Correlation between pH dependence of O2 evolution and sensitivity of Mn cations in the oxygen-evolving complex to exogenous reductants

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 Mn4CaO5 which can be extracted from PSII and then reconstructed using exogenous cations Mn(II) and Ca2+. 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. Binding of Fe(II) cations first requires its light-dependent oxidation. In general, the interaction of Fe(II) with Mn-depleted PSII has a number of features similar to the two-quantum model of photoactivation of the complex with the release of oxygen. Interestingly, incubation of Ca-depleted PSII with Fe(II) cations under certain conditions is accompanied by the formation of a chimeric cluster Mn/Fe in the oxygen-evolving complex. PSII with the cluster 2Mn2Fe was found to be capable of water oxidation, but only to the H2O2 intermediate. However, the cluster 3Mn1Fe can oxidize water to O2 with an efficiency about 25% of the original in the absence of extrinsic proteins PsbQ and PsbP. In the presence of these proteins, the efficiency of O2 evolution can reach 80% of the original when adding exogenous Ca2+. In this review, we summarized information on the formation of chimeric Mn-Fe clusters in the oxygen-evolving complex. The data cited may be useful for detailing the mechanism of water oxidation.

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 Ca²⁺ in the Mn₄CaO₅ 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. We found that Fe(II) cation replaces Mn cation at pH 6.5, however, PSII(-Ca/low pH) membranes with the 3Mn1Fe chimeric cluster in the oxygen-evolving complex evolve O₂ with high intensity in the presence of exogenous Ca²⁺. The O₂ evolution rate is about 80% of the same rate in PSII(-Ca/low pH) membranes.

Ascorbate inactivates the oxygen-evolving complex in prolonged darkness

Ascorbate (Asc, vitamin C) is an essential metabolite participating in multiple physiological processes of plants, including environmental stress management and development. In this study, we acquired knowledge on the role of Asc in dark-induced leaf senescence using Arabidopsis thaliana as a model organism. One of the earliest effects of prolonged darkness is the inactivation of oxygen-evolving complexes (OEC) as demonstrated here by fast chlorophyll a fluorescence and thermoluminescence measurements. We found that inactivation of OEC due to prolonged darkness was attenuated in the Asc-deficient vtc2-4 mutant. On the other hand, the severe photosynthetic phenotype of a psbo1 knockout mutant, lacking the major extrinsic OEC subunit PSBO1, was further aggravated upon a 24-h dark treatment. The psbr mutant, devoid of the PSBR subunit of OEC, performed only slightly disturbed photosynthetic activity under normal growth conditions, whereas it showed a strongly diminished B thermoluminescence band upon dark treatment. We have also generated a double psbo1 vtc2 mutant, and it showed a slightly milder photosynthetic phenotype than the single psbo1 mutant. Our results, therefore, suggest that Asc leads to the inactivation of OEC in prolonged darkness by over-reducing the Mn-complex that is probably enabled by a dark-induced dissociation of the extrinsic OEC subunits. Our study is an example that Asc may negatively affect certain cellular processes and thus its concentration and localization need to be highly controlled.

pH dependence of photosystem II photoinhibition: relationship with structural transition of oxygen-evolving complex at the pH of thylakoid lumen

Ca-depleted photosystem II membranes (PSII[-Ca]) do not contain PsbP and PsbQ proteins protecting the Mn₄CaO₅ cluster of the PSII oxygen-evolving complex (OEC). Therefore, the Mn ions in the PSII(-Ca) membranes can be reduced by exogenous bulky reductants or the charged reductant Fe(II). We have recently found that the resistance of Mn ions in the OEC to the Fe(II) action is pH dependent and that this reductant is less effective at pH 5.7 than at pH 6.5 (Semin et al. J Photochem Photobiol B 178:192, 2018). Taking these data into account, we investigated the photoinhibition in different PSII membranes at pH 5.7 and 6.5 and found that the resistance to photoinhibition of PSII and PSII(-Ca) membranes with a Mn cluster is higher at pH 5.7 than at pH 6.5, whereas the resistance of the Mn-depleted PSII membranes is pH independent. In thylakoids, light generates the transmembrane ΔpH, leading to the acidulation of lumen that results in pH 5.7. The uncouplers (NH₄Cl or nigericin) that significantly prevent acidulation increase the rate of PSII photoinhibition in thylakoids. We suggest that the structural transition in the OEC at pH 5.7 plays a role of a built-in mechanism increasing the resistance of OEC to photoinhibition under illumination, since it is accompanied by a pH decrease in lumen to 5.7. The coincidence of these pH values, i.e. lumen pH under illumination and pH of the maximal resistance of the Mn cluster to the reduction by reductants, can point at the pH-dependent mechanism of PSII self-protection from photoinactivation.

Creation of a 3Mn/1Fe cluster in the oxygen-evolving complex of photosystem II and investigation of its functional activity

Extraction of Mn cations from the oxygen-evolving complex (OEC) of Ca-depleted PSII membranes (PSII[-Ca,4Mn]) by reductants like hydroquinone (H₂Q) occurs with lower efficiency at acidic pH (2Mn/reaction center [RC] are extracted at pH5.7) than at neutral pH (3Mn/RC are extracted at pH6.5) [Semin et al. Photosynth. Res. 125 (2015) 95]. Fe(II) also extracts Mn cations from PSII(-Ca,4Mn), but only 2Mn/RC at pH6.5, forming a heteronuclear 2Mn/2Fe cluster [Semin and Seibert, J. Bioenerg. Biomembr. 48 (2016) 227]. Here we investigated the efficiency of Mn extraction by Fe(II) at acidic pH and found that Fe(II) cations can extract only 1Mn/RC from PSII(-Ca,4Mn) membranes at pH 5.7, forming a 3Mn/1Fe cluster. Also we found that the presence of Fe cations in a heteronuclear cluster (2Mn/2Fe) increases the resistance of the remaining Mn cations to H₂Q action, since H₂Q can extract Mn cations from homonuclear Mn clusters of PSII(-Ca,4Mn) and PSII(-Ca,2Mn) membranes but not from the heteronuclear cluster in PSII(-Ca,2Mn,2Fe) membranes. H₂Q also cannot extract Mn from PSII membranes obtained by incubation of PSII(-Ca,4Mn) membranes with Fe(II) cations at pH5.7, which suggests the formation of a heteronuclear 3Mn/1Fe cluster in the OEC. Functional activity of PSII with a 3Mn/1Fe cluster was investigated. PSII preparations with a 3Mn/1Fe cluster in the OEC are able to photoreduce the exogenous electron acceptor 2,6-dichlorophenolindophenol, possibly due to incomplete oxidation of water molecules as is the case with PSII(-Ca,2Mn,2Fe) samples. However, in the contrast to PSII(-Ca,2Mn,2Fe) samples PSII(-Ca,3Mn,1Fe) membranes can evolve O₂ at a low rate in the presence of exogenous Ca²⁺ (at about 27% of the rate of O₂ evolution in native PSII membranes). The explanation for this phenomenon (either water splitting and production of molecular O₂ by the 3Mn/1Fe cluster or apparent O₂ evolution due to minor contamination of PSII(3Mn,1Fe) samples with PSII(-Ca,4Mn) membranes) is discussed.