Inter-quinone electron transfer in Photosystem I as evidenced by altering the hydrogen bond strength to the phylloquinone(s)
Stefano Santabarbara1,2,3,5, Kiera Reifschneider1, Audrius Jasaitis2,3, Feifei Gu3, Giancarlo Agostini4, Donatella Carbonera4, Fabrice Rappaport2, and Kevin E. Redding1
1) Department of Chemistry and Biochemistry, Arizona State University, Tempe AZ 85287-1604
2) Institut de Biologie Physico-Chimique, UMR 7141 CNRS-Univ. P. et M. Curie, 75005 Paris, France
3) Department of Chemistry, University of Alabama, Tuscaloosa, AL 35487
4) Department of Chemical Sciences, University of Padua, Italy
5) Present address: National Institute of Biophysics, CNR, Italy
The kinetics of electron transfer from phyllosemiquinone (PhQ•-) to the iron sulphur cluster FX in Photosystem I (PS I) are described by lifetimes of ~20-ns and ~250-ns. These two rates are attributed to reactions involving the quinones bound primarily by the PsaB (PhQB) and PsaA (PhQA) subunits, respectively. The factors leading to a ~10-fold difference between the observed lifetimes are not yet clear. The peptide nitrogen of conserved residues PsaA-Leu722 and PsaB-Leu706 is involved in asymmetric hydrogen-bonding to PhQA and PhQB, respectively. Upon mutation of these residues in PS I of the green alga, Chlamydomonas reinhardtii, we observe an acceleration of the oxidation kinetics of the PhQ•- interacting with the targeted residue: from ~255 ns to ~180 ns in PsaA-L722Y/T and from ~24 ns to ~10 ns in PsaB-L706Y. The acceleration of the kinetics in the mutants is consistent with a perturbation of the H-bond, de-stabilizing the PhQ•- state, and increasing the driving force of its oxidation. Surprisingly, the relative amplitudes of the phases reflecting PhQA•- and PhQB•- oxidation were also affected by these mutations: the apparent PhQA•-:•- ratio is shifted from 0.65:0.35 in wild-type reaction centres to 0.5:0.5 in PsaA-L722Y/T and to 0.85:0.15 in PsaB-L706Y. The most consistent account for all these observations involves considering reversibility of PhQA•-, PhQB•- oxidation by FX, as well as asymmetry in the driving forces for these electron transfer reactions, which, in turn, leads to Fx-mediated inter-quinone electron transfer.
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