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) b₆f 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 b₆f complex, which stands between photosystems II and I (PSII and PSI), are briefly reviewed. The Cyt b₆f complex oxidizes plastoquinol (PQH2) molecules formed in PSII, and reduces plastocyanin, which serves as an electron donor to PSI. PQH2 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 PQH2 in the catalytic center Q_o of the Cyt b₆f complex. Results of DFT calculations are consistent with the fact that the first step of PQH2 oxidation, electron transfer to the Fe2S2 cluster of the iron-sulfur protein (ISP), is an endergonic (energy-accepting) process (ΔE ≈ 15 kJ·mol^-1) that can limit turnover of the Cyt b₆f complex. The second stage of bifurcated oxidation of PQH2 - electron transfer from semiquinone (PQH^•, formed after the first step of PQH2 oxidation) to heme b₆^L - is the exergonic (energy-donating) process (ΔE < 0). DFT modeling of this stage revealed that semiquinone oxidation should accelerate after the PQH^• radical shift towards the heme b₆^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 PQH2 oxidation at the Q_o site of the Cyt b₆f complex.