부산대학교 도서관

The reduction kinetics of the photo-oxidized primary electron donor P.sub.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.sub.700, secondary quinone acceptor A.sub.1, iron-sulfur clusters and external electron donor and acceptors - methylviologen (MV), 2,3-dichloro-naphthoquinone (Cl.sub.2NQ) and oxygen. PS I complexes containing various quinones in the A.sub.1-binding site (phylloquinone PhQ, plastoquinone-9 PQ and Cl.sub.2NQ) as well as F.sub.X-core complexes, depleted of terminal iron-sulfur F.sub.A/F.sub.B clusters, were studied. The acceleration of charge recombination in F.sub.X-core complexes by PhQ/PQ substitution indicates that backward ET from the iron-sulfur clusters involves quinone in the A.sub.1-binding site. The kinetic parameters of ET reactions were obtained by global fitting of the P.sub.700.sup.+ reduction with the kinetic model. The free energy gap [DELTA]G.sub.0 between F.sub.X and F.sub.A/F.sub.B clusters was estimated as -130 meV. The driving force of ET from A.sub.1 to F.sub.X was determined as -50 and -220 meV for PhQ in the A and B cofactor branches, respectively. For PQ in A.sub.1A-site, this reaction was found to be endergonic ([DELTA]G.sub.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.sub.A/F.sub.B, F.sub.X and Cl.sub.2NQ in the A.sub.1-site of PS I to external acceptors were estimated. The side production of superoxide radical in the A.sub.1-site by oxygen reduction via the Mehler reaction might comprise [greater than or equal to]0.3% of the total electron flow in PS I.
Author(s): Georgy E. Milanovsky [sup.1] , Anastasia A. Petrova [sup.1] , Dmitry A. Cherepanov [sup.1] [sup.2] , Alexey Yu. Semenov [sup.1] [sup.3] Author Affiliations: (1) A.N. Belozersky Institute of Physical-Chemical [...]