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...