Аннотация: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. Composition of the chloroplast ETC and molecular mechanisms of functioning of the cytochrome b6f complex, which is located 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. Plastoquinol oxidation is the rate-limiting step in the process of electron transfer between PSII and PSI. Using the density functional theory (DFT) method, we have analyzed the two-electron (bifurcated) processes of PQH2 oxidation in the Qo center of the Cyt b6f complex. Results of the 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 quinol – electron transfer from semiquinone (PQH•, formed after the first stage of electron transfer) to heme b6L – is the exergonic (energy-donating) process (ΔE < 0). DFT modeling of this stage revealed that semiquinone oxidation was accelerated after the PQH• shift towards the heme b6L (an electron acceptor) and the carboxy group of Glu78 (a proton acceptor). The obtained data are discussed within the framework of the Mitchell Q cycle model describing plastoquinol oxidation at the Qo site of the Cyt b6f complex.
