Roles of Ca2+ in O2 evolution in higher plant photosystem II: effects of replacement of Ca2+ site by other cations.

The properties of the S2 states formed in Ca2+-extracted, Sr2+-substituted, and Cd2+-substituted photosystem II (PSII) were comparatively studied by means of thermoluminescence (TL) and low temperature EPR spectroscopy. The following results were obtained: (i) Ca-extracted PSII showed neither EPR multiline nor g = 4.1 signals but showed an abnormal TL band with an upshifted peak temperature, which did not oscillate on excitation with two or more flashes. Addition of Ca2+ to the extracted PSII reversed most of these effects concomitant with marked restoration of O2 evolution, but the EPR g = 4.1 signal remained lost. (ii) Sr2+ substitution largely restored the g = 4.1 signal and partly restored the multiline signal in a modified form having a reduced hyperfine line spacing concomitant with partial restoration of O2 evolution. Sr2+ substitution also reversed the abnormal TL peak temperature to normal, but the oscillation of the restored TL peak was much damped. (iii) Cd2+ substitution restored neither of the two EPR signals nor O2 evolution but reversed the abnormal TL peak temperature to normal. However, the reversed TL peak did not show any oscillatory behavior. Given these results, the function of the weakly bound Ca2+ in S-state turnovers in higher plant PSII is discussed.