ИСТИНА

Photosystem I (PS I) is one of the pigment-protein complexes of photosynthetic electron transfer chain, which is capable of light energy conversion. PS I participates in the linear, cyclic and pseudo-cyclic electron flows. One of the most important functions of PS I is redistribution between these pathways under changing environmental conditions. This is determined by the PS I ability to interact with different electron acceptors. Recently the ability of the quinone cofactor of PS I to directly interact with O2 and ascorbate has been shown (Kozuleva et al., 2014, Trubitsin et al., 2014). We used purified PS I complexes with different modifications to analyze how thermodynamic properties of the PS I cofactors affect its ability to interact with different exogenous electron acceptors. The kinetics of P700+ dark reduction was registered using optical and EPR spectroscopy under single-flash excitation and continuous illumination in PS I complexes containing different number of iron-sulfur clusters (intact PS I complexes from the wild type strain of Synechocystis sp. PCC 6803 and FX-core complexes depleted of PsaC subunit and terminal iron-sulfur clusters FA/FB ) and different quinones in the A1-site (phylloquinone in PS I from the wild type strain and plastoquinone or 2,3-dichloro-1,4-naphtoquinone (Cl2NQ) in PS I from the menB mutant strain). Methylviologen (MV) at different concentration was used as electron acceptor. The kinetic model of electron transfer reactions in PS I was developed basing on this data. The model allowed us to estimate how modification of the kinetic constants of electron transfer reactions in PS I affects the efficiency of interaction of the complexes with exogenous acceptors. It was demonstrated that one of the crucial factors governing the interaction of PS I with external acceptors is the Em value of the quinone in the A1-binding site. We estimated the Em values of the phylloquinone and plastoquinone in the A1 site of PS I. The gradual decrease of the efficiency of MV interaction with PS I was demonstrated in the following order of complexes: intact WT>menB>FX-core. The model allowed to describe the interaction of PS I complexes with exogenous acceptors in terms of Michaelis-Menten approximation, to reveal the direct reduction of the MV by high-potential Cl2NQ in the A1-site of PS I and propose a mechanism of the latter reaction. The cyclic electron transport around PS I mediated by 2,6-dichloro-phenolindophenol (DCPIP) and Cl2NQ was demonstrated under continuous illumination. The obtained data extend our knowledge about the mechanisms of PS I interaction with different electron acceptors.