Proton Matrix ENDOR studies on Ca²⁺-depleted and Sr²⁺-substituted manganese cluster in photosystem II

Proton matrix ENDOR spectra were measured for Ca²⁺-depleted and Sr²⁺-substituted photosystem II (PSII) membrane samples from spinach and core complexes from Thermosynechococcus vulcanus in the S2 state. The ENDOR spectra obtained were similar for untreated PSII from T. vulcanus and spinach, as well as for Ca²⁺-containing and Sr²⁺-substituted PSII, indicating that the proton arrangements around the manganese cluster in cyanobacterial and higher plant PSII and Ca²⁺-containing and Sr²⁺-substituted PSII are similar in the S2 state, in agreement with the similarity of the crystal structure of both Ca²⁺-containing and Sr²⁺-substituted PSII in the S1 state. Nevertheless, slightly different hyperfine separations were found between Ca²⁺-containing and Sr²⁺-substituted PSII because of modifications of the water protons ligating to the Sr²⁺ ion. Importantly, Ca²⁺ depletion caused the loss of ENDOR signals with a 1.36-MHz separation because of the loss of the water proton W4 connecting Ca²⁺ and YZ directly. With respect to the crystal structure and the functions of Ca²⁺ in oxygen evolution, it was concluded that the roles of Ca²⁺ and Sr²⁺ involve the maintenance of the hydrogen bond network near the Ca²⁺ site and electron transfer pathway to the manganese cluster.