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-700 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-700, secondary quinone acceptor A(1), iron-sulfur clusters and external electron donor and acceptors - methylviologen (MV), 2,3-dichloro-naphthoquinone (Cl(2)NQ) and oxygen. PS I complexes containing various quinones in the A(1)-binding site (phylloquinone PhQ, plastoquinone-9 PQ and Cl(2)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 A(1)-binding site. The kinetic parameters of ET reactions were obtained by global fitting of the P-700 (+) reduction with the kinetic model. The free energy gap Delta G (0) between F (X) and F (A)/F (B) clusters was estimated as -130 meV. The driving force of ET from A(1) to F (X) was determined as -50 and -220 meV for PhQ in the A and B cofactor branches, respectively. For PQ in A(1A)-site, this reaction was found to be endergonic (Delta G (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 Cl(2)NQ in the A(1)-site of PS I to external acceptors were estimated. The side production of superoxide radical in the A(1)-site by oxygen reduction via the Mehler reaction might comprise ae<yen>0.3% of the total electron flow in PS I.