S-state dependence of chloride binding affinities and exchange dynamics in the intact and polypeptide-depleted O2 evolving complex of photosystem II

The Cl- binding properties in the successive oxidation states of the O-2 evolving complex of photosystem II were investigated by measurements of UV absorbance changes, induced by a series of saturating flashes, that monitor manganese oxidation state transitions. In dark-adapted, intact photosystem II, Cl- can be replaced by NO3- in minutes, in an exchange reaction that depends on the NO3- concentration and that is not rate-limited by dissociation of Cl- from its binding site. Preillumination of dark-adapted photosystem II by one or two flashes accelerated the NO3- substitution reaction by an order of magnitude. A quantitative analysis of the Cl- concentration dependence of UV absorbance changes, measured in photosystem II preparations depleted of extrinsic 17 and 23 kDa polypeptides, shows that the Cl- binding properties of photosystem II change with the oxidation state of the oxygen evolving complex. Although the affinity for the individual S-states could not be determined with precision, it is shown that the affinity is an order of magnitude lower in the S-2 state than in the S-1 state. Comparison of the results obtained using intact photosystem II and preparations depleted of the 17 and 23 kDa extrinsic polypeptides suggests that these proteins constitute a diffusion barrier, which prevents fast equilibration of the Cl- binding site with the medium, but does not change the Cl- affinity of the binding site.