CA2+ DEPLETION MODIFIES THE ELECTRON-TRANSFER ON BOTH DONOR AND ACCEPTOR SIDES IN PHOTOSYSTEM-II FROM SPINACH

Ca2+ depletion of Photosystem II from spinach results in reversible retardation of electron transfer on both donor and acceptor sides. On the donor side, a decrease of the electron transfer rate from TyrZ results in an enhanced charge recombination between the oxidized primary donor, P680(+), and the reduced acceptor quinone, Q(A)(-), which in turn leads to a decrease in the amplitude of the fluorescence yield. In addition, slow electron transfer from the manganese cluster in the dark-stable S-2 State results in the appearance of a transient EPR signal from TyrZ(ox) which decays with half-times of 600 ms and 5 s. On the acceptor side, the disappearance of the 400 mu s decay transient in the fluorescence yield indicates that the electron transfer from Q(A)(-) to Q(B) has been severely inhibited. These results suggests that removal of a Ca2+ ion from the donor side in PS II, which results in the inhibition of oxygen evolution and in the appearance of an EPR signal in the S-3' state leads to structural changes which are transmitted to the acceptor side. The strikingly similar behavior after depletion of Ca2+ of the TyrZ(ox) EPR signal and the split radical signal from the S-3' state suggests that both signals involves the same oxidized amino acid residue, TyrZ(ox). The absence of large effects on the EPR properties of the non-heme iron suggests that the structural changes on the acceptor side are subtle in nature. Chemical modification of histidine results in inhibition of Q(A)(-) to Q(B) electron transfer and to changes in the magnetic properties of the oxidized non-heme iron but only to minor perturbations of the donor-side. This suggests that histidine, susceptible to chemical modification, is located mainly on the acceptor side of PS II.