The cytochrome b6f complex: DFT modeling of the first step of plastoquinol oxidation by the iron-sulfur protein

Publication date: Available online 31 January 2018 Source:Journal of Organometallic Chemistry Author(s): Leila Y. Ustynyuk, Alexander N. Tikhonov In chloroplasts, the cytochrome (Cyt) b 6 f complex (plastoquinol-plastocyanin-oxidoreductase) provides connectivity between photosystems (PS) II and I, oxidizing plastoquinol (PQH2) formed in PSII and reducing plastocyanin (electron donor to PSI). The overall rate of the intersystem electron transport is determined by PQH2 oxidation by the Cyt b 6 f complex. In this work, using the DFT method, we have modeled the first step of PQH2 oxidation by the iron-sulfur protein (ISP) of the Cyt b 6 f complex. The model system contained the iron–sulfur cluster [Fe2S2], surrounding amino acid residues, and 2,3,5-trimethylbenzoquinol (TMBQH2), the tailless analog of PQH2. The energy profiles of the H atom transfer from TMBQH2 to the iron-sulfur protein (ISP) were calculated for two modes of the H-transfer, “diabatic” and “adiabatic”. The energies of transient states were estimated as 18.4 and 14.4 kcal mol−1. The energy effects of the reaction were evaluated as 7.7 and −0.2 kcal mol−1, respectively. The analysis of partial spin densities and electric charges on the atoms of the model system supports the bidirectional mechanism of the H-transfer reaction: an electron is directed to the Fe(1) atom of the [Fe2S2] cluster of the ISP, and a proton is accepted by the Nε atom of the His155 residue liganding the Fe(1) atom....
Source: Journal of Organometallic Chemistry - Category: Chemistry Source Type: research
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