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) bf 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 bf complex, which stands between photosystems II and I (PSII and PSI), are briefly reviewed. The Cyt bf 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 of the Cyt bf 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) that can limit turnover of the Cyt bf complex. The second stage of bifurcated oxidation of PQH2 - electron transfer from semiquinone (PQH, formed after the first step of PQH2 oxidation) to heme b - 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 b (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 site of the Cyt bf complex.