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 P 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 P, secondary quinone acceptor A, iron-sulfur clusters and external electron donor and acceptors - methylviologen (MV), 2,3-dichloro-naphthoquinone (ClNQ) and oxygen. PS I complexes containing various quinones in the A-binding site (phylloquinone PhQ, plastoquinone-9 PQ and ClNQ) as well as F -core complexes, depleted of terminal iron-sulfur F /F clusters, were studied. The acceleration of charge recombination in F -core complexes by PhQ/PQ substitution indicates that backward ET from the iron-sulfur clusters involves quinone in the A-binding site. The kinetic parameters of ET reactions were obtained by global fitting of the P reduction with the kinetic model. The free energy gap ΔG between F and F /F clusters was estimated as -130 meV. The driving force of ET from A to F was determined as -50 and -220 meV for PhQ in the A and B cofactor branches, respectively. For PQ in A-site, this reaction was found to be endergonic (ΔG  = +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 /F , F and ClNQ in the A-site of PS I to external acceptors were estimated. The side production of superoxide radical in the A-site by oxygen reduction via the Mehler reaction might comprise ≥0.3% of the total electron flow in PS I.