title: Bidirectional electron transfer in photosystem I: replacement of the symmetry-breaking tryptophan close to the PsaB-bound phylloquinone A1B with a glycine residue alters the redox properties of A1B and blocks forward electron transfer at cryogenic temperatures.
abstract: A conserved tryptophan residue located between the A(1B) and F(X) redox centres on the PsaB side of the Photosystem I reaction centre has been mutated to a glycine in Chlamydomonas reinhardtii, thereby matching the conserved residue found in the equivalent position on the PsaA side. This mutant (PsaB:W669G) was studied using EPR spectroscopy with a view to understanding the molecular basis of the reported kinetic differences in forward electron transfer from the A(1A) and the A(1B) phyllo(semi)quinones. The kinetics of A(1)(-) reoxidation due to forward electron transfer or charge recombination were measured by electron spin echo spectroscopy at 265 K and 100 K, respectively. At 265 K, the reoxidation kinetics are considerably lengthened in the mutant in comparison to the wild-type. Under conditions in which F(X) is initially oxidised the kinetics of charge recombination at 100 K are found to be biphasic in the mutant while they are substantially monophasic in the wild-type. Pre-reduction of F(X) leads to biphasic kinetics in the wild-type, but does not alter the already biphasic kinetic properties of the PsaB:W669G mutant. Reduction of the [4Fe-4S] clusters F(A) and F(B) by illumination at 15 K is suppressed in the mutant. The results provide further support for the bi-directional model of electron transfer in Photosystem I of C. reinhardtii, and indicate that the replacement of the tryptophan residue with glycine mainly affects the redox properties of the PsaB bound phylloquinone A(1B).
authors.affiliation: Department of Biology, University College London, Gower Street, London WC1E 6BT, UK.
authors.forename: Kulsam
authors.initials: K
authors.lastname: Ali
authors.forename: Stefano
authors.initials: S
authors.lastname: Santabarbara
authors.forename: Peter
authors.initials: P
authors.lastname: Heathcote
authors.forename: Michael C W
authors.initials: MC
authors.lastname: Evans
authors.forename: Saul
authors.initials: S
authors.lastname: Purton
mesh_terms: D020418:Algal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D019943:Amino Acid Substitution; D000818:Animals; D001665:Binding Sites / Q000235:genetics; D016825:Chlamydomonas reinhardtii / Q000235:genetics / Q000378:metabolism; D017124:Conserved Sequence; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D005615:Freezing; D005998:Glycine / Q000737:chemistry; D007700:Kinetics; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D045331:Photosystem I Protein Complex / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D052817:Phototrophic Processes; D015800:Protozoan Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D014364:Tryptophan / Q000737:chemistry; D010837:Vitamin K 1 / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D020418:Algal Proteins; D045331:Photosystem I Protein Complex; D015800:Protozoan Proteins; C089909:photosystem I, psaB subunit; D010837:Vitamin K 1; D014364:Tryptophan; D005998:Glycine