title: Biosynthetic exchange of bromide for chloride and strontium for calcium in the photosystem II oxygen-evolving enzymes.
abstract: The active site for water oxidation in photosystem II goes through five sequential oxidation states (S(0) to S(4)) before O(2) is evolved. It consists of a Mn(4)Ca cluster close to a redox-active tyrosine residue (Tyr(Z)). Cl(-) is also required for enzyme activity. To study the role of Ca(2+) and Cl(-) in PSII, these ions were biosynthetically substituted by Sr(2+) and Br(-), respectively, in the thermophilic cyanobacterium Thermosynechococcus elongatus. Irrespective of the combination of the non-native ions used (Ca/Br, Sr/Cl, Sr/Br), the enzyme could be isolated in a state that was fully intact but kinetically limited. The electron transfer steps affected by the exchanges were identified and then investigated by using time-resolved UV-visible absorption spectroscopy, time-resolved O(2) polarography, and thermoluminescence spectroscopy. The effect of the Ca(2+)/Sr(2+) and Cl(-)/Br(-) exchanges was additive, and the magnitude of the effect varied in the following order: Ca/Cl < Ca/Br < Sr/Cl < Sr/Br. In all cases, the rate of O(2) release was similar to that of the S(3)Tyr(Z)(.) to S(0)Tyr(Z) transition, with the slowest kinetics (i.e. the Sr/Br enzyme) being approximately 6-7 slower than in the native Ca/Cl enzyme. This slowdown in the kinetics was reflected in a decrease in the free energy level of the S(3) state as manifest by thermoluminescence. These observations indicate that Cl(-) is involved in the water oxidation mechanism. The possibility that Cl(-) is close to the active site is discussed in terms of recent structural models.
authors.affiliation: Institut de Biologie et Technologie de Saclay, URA CNRS 2096, Commissariat à l'Energie Atomique Saclay, 91191 Gif-sur-Yvette, France.
authors.forename: Naoko
authors.initials: N
authors.lastname: Ishida
authors.forename: Miwa
authors.initials: M
authors.lastname: Sugiura
authors.forename: Fabrice
authors.initials: F
authors.lastname: Rappaport
authors.forename: Thanh-Lan
authors.initials: TL
authors.lastname: Lai
authors.forename: A William
authors.initials: AW
authors.lastname: Rutherford
authors.forename: Alain
authors.initials: A
authors.lastname: Boussac
mesh_terms: D001965:Bromides / Q000737:chemistry / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002712:Chlorides / Q000737:chemistry / Q000378:metabolism*; D000458:Cyanobacteria / Q000201:enzymology; D007700:Kinetics; D010100:Oxygen / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000737:chemistry / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D013324:Strontium / Q000737:chemistry / Q000378:metabolism*; D013997:Time Factors
chemical_list: D001965:Bromides; D002712:Chlorides; D045332:Photosystem II Protein Complex; D010100:Oxygen; D002118:Calcium; D013324:Strontium