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 FX 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 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 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(+)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 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(+)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.
Cohen
Rachel O
RO
Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
Shen
Gaozhong
G
Golbeck
John H
JH
Xu
Wu
W
Chitnis
Parag R
PR
Valieva
Alfia I
AI
van der Est
Art
A
Pushkar
Yulia
Y
Stehlik
Dietmar
D
D001665:Binding Sites / Q000235:genetics; D002734:Chlorophyll / Q000235:genetics / Q000378:metabolism; D000458:Cyanobacteria / Q000235:genetics / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport / Q000235:genetics; D005615:Freezing; D007700:Kinetics; D007930:Leucine / Q000235:genetics*; D008024:Ligands; D008715:Methionine / Q000235:genetics*; D016297:Mutagenesis, Site-Directed; D010782:Photolysis; D045331:Photosystem I Protein Complex / Q000235:genetics / Q000378:metabolism*; D021122:Protein Subunits / Q000235:genetics / Q000378:metabolism; D013055:Spectrophotometry, Infrared; D013696:Temperature
D008024:Ligands; D045331:Photosystem I Protein Complex; D021122:Protein Subunits; C089909:photosystem I, psaB subunit; D002734:Chlorophyll; C001785:chlorophyll P 700; D008715:Methionine; D007930:Leucine