Kinetic modeling of electron transfer reactions in photosystem I complexes of various structures with substituted quinone acceptors.

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 (Cl2 NQ) and oxygen. PS I complexes containing various quinones in the A1 -binding site (phylloquinone PhQ, plastoquinone-9 PQ and Cl2 NQ) as well as F X -core complexes, depleted of terminal iron-sulfur F A /F B clusters, were studied. The acceleration of charge recombination in F X -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 ΔG 0 between F X and F A /F B clusters was estimated as -130 meV. The driving force of ET from A1 to F X 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 (ΔG 0  = +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 F A /F B , F X and Cl2 NQ 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.