Light increases resistance of thylakoid membranes to thermal inactivation.

In the region of slightly acidic pH (рН 5.7), the manganese cluster in oxygen-evolving complex of photosystem II (PSII) is more resistant to exogenous reductants. The effect of such pH on the heat inactivation efficiency of the electron transport chain (O evolution and 2,6-dichlorophenolindophenol reduction) in PSII membranes and thylakoid membranes was investigated.

Current analysis of cations substitution in the oxygen-evolving complex of photosystem II.

Water oxidation in photosystem II (PSII) is performed by the oxygen-evolving complex MnCaO which can be extracted from PSII and then reconstructed using exogenous cations Mn(II) and Ca. The binding efficiency of other cations to the Mn-binding sites in Mn-depleted PSII was investigated without any positive results. At the same time, a study of the Fe cations interaction with Mn-binding sites showed that it binds at a level comparable with the binding of Mn cations.

Effect of lanthanides on oxidation of Mn cations via a high-affinity Mn-binding site in photosystem II membranes.

Lanthanide cations (La and Tb) bind to the Ca-binding site of the oxygen-evolving complex in Ca-depleted PSII membranes and irreversibly inhibit the oxygen evolution. Оn the other hand, EPR measurement of Mn concentration in buffer revealed that lanthanide cations inhibit the light-dependent oxidation of Mn cations via the high-affinity Mn-binding site in Mn-depleted PSII membranes, which suggests that they bind to and inhibit the high-affinity Mn-binding site of the oxygen-evolving complex. The inhibition is irreversible, bound Ln cation could not be washed out from the sample.

High-efficiency oxygen evolution by photosystem II oxygen-evolving complex containing 3Mn per reaction center.

Ca-depleted photosystem II membranes obtained by treatment with acidic buffer do not contain Ca in the MnCaO cluster but contain all extrinsic proteins protecting this cluster (PSII(-Ca/low pH)). However, unlike native photosystem II, Mn cluster in PSII(-Ca/low pH) samples is available for small-sized reductants. Using this property, we investigated the substitution possibility of Mn cation(s) with Fe cation(s) to obtain a chimeric cluster in PSII(-Ca/low pH) samples containing extrinsic proteins.

Competitive interaction of Mn(II) and Fe(II) cations with the high-affinity Mn-binding site of the photosystem II: evolutionary aspect.

The evolutionary origin of the oxygen-evolving complex (OEC) in the photosystem II (PSII) is still unclear, as is the nature of electron source for the photosystem before the OEC had appeared. Johnson et al. (in PNAS 110:11238, 2013) speculated that Mn(II) cations were the source of electrons for transitional photosystems.