ИСТИНА

It seems that the most important factor, that affects the electron transpher (ET), is the free energy of ET (ΔG). The ΔG values depend on midpoint potentials (Em) of redox-cofactors in photosystem I (PS I). The Em of electron carrier in the protein environment depends on amino acid residue, which binds electron transfer cofactor. Electron carriers in PS I organized in two psewdosymmetric branches, A- and B-branches, which associated with PsaA and PsaB protein subunits of PS I. In this work, the impact of mutation of asparagine residues N591PsaA and N604PsaB associated with second chlorophyll molecules Chl2 (or A-1) in the A and B-branch correspondingly, on the initial steps of electron transfer was studied. To study the impact of amino acid residue on the ET these asparagines were mutated to leucines and histidines. The following pairs of cyanobacteria Synechocystis sp. PCC 6803 mutants were used: ANL/BNL, ANH/BNH (where the A or B notation indicates the subunit in which the mutation is generated). The process of ion-radical pair P700+-A1- formation in these mutants was studied by differential absorption spectrometry and pump-probe femtosecond absorption flash-spectrometry. The data obtained by steady state differential absorption spectrometry revealed some differences between the PS I spectra of A-chain and B-chain mutants. The amplitudes of the bleaching at 700 nm for PS I from ANH and ANL mutants were much smaller than for PS I from BNH and BNL mutants. These discrepancies indicate that replacement of asparagin amino acid residue in A-branch somehow affects the process of P700 oxidation, which is impaired in the case of A-branch mutants. The results obtained by pump-probe femtosecond differential spectroscopy at 500 ps time delay demonstrate that in case of ANH and ANL mutants, ET to A1 is impaired, while for BNH and BNL mutants the ET rates are similar to those from the wild type PS I. These data reveal significant influence of asparagin N591PsaA mutation on the ET in PS I and confirm the assumption of Chl2 participation in ET.