The photosystem II-associated Cah3 in Chlamydomonas enhances the O ₂ evolution rate by proton removal

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 H ₂O/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.