Interquinone Electron Transfer in Photosystem I As Evidenced by Altering the Hydrogen Bond Strength to the Phylloquinone(s)

The kinetics of electron transfer from phyllosemiquinone (PhQ•−) to the iron sulfur cluster FX in Photosystem I (PS I) are described by lifetimes of ∼20 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 to ∼180 ns in PsaA-L722Y/T and from ∼24 to ∼10 ns in PsaB-L706Y. The acceleration of the kinetics in the mutants is consistent with a perturbation of the H-bond, destabilizing 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 •−/PhQB •− ratio is shifted from 0.65:0.35 in wild-type reaction centers to 0.5:0.5 in PsaA-L722Y/T and to 0.8:0.2 in PsaB-L706Y. The most consistent account for all these observations involves considering reversibility of oxidation of PhQA •− and PhQB •− by FX, and asymmetry in the driving forces for these electron transfer reactions, which in turn leads to F x -mediated interquinone electron transfer.