Ca2+ depletion modifies the electron transfer on both donor and acceptor sides in Photosystem II from spinach

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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, QA-, 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 S2 state results in the appearance of a transient EPR signal from TyrZox which decays with half-times of 600 ms and 5 s. On the acceptor side, the disappearance of the 400 μs decay transient in the fluorescence yield indicates that the electron transfer from QA- to QB 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 TyrZox EPR signal and the split radical signal from the S′3 state suggests that both signals involves the same oxidized amino acid residue, TyrZox. 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 QA- to QB 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. © 1995.




Andréasson, L. E., Vass, I., & Styring, S. (1995). Ca2+ depletion modifies the electron transfer on both donor and acceptor sides in Photosystem II from spinach. BBA - Bioenergetics, 1230(3), 155–164.

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