title: Tyrozine D oxidation and redox equilibrium in photosystem II.
abstract: Tyrosine D (TyrD) is an auxiliary redox active tyrosine residue in photosystem II (PSII). The mechanism of TyrDoxidation was investigated by EPR spectroscopy, flash-induced fluorescence decay and thermoluminescence measurements in PSII enriched membranes from spinach. PSII membranes were chemically treated with 3mM ascorbate and 1mM diaminodurene and subsequent washing, leading to the complete reduction of TyrD. TyrDoxidation kinetics and competing recombination reactions were measured after a single saturating flash in the absence and presence of DCMU (inhibitor of the QB-site) in the pH range of 4.7-8.5. Two kinetic phases of TyrDoxidation were observed by the time resolved EPR spectroscopy - the fast phase (msec-sec time range) and the pH dependent slow phase (tens of seconds time range). In the presence of DCMU, TyrDoxidation kinetics was monophasic in the entire pH range, i.e. only the fast kinetics was observed. The results obtained from the fluorescence and thermoluminescence analysis show that when forward electron transport is blocked in the presence of DCMU, the S2QA-recombination outcompetes the slow phase of TyrDoxidation by the S2state. Modelling of the whole complex of these electron transfer events associated with TyrDoxidation fitted very well with our experimental data. Based on these data, structural information and theoretical considerations we confirm our assignment of the fast and slow oxidation kinetics to two populations of PSII centers with different water positions (proximal and distal) in the TyrDvicinity.
authors.affiliation: Molecular Biomimetics, Department of Chemistry - Ångström Laboratory, Box 523 Uppsala University, 751 20 Uppsala, Sweden.
authors.forename: Nigar
authors.initials: N
authors.lastname: Ahmadova
authors.affiliation: Molecular Biomimetics, Department of Chemistry - Ångström Laboratory, Box 523 Uppsala University, 751 20 Uppsala, Sweden.
authors.forename: Felix M
authors.initials: FM
authors.lastname: Ho
authors.affiliation: Molecular Biomimetics, Department of Chemistry - Ångström Laboratory, Box 523 Uppsala University, 751 20 Uppsala, Sweden.
authors.forename: Stenbjörn
authors.initials: S
authors.lastname: Styring
authors.affiliation: Molecular Biomimetics, Department of Chemistry - Ångström Laboratory, Box 523 Uppsala University, 751 20 Uppsala, Sweden. Electronic address: fikret.mamedov@kemi.uu.se.
authors.forename: Fikret
authors.initials: F
authors.lastname: Mamedov
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D005470:Fluorometry; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008163:Luminescent Measurements; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010777:Photochemistry; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000528:radiation effects; D018724:Spinacia oleracea; D013696:Temperature; D014443:Tyrosine / Q000737:chemistry*
chemical_list: D045332:Photosystem II Protein Complex; D014443:Tyrosine
keywords: Electron transfer; Photosystem II; Q(A)(−) S(2) state recombination; Tyrosine D