Evidence for Asymmetric Electron Transfer in Cyanobacterial Photosystem I: Analysis of a Methionine-to-Leucine Mutation of the Ligand to the Primary Electron Acceptor A₀

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 Met688_PsaA and Met668_PsaB, which provide the axial ligands to the Mg²⁺ 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 A₀- to A ₁. 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 F _A/F_B iron-sulfur clusters at room temperature; (iii) an increased formation of the ³P₇₀₀ triplet state due to P₇₀₀+A₀^- recombination; and (iv) a change in the intensity and shape of the polarization patterns of the consecutive radical pair states P₇₀₀+A₁^- and P₇₀₀+F _x^-. 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 A₀^- 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⁺F _x^-. 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.