The photosystem II-associated Cah3 in Chlamydomonas enhances the O2 evolution rate by proton removal.
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.
Shutova
Tatiana
T
Department of Plant Physiology, Umeå Plant Science Centre, Umeå University, Umeå, Sweden.
Kenneweg
Hella
H
Buchta
Joachim
J
Nikitina
Julia
J
Terentyev
Vasily
V
Chernyshov
Sergey
S
Andersson
Bertil
B
Allakhverdiev
Suleyman I
SI
Klimov
Vyacheslav V
VV
Dau
Holger
H
Junge
Wolfgang
W
Samuelsson
Göran
G
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
D001639:Bicarbonates; D045332:Photosystem II Protein Complex; D011522:Protons; D011993:Recombinant Fusion Proteins; D002734:Chlorophyll; D002256:Carbonic Anhydrases; D010100:Oxygen