No abstract

The composition and catalase-like activity of Mn2+ complexes with bicarbonate were investigated with voltammetry and kinetic methods (by the rate of O2 production from H2O2). Three linear sections were revealed on the dependence of the reduction potential of Mn2+ on logarithm of bicarbonate concentration (logC(NaHCO3)) having slopes equal to 0 mV/logC(NaHCO3), -14 mV/logC(NaHCO3), and -59 mV/logC(NaHCO3), corresponding to Mn2+ aqua complex (Mn2+(aq)) and to Mn2+-bicarbonate complexes of the composition [Mn2+(HCO3(-))]+ (at concentration of HCO3(-) 10-100 mM) and [Mn2+(HCO3(-))2]0 (at concentration of HCO3(-) 100-600 mM). Comparison of HCO3(-) concentration needed for the catalase-like activity of Mn2+ with the electrochemical data showed that only electroneutral complex Mn2+(HCO3(-))2 catalyzed decomposition of H2O2, whereas positively charged Mn2+(aq) complex and [Mn2+(HCO3(-))]+ were not active. The catalase-like activity of Mn2+ did not appear upon substitution of anions of carbonic acids (acetate and formate) for HCO3(-). The rate of O2 production in the system Mn2+-HCO3(-)-H2O2 (pH 7.4) is proportional to the second power of Mn2+ concentration and to the fourth power of HCO3(-) concentration that indicates simultaneous involvement of two Mn2+(HCO3(-))2 complexes in the reaction of H2O2 decomposition.

A protective effect of bicarbonate (BC) against extraction of the extrinsic proteins, predominantly the Mn-stabilizing protein (PsbO protein), during treatment of Photosystem II (PS II) membrane fragment from pea with 2 M urea, and at low pH (using incubation in 0.2 M glycine-HCl buffer, pH 3.5 or 0.5 M citrate buffer, pH 4.0-4.5) was detected. It was shown that the extraction of the proteins with Mw 24 kDa (PsbP protein) and 18 kDa (PsbQ protein) by the use of highly concentrated solutions of NaCl does not depend on the presence of BC in the medium. An optimal concentration of BC at which it produces the maximum protecting effect was shown to be between 1 mM and 10 mM. The addition of formate did not influence the protein extraction but it reduced the stabilizing effect of BC. Independence of the stabilizing effect on the presence of the functionally active Mn within the water-oxidizing complex indicates that the protecting effect of BC is not related to its interaction with Mn ions. The fact that there is a preferable sensitivity of the PsbO protein to the absence of BC in the medium during all the treatments makes it possible to suggest that either BC interacts directly with the PsbO protein or it binds to some other sites within PS II and this binding facilitates the preservation of the native structure of this protein.

Photosystem II (PSII) is a pigment-protein complex of thylakoid membrane of higher plants, algae, and cyanobacteria where light energy is used for oxidation of water and reduction of plastoquinone. Light-dependent reactions (generation of excited states of pigments, electron transfer, water oxidation) taking place in PSII can lead to the formation of reactive oxygen species. In this review attention is focused on the problem of interaction of molecular oxygen with the donor site of PSII, where after the removal of manganese from the water-oxidizing complex illumination induces formation of long-lived states (P680(+•) and TyrZ(•)) capable of oxidizing surrounding organic molecules to form radicals.