Evidence for Asymmetric Electron Transfer in Cyanobacterial Photosystem I: Analysis of a Methionine-to-Leucine Mutation of the Ligand to the Primary

The X-ray crystal structure of photosystem I (PS I) depicts six chlorophyll a molecules (in three pairs), two phylloquinones, and a (4Fe-4S) cluster arranged in two pseudo C2-symmetric branches that diverge at the P700 special pair and reconverge at the interpolypeptide F X cluster. At present, there is agreement that light-induced electron transfer proceeds via the PsaA branch, but there is conflicting evidence whether, and to what extent, the PsaB branch is active. This problem is addressed in cyanobacterial PS I by changing Met688PsaA and Met668PsaB, which provide the axial ligands to the Mg 2+ of the eC-A3 and eC-B3-chlorophylls, to Leu. The premise of the experiment is that alteration or removal of the ligand should alter the midpoint potential of the A0 - /A0 redox pair and thereby result in a change in the forward electron-transfer kinetics from A 0 - to A1. In comparison with the wild type, the PsaA-branch mutant shows: (i) slower growth rates, higher light sensitivity, and reduced amounts of PS I; (ii) a reduced yield of electron transfer from P700 to the FA/FB iron-sulfur clusters at room temperature; (iii) an increased formation of the 3 P700 triplet state due to P 700 + A0 - recombination; and (iv) a change in the intensity and shape of the polarization patterns of the consecutive radical pair states P 700 + A1 - and P700 + FX - . The latter changes are temperature dependent and most pronounced at 298 K. These results are interpreted as being due to disorder in the A0 binding site, which leads to a distribution of lifetimes for A 0 - in the PsaA branch of cofactors. This allows a greater degree of singlet-triplet mixing during the lifetime of the radical pair P700 + A0 - , which changes the polarization patterns of P700 + A1 - and P700 + FX - . The lower quantum yield of electron transfer is also the likely cause of the physiological changes in this mutant. In contrast, the PsaB-branch mutant showed only minor changes in its physiological and spectroscopic properties. Because the environments of eC-A3 and eC-B3 are nearly identical, these results provide evidence for asymmetric electron-transfer activity primarily along the PsaA branch in cyanobacterial PS I.