title: Mechanism of photoinhibition of photosynthetic water oxidation by Cl- depletion and F- substitution: oxidation of a protein residue.
abstract: New evidence on the chloride requirement for photosynthetic O2 evolution has indicated that Cl- facilitates oxidation of the manganese cluster by the photosystem II (PSII) Tyr-Z+ radical. Illumination above 250 K of spinach PSII centers which are inhibited in O2 evolution by either Cl- depletion or F- substitution produces a new EPR signal which has magnetic characteristics similar to one recently discovered in samples inhibited by depletion of Ca2+ only [Boussac et al. (1989) Biochemistry 28, 8984; Sivaraja et al. (1989) Biochemistry 28, 9459]. The physiological roles of Cl- and Ca2+ in water oxidation are thus linked. The characteristics include a nearly isotropic g = 2.00 +/- 0.005, a symmetric line shape with line width = 16 +/- 2 mT, almost stoichiometric spin concentration relative to Try-D+ = 0.6 +/- 0.3 spin/PSII, very rapid spin relaxation at all temperatures measured down to 6 K, and an undetectable change in magnetic susceptibility upon formation (less than 1 mu B2). The signal appears to originate from a spin doublet (radical) in magnetic dipolar contact with a transition-metal ion, most probably a photooxidized protein residue within 10 A of the Mn cluster (Mn-proximal radical). It is distinct from the three other protein-bound radical-type electron donors found in the PSII reaction center: Tyr-D+, Tyr-Z+, and C+. This signal photoaccumulates to a stable level under continuous illumination at 270 K and decays only after illumination stops.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Chemistry, Princeton University, New Jersey 08544.
authors.forename: M
authors.initials: M
authors.lastname: Baumgarten
authors.forename: J S
authors.initials: JS
authors.lastname: Philo
authors.forename: G C
authors.initials: GC
authors.lastname: Dismukes
mesh_terms: D002712:Chlorides / Q000494:pharmacology*; D004578:Electron Spin Resonance Spectroscopy; D005459:Fluorides / Q000494:pharmacology; D005609:Free Radicals; D008027:Light; D008345:Manganese / Q000378:metabolism; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D045332:Photosystem II Protein Complex; D010944:Plants / Q000378:metabolism*; D014443:Tyrosine / Q000378:metabolism; D014867:Water / Q000378:metabolism
chemical_list: D002712:Chlorides; D005609:Free Radicals; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D014867:Water; D014443:Tyrosine; D008345:Manganese; D005459:Fluorides; D010100:Oxygen