, 2)). In order to answer these questions, we studied electron transfer kinetics in purified PS-C in ns-ms time range by flash absorption spectroscopy. We also measured the reduction rate of photooxidized P840 by Cyt c₅₅₁ bound to PS-C.