LAUSR

The reduction kinetics of the photo-oxidized primary electron donor P700 in photosystem I (PS I) complexes from cyanobacteria Synechocystis sp. PCC 6803 were analyzed within the kinetic model, which considers electron transfer (ET) reactions between P700, secondary quinone acceptor A1, iron-sulfur clusters and external electron donor and acceptors – methylviologen (MV), 2,3-dichloro-naphthoquinone (Cl2NQ) and oxygen. PS I complexes containing various quinones in the A1-binding site (phylloquinone PhQ, plastoquinone-9 PQ and Cl2NQ) as well as FX-core complexes, depleted of terminal iron–sulfur FA/FB clusters, were studied. The acceleration of charge recombination in FX-core complexes by PhQ/PQ substitution indicates that backward ET from the iron–sulfur clusters involves quinone in the A1-binding site. The kinetic parameters of ET reactions were obtained by global fitting of the P700+ reduction with the kinetic model. The free energy gap ΔG0 between FX and FA/FB clusters was estimated as −130 meV. The driving force of ET from A1 to FX was determined as −50 and −220 meV for PhQ in the A and B cofactor branches, respectively. For PQ in A1A-site, this reaction was found to be endergonic (ΔG0 = +75 meV). The interaction of PS I with external acceptors was quantitatively described in terms of Michaelis–Menten kinetics. The second-order rate constants of ET from FA/FB, FX and Cl2NQ in the A1-site of PS I to external acceptors were estimated. The side production of superoxide radical in the A1-site by oxygen reduction via the Mehler reaction might comprise ≥0.3% of the total electron flow in PS I.