Monovalent cations (Na⁺, K⁺, Cs⁺) inhibit calcium activation of photosynthetic oxygen evolution

In the absence of the 17 and 23 kDa water‐soluble polypeptides, the oxygen‐evolving complex of photosystem II requires addition of Cl − and Ca 2+ to maximize rates of oxygen evolution activity. We have found that monovalent cations (Na + , K + , Cs + ) inhibit the calcium‐binding site responsible for activation of the oxygen‐evolving complex. K + inhibits in a purely competitive manner, while Na + , with an ionic radius much closer to that of calcium, displays mixed‐type inhibition kinetics. The K i values for Na + and K + have been estimated to be in the range of 8–10 mM. These values are well below the concentrations of these ions commonly used in buffer titrations and Cl − additions to standard oxygen evolution assay media and as such could alter the interpretation of data regarding calcium activation of oxygen evolution activity. An organic cation, tetramethylammonium, does not inhibit calcium activation and can be substituted for the inorganic cation as a Cl − counterion in oxygen evolution assays.