Probing the role of chloride in Photosystem II from Thermosynechococcus elongatus by exchanging chloride for iodide

The active site for water oxidation in Photosystem II (PSII) goes through five sequential oxidation states (S-0 to S-4) before O-2 is evolved. It consists of a Mn4CaO5 cluster and Tyr(Z), a redox-active tyrosine residue. Chloride ions have been known for long time to be required for the function of the enzyme. However, X-ray data have shown that they are located about 7 angstrom away from the Mn4CaO5 cluster, a distance that seems too large to be compatible with a direct involvement of chloride in the water splitting chemistry. We have investigated the role of this anion by substituting I- for Cl- in the cyanobacterium Thermosynechococcus elongatus with either Ca2+ or Sr2+ biosynthetically assembled into the Mn-4 cluster. The electron transfer steps affected by the exchanges were investigated by time-resolved UV-visible absorption spectroscopy, time-resolved EPR at room temperature and low temperature cw-EPR spectroscopy. In both Ca-PSII and Sr-PSII, the Cl-/I- exchange considerably slowed down the two S(3)Tyr(Z)(center dot) -> (S(3)Tyr(Z)(center dot))' -> So reactions in which the fast phase, S(3)Tyr(Z)(center dot) -> (S(3)Tyr(Z)(center dot))', reflects the electrostatically triggered expulsion of one proton from the catalytic center caused by the positive charge near/on Tyr(Z)(center dot) and the slow phase corresponds to the S-0 and O-2 formations and to a second proton release. The t(1/2) for S-0 formation increased from 1.1 ms in Ca/Cl-PSII to approximate to 6 ms in Ca/I-PSII and from 4.8 ms in Sr/Cl-PSII to approximate to 45 ms in Sr/I-PSII. In all cases the Tyr(Z)(center dot) reduction was the limiting step. The kinetic effects are interpreted by a model in which the Ca2+ binding site and the Cl- binding site, although spatially distant, interact. This interaction is likely mediated by the H-bond and/or water molecules network(s) connecting the Cl- and Ca2+ binding sites by which proton release may be channelled. (C) 2012 Elsevier B.V. All rights reserved.