Abstract
Despite the high level of symmetry between the PsaA and PsaB polypeptides in Photosystem I, some amino acids pairs are strikingly different, such as PsaA-Gly693 and PsaB-Trp673, which are located near a cluster of 11 water molecules between the A1A and A1B quinones and the FX iron-sulfur cluster. In this work, we changed PsaB-Trp673 to PsaB-Phe673 in Synechocystis sp. PCC 6803. The variant contains ~ 85% of wild-type (WT) levels of Photosystem I but is unable to grow photoautotrophically. Both time-resolved and steady-state optical measurements show that in the PsaB-W673F variant less than 50% of the electrons reach the terminal iron-sulfur clusters FA and FB; the majority of the electrons recombine from A1A− and A1B−. However, in those reaction centers which pass electrons forward the transfer is heterogeneous: a minor population shows electron transfer rates from A1A− and A1B− to FX slightly slower than that of the WT, whereas a major population shows forward electron transfer rates to FX slowed to the ~ 10 µs time range. Competition between relatively similar forward and backward rates of electron transfer from the quinones to the FX cluster account for the relatively low yield of long-lived charge separation in the PsaB-W673F variant. A higher water content and its increased mobility observed in MD simulations in the interquinone cavity of the PsaB-W673F variant shifts the pK of PsaB-Asp575 and allows its deprotonation in situ. The heterogeneity found may be rooted in protonation state of PsaB-Asp575, which controls whether electron transfer can proceed beyond the phylloquinone cofactors.
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Acknowledgements
The molecular biology was supported by grants from NSF EPSCoR (NSF(2010)-PFUND-217), the Louisiana RCS and Enhancement Program (LEQSF(2013-16)-RD-A-15), (LEQSF(2014-15)-ENH-TR-28) and (LEQSF(2015-16)-ENH-TR-34) to W.X. The time-resolved optical studies were supported by NSF Grant MCB-1613022 to J.H.G. The (P700+ − P700) measurements were supported by DOE grant DE-SC0018239 to S.S. The theoretical calculations were supported by the Russian Science Foundation Grant RSF 19-14-00366 to D.C. and A.S. A.S. and G.M. acknowledge partial support from Lomonosov Moscow State University Program of Development. The authors thank Andrei Y. Chistoserdov from the Department of Biology at University of Louisiana at Lafayette for his help and support on this project. An undergraduate student, Lillian D. Lestelle, cultured cells and isolated plasmid DNAs.
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Kurashov, V., Milanovsky, G., Luo, L. et al. Conserved residue PsaB-Trp673 is essential for high-efficiency electron transfer between the phylloquinones and the iron-sulfur clusters in Photosystem I. Photosynth Res 148, 161–180 (2021). https://doi.org/10.1007/s11120-021-00839-x
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DOI: https://doi.org/10.1007/s11120-021-00839-x