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

Leila Yu. Ustynyuk1 and Alexander N. Tikhonov2,a*

1Faculty of Chemistry, Lomonosov Moscow State University, 119991 Moscow, Russia

2Faculty of Physics, Lomonosov Moscow State University, 119991 Moscow, Russia

* To whom correspondence should be addressed.

Received June 22, 2022; Revised August 4, 2022; Accepted August 25, 2022
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 Qo 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 b6L – 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 b6L (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 Qo site of the Cyt b6f complex.
KEY WORDS: photosynthesis, cytochrome complex, plastoquinol oxidation, modeling

DOI: 10.1134/S0006297922100029