The Effect of Na+ on the Activation of Water Oxidation by Ca2+ and Sr2+

Washing PSII preparations in 1 M NaCl removes about 1 of the 2–3 bound Ca2+ per Photosystem II reaction center and causes the loss of water oxidation (1). Partial recovery of this activity is specific to the addition of Ca2+ (2) or, less effectively, to Sr2+ (3,4). This salt wash also removes the P17 and P23 proteins from the preparation and it has been hypothesized (4,5) that the loss of Ca2+ results from a decreased affinity of the cation for the P23-less reaction centers. However, the presence of a high affinity (10 µM) and a low affinity (1 mM) Ca2+ binding sites have been observed in P17, P23-depleted PSII preparations (6,7). Alternatively, it has been proposed that the relative proportion of Ca2+ removed from these sites depends on the presence of light or EGTA during the salt wash (1,7). Experiments show that the high affinity Ca2+ binding site is required to form a functional S3 state (8,9) while the low affinity site is involved with photoactivation (10), H2O2-driven S1/S-1 cycling (11) and for electron transfer to Z+ (12).