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  • The EMBO Journal (2008) 27, 782 - 791
  • doi:10.1038/emboj.2008.12

Published online: 31 January 2008

The photosystem II-associated Cah3 in Chlamydomonas enhances the O2 evolution rate by proton removal

Tatiana Shutova1, Hella Kenneweg2, Joachim Buchta3, Julia Nikitina4, Vasily Terentyev4, Sergey Chernyshov5, Bertil Andersson1,6, Suleyman I Allakhverdiev4, Vyacheslav V Klimov4, Holger Dau3, Wolfgang Junge2 and Göran Samuelsson1

  1. Department of Plant Physiology, Umeå Plant Science Centre, Umeå University, Umeå, Sweden
  2. Department of Biophysics, University of Osnabrück, Osnabrück, Germany
  3. Department of Physics, Free University of Berlin, Berlin, Germany
  4. Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino, Russia
  5. Branch of Shemyakin and Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Pushchino, Russia
  6. Division of Cell Biology, Linköping University, Linköping, Sweden

Correspondence to:

Göran Samuelsson, Department of Plant Physiology, Umeå Plant Science Centre, Umeå University, Umeå 90187, Sweden. Tel.: +46 090 786 5694; Fax: +46 090 786 6676; E-mail: goran.samuelsson@plantphys.umu.se

Received 16 May 2007; Accepted 9 January 2008

Water oxidation in photosystem II (PSII) is still insufficiently understood and is assumed to involve HCO3 -. A Chlamydomonas mutant lacking a carbonic anhydrase associated with the PSII donor side shows impaired O2 evolution in the absence of HCO3 -. The O2 evolution for saturating, continuous illumination (R O2) was slower than in the wild type, but was elevated by HCO3 - and increased further by Cah3. The R O2 limitation in the absence of Cah3/HCO3 - was amplified by H2O/D2O exchange, but relieved by an amphiphilic proton carrier, suggesting a role of Cah3/HCO3 - in proton translocation. Chlorophyll fluorescence indicates a Cah3/HCO3 - effect at the donor side of PSII. Time-resolved delayed fluorescence and O2-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 HCO3 -, which may function as proton carrier. Without Cah3, proton removal could become rate limiting during O2 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.

  • Keywords:

    • carbonic anhydrase,
    • Chlamydomonas reinhardtii,
    • photosystem II,
    • proton removal,
    • water oxidation