Redox potential of the non-heme iron complex in bacterial photosynthetic reaction center

In bacterial photosynthetic reaction centers (bRC), the electron is transferred from the special pair (P) via accessory bacteriochlorophyll (BA), bacteriopheopytin (HA), the primary quinone (QA) to the secondary quinone (QB). Although the non-heme iron complex (Fe complex) is located between QA and QB, it was generally supposed not to be redox-active. Involvement of the Fe complex in electron transfer (ET) was proposed in recent FTIR studies [A. Remy and K. Gerwert, Coupling of light-induced electron transfer to proton uptake in photosynthesis, Nat. Struct. Biol. 10 (2003) 637–644]. However, other FTIR studies resulted in opposite results [J. Breton, Steady-state FTIR spectra of the photoreduction of QA and QB in Rhodobacter sphaeroides reaction centers provide evidence against the presence of a proposed transient electron acceptor X between the two quinones, Biochemistry 46 (2007) 4459–4465]. In this study, we calculated redox potentials of QA/B (Em(QA/B)) and the Fe complex (Em(Fe)) based on crystal structure of the wild-type bRC (WT-bRC), and we investigated the energetics of the system where the Fe complex is assumed to be involved in the ET. Em(Fe) in WT-bRC is much less pH-dependent than that in PSII. In WT-bRC, we observed significant coupling of ET with Glu-L212 protonation upon oxidation of the Fe complex and a dramatic Em(Fe) downshift by 230 mV upon formation of QA− (but not QB−) due to the absence of proton uptake of Glu-L212. Changes in net charges of the His ligands of the Fe complex appear to be the nature of the redox event if we assume the involvement of the Fe complex in the ET.