Abstract
Ferrous iron cations Fe(II) can effectively bind to the donor side of the manganese-depleted photosystem II (PSII(-Mn)) and in this way block electron transfer from diphenylcarbazide (DPC) to the major donor for P680, YZ. The present study was focused on the characteristic features of this process. The oxidation and subsequent binding of Fe(II) cations to PSII(-Mn) may proceed in the absence of an artificial electron acceptor, and therefore we investigated the role of O2 as a putative endogenous acceptor. Oxygen was shown to participate in the blockade of YZ by Fe cations, apparently as a structural element of Fe cluster formed at the donor side of PSII(-Mn). The kinetic study of blocking YZ by Fe(II) as dependent on light intensity demonstrated that the quantum efficiency of Fe cations binding to the donor side of PSII(-Mn) considerably exceeded that of Mn cations. We also compared the possibilities of extracting the native Mn cluster and reconstructed Fe cations from PSII and an alternative electron transport from DPC to P680+ under the conditions of the YZ blockade by Fe cations. Neither an alternative donor for P680, YD , nor cytochrome b559 participated in the latter process. As a whole, our evidence shows that many features of binding Fe cation to the donor side of PSII(-Mn) are in common with photoassembling the Mn cluster.
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