Dark Chemical Blocking of the High-Affinity Mn-Binding Site of Photosystem II by Fe(II) Cations

Binding of an Fe(II) cation to the high-affinity Mn-binding site (HAS) of photosystem II without the oxygen-evolving complex (PSII(-Mn)) consists of several stages: highly specific association of the Fe(II) cation with HAS, oxidation of the bound cation by the tyrosine radical $${\text{Y}}_{{\text{Z}}}^{ \bullet }$$ generated as a result of absorption of a quantum of light by the primary donor P680 and charge separation in the reaction center, strong binding of the Fe(III) cation to HAS, which leads to HAS blocking (photochemical). In the present work, the authors showed that HAS blocking by the Fe cation can be achieved not only photochemically but also chemically: the Fe(II) cation weakly bound to HAS can be oxidized by H2O2, which leads to the formation of Fe(III) and its blocking of HAS as a result of strong binding. However, the efficiency of chemical blocking is significantly less than that of photochemical one. This fact indicates that photochemical blocking is not a single quantum process but at least a two-quantum process. The obtained data testify in favor of the two quantum mechanism of photoactivation (reconstruction of the manganese cluster in PSII(-Mn) particles during their incubation under illumination with Mn(II) cations), a physiologically very important process.