title: The photosystem II-associated Cah3 in Chlamydomonas enhances the O2 evolution rate by proton removal.
abstract: Water oxidation in photosystem II (PSII) is still insufficiently understood and is assumed to involve HCO(3)(-). A Chlamydomonas mutant lacking a carbonic anhydrase associated with the PSII donor side shows impaired O(2) evolution in the absence of HCO(3)(-). The O(2) evolution for saturating, continuous illumination (R(O2)) was slower than in the wild type, but was elevated by HCO(3)(-) and increased further by Cah3. The R(O2) limitation in the absence of Cah3/HCO(3)(-) was amplified by H(2)O/D(2)O exchange, but relieved by an amphiphilic proton carrier, suggesting a role of Cah3/HCO(3)(-) in proton translocation. Chlorophyll fluorescence indicates a Cah3/HCO(3)(-) effect at the donor side of PSII. Time-resolved delayed fluorescence and O(2)-release measurements suggest specific effects on proton-release steps but not on electron transfer. We propose that Cah3 promotes proton removal from the Mn complex by locally providing HCO(3)(-), which may function as proton carrier. Without Cah3, proton removal could become rate limiting during O(2) formation and thus, limit water oxidation under high light. Our results underlie the general importance of proton release at the donor side of PSII during water oxidation.
authors.affiliation: Department of Plant Physiology, Umeå Plant Science Centre, Umeå University, Umeå, Sweden.
authors.forename: Tatiana
authors.initials: T
authors.lastname: Shutova
authors.forename: Hella
authors.initials: H
authors.lastname: Kenneweg
authors.forename: Joachim
authors.initials: J
authors.lastname: Buchta
authors.forename: Julia
authors.initials: J
authors.lastname: Nikitina
authors.forename: Vasily
authors.initials: V
authors.lastname: Terentyev
authors.forename: Sergey
authors.initials: S
authors.lastname: Chernyshov
authors.forename: Bertil
authors.initials: B
authors.lastname: Andersson
authors.forename: Suleyman I
authors.initials: SI
authors.lastname: Allakhverdiev
authors.forename: Vyacheslav V
authors.initials: VV
authors.lastname: Klimov
authors.forename: Holger
authors.initials: H
authors.lastname: Dau
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Junge
authors.forename: Göran
authors.initials: G
authors.lastname: Samuelsson
mesh_terms: D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D002256:Carbonic Anhydrases / Q000235:genetics / Q000378:metabolism*; D016825:Chlamydomonas reinhardtii / Q000378:metabolism*; D002734:Chlorophyll / Q000378:metabolism; D009154:Mutation; D010100:Oxygen / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000378:metabolism*; D011522:Protons; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D001639:Bicarbonates; D045332:Photosystem II Protein Complex; D011522:Protons; D011993:Recombinant Fusion Proteins; D002734:Chlorophyll; D002256:Carbonic Anhydrases; D010100:Oxygen