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

Abstract

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.

Photocurrent generation by a photosystem I-NiO photocathode for a p-type biophotovoltaic tandem cell

“…This is because the reaction rate of MV 2+ with PSI is higher than that with oxygen. 39 For comparison, when using the n-type semiconductor TiO 2 , the resulting photocurrent is anodic. 21,40 Overall, these results indicate that the PSI-NiO electrode acts as a photocathode.…”

The Mehler reaction site is the Phylloquinone within Photosystem I

“…The exact rate constant of the Mehler reaction is not currently known and estimates for the kinetic parameters for O2 photoreduction by PSI range from ~10 3 M −1 s −1 (22) to 10 7 M −1 s −1 (23). The mid value (7 × 10 4 M −1 × s −1 ) was obtained by kinetic modeling of charge recombination kinetics observed after a light flash (24). We measured the apparent k2 for intact fully mature PSI complexes purified from the wild type (WT) strain (PSI WT ), under conditions where O2 was the principal electron acceptor.…”

Kinetics and Energetics of Phylloquinone Reduction in Photosystem I: Insight From Modeling of the Site Directed Mutants

“…On the other hand, direct titration of A1 has proven to be very difficult because of its very negative redox midpoint potential. Henceforth, to acquire information on the redox properties of A1textA and A1textB, alternative approaches based either on theoretical chemical methods (Ishikita and Knapp, 2003; Karyagina et al, 2007; Ptushenko et al, 2008; Kawashima and Ishikita, 2017) or on modeling of the ET reactions kinetics in the framework of non-adiabatic ET tunneling theory (Santabarbara et al, 2005a; Moser and Dutton, 2006; Makita et al, 2015; Santabarbara and Zucchelli, 2016; Cherepanov et al, 2017; Makita and Hastings, 2017; Milanovsky et al, 2017) have been adopted.…”