Plastoquinol Oxidation: Rate-Limiting Stage in the Electron Transport Chain of Chloroplasts

This work is devoted to theoretical study of functioning of the cytochrome (Cyt) b(6)f complex (plastoquinol:plastocyanin oxidoreductase) of the electron transport chain (ETC) in oxygenic photosynthesis. A composition of the chloroplast ETC and molecular mechanisms of functioning of the Cyt b(6)f complex, which stands between photosystems II and I (PSII and PSI), are briefly reviewed. The Cyt b(6)f complex oxidizes plastoquinol (PQH(2)) molecules formed in PSII, and reduces plastocyanin, which serves as an electron donor to PSI. PQH(2) oxidation is the rate-limiting step in the chain of electron transfer processes between PSII and PSI. Using the density functional theory (DFT) method, we have analyzed the two-electron (bifurcated) oxidation of PQH(2) in the catalytic center Q(o) of the Cyt b(6)f complex. Results of DFT calculations are consistent with the fact that the first step of PQH(2) oxidation, electron transfer to the Fe2S2 cluster of the iron-sulfur protein (ISP), is an endergonic (energy-accepting) process (Delta E approximate to 15 kJ center dot mol(-1)) that can limit turnover of the Cyt b(6)f complex. The second stage of bifurcated oxidation of PQH(2) - electron transfer from semiquinone (PQH(center dot), formed after the first step of PQH(2) oxidation) to heme b(6)(L) - is the exergonic (energy-donating) process (Delta E < 0). DFT modeling of this stage revealed that semiquinone oxidation should accelerate after the PQH(center dot) radical shift towards the heme b(6)(L) (an electron acceptor) and the carboxy group of Glu78 (a proton acceptor). The data obtained are discussed within the framework of the Mitchell Q-cycle model describing PQH(2) oxidation at the Q(o) site of the Cyt b(6)f complex.