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 F_X 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 (PhQ_B) and PsaA (PhQ_A) 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 PhQ_A and PhQ _B, 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 PhQ_A^̇- and PhQ_B^̇- oxidation were also affected by these mutations: the apparent PhQ_A^̇-/PhQ_B ^̇- 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 PhQ_A^̇- and PhQ_B ^̇- by F_X, and asymmetry in the driving forces for these electron transfer reactions, which in turn leads to F_x-mediated interquinone electron transfer.