Proton Matrix ENDOR Studies on Ca2+-Depleted and Sr2+-Substituted Mn Cluster in Photosystem II [Bioenergetics]

October 5th, 2015 by Nagashima, H., Nakajima, Y., Shen, J.-R., Mino, H.

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 Mn-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 due to 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 due to 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 Mn cluster.