title: A photosystem II-associated carbonic anhydrase regulates the efficiency of photosynthetic oxygen evolution.
abstract: We show for the first time that Cah3, a carbonic anhydrase associated with the photosystem II (PSII) donor side in Chlamydomonas reinhardtii, regulates the water oxidation reaction. The mutant cia3, lacking Cah3 activity, has an impaired water splitting capacity, as shown for intact cells, thylakoids and PSII particles. To compensate this impairment, the mutant overproduces PSII reaction centres (1.6 times more than wild type). We present compelling evidence that the mutant has an average of two manganese atoms per PSII reaction centre. When bicarbonate is added to mutant thylakoids or PSII particles, the O2 evolution rates exceed those of the wild type by up to 50%. The donor side of PSII in the mutant also exhibits a much higher sensitivity to overexcitation than that of the wild type. We therefore conclude that Cah3 activity is necessary to stabilize the manganese cluster and maintain the water-oxidizing complex in a functionally active state. The possibility that two manganese atoms are enough for water oxidation if bicarbonate ions are available is discussed.
authors.affiliation: Umeå Plant Science Center, Department of Plant Physiology, University of Umeå, S-901 87 Umeå, Sweden.
authors.forename: Arsenio
authors.initials: A
authors.lastname: Villarejo
authors.forename: Tatiana
authors.initials: T
authors.lastname: Shutova
authors.forename: Oleg
authors.initials: O
authors.lastname: Moskvin
authors.forename: Magnus
authors.initials: M
authors.lastname: Forssén
authors.forename: Vyacheslav V
authors.initials: VV
authors.lastname: Klimov
authors.forename: Göran
authors.initials: G
authors.lastname: Samuelsson
mesh_terms: D000818:Animals; D002256:Carbonic Anhydrases / Q000235:genetics / Q000378:metabolism*; D016825:Chlamydomonas reinhardtii / Q000201:enzymology* / Q000235:genetics; D008027:Light; D010100:Oxygen / Q000378:metabolism; D010788:Photosynthesis / Q000502:physiology*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D045332:Photosystem II Protein Complex
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D002256:Carbonic Anhydrases; D010100:Oxygen