title: Reduction of the Mn cluster of the water-oxidizing enzyme by nitric oxide: formation of an S(-2) state.
abstract: The manganese cluster of the oxygen-evolving enzyme of photosystem II is chemically reduced upon interaction with nitric oxide at -30 degrees C. The state formed gives rise to an S = 1/2 multiline EPR signal [Goussias, Ch., Ioannidis, N., and Petrouleas, V. (1997) Biochemistry 36, 9261] that is attributed to a Mn(II)- Mn(III) dimer [Sarrou, J., Ioannidis, N., Deligiannakis, Y., and Petrouleas, V. (1998) Biochemistry 37, 3581]. In this work, we sought to establish whether the state could be assigned to a specific, reduced S state by using flash oxymetry, chlorophyll a fluorescence, and electron paramagnetic resonance spectroscopy. With the Joliot-type O(2) electrode, the first maximum of oxygen evolution was observed on the sixth or seventh flash. Three saturating pre-flashes were required to convert the flash pattern characteristic of NO-reduced samples to that of the untreated control (i.e., O(2) evolution maximum on the third flash). Measurements of the S state-dependent level of chlorophyll fluorescence in NO-treated PSII showed a three-flash downshift compared to untreated controls. In the EPR study, the maximum S(2) multi-line EPR signal was observed after the fourth flash. The results from all three methods are consistent with the Mn cluster being in a redox state corresponding to an S(-2) state in a majority of centers after treatment with NO. We were unable to generate the Mn(II)-Mn(III) multi-line signal using hydrazine as a reductant; it appears that the valence distribution and possibly the structure of the Mn cluster in the S(-2) state are dependent on the nature of the reductant that is used.
authors.affiliation: Institute of Materials Science, NCSR Demokritos, 15310 Aghia Paraskevi, Athens, Greece.
authors.forename: Gert
authors.initials: G
authors.lastname: Schansker
authors.forename: Charilaos
authors.initials: C
authors.lastname: Goussias
authors.forename: Vasili
authors.initials: V
authors.lastname: Petrouleas
authors.forename: A William
authors.initials: AW
authors.lastname: Rutherford
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D004798:Enzymes / Q000737:chemistry / Q000378:metabolism; D005453:Fluorescence; D045342:Light-Harvesting Protein Complexes; D008345:Manganese / Q000737:chemistry*; D009569:Nitric Oxide / Q000737:chemistry*; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism; D045332:Photosystem II Protein Complex; D013997:Time Factors; D014867:Water / Q000737:chemistry*
chemical_list: D004798:Enzymes; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D014867:Water; D009569:Nitric Oxide; D008345:Manganese