Dark chemical blocking of the high-affinity Mn-binding site of photosystem II by Fe(II) cations

Binding of Fe(II) iron cation to high-affinity Mn-binding site (HAS) of photosystem II without oxygen-evolving complex (PSII(-Mn)) consists of several steps – a highly specific association of the Fe(II) cation with the HAS, oxidation of the bound cation with the tyrosine radical YZ●, generated as a result of light absorption by the primary donor P680 and charge separation in the reaction center, strong binding of the Fe(III) cation with HAS, which leads to HAS blocking (photochemical) with an iron cation. In the present work, we have shown that blocking HAS with an iron cation can be achieved not only by a photochemical process, but also by a chemical one: the Fe(II) cation weakly bound to HAS can be oxidized by hydrogen peroxide, which leads to the formation of Fe(III) and blocking of HAS by oxidized Fe cation as a result of strong binding. However, the effectiveness of chemical blocking is less than photochemical. This fact indicates that photochemical blocking is not a single quantum, but at least a two-quantum process. The data obtained indicate the two-quantum mechanism of a physiologically very important reaction - photoactivation (reconstruction of the manganese cluster in PSII(-Mn) particles during their incubation with Mn(II) cations).