ИСТИНА

Flash-induced reduction of the primary electron donor P700+ in photosystem I (PS I) from menB mutant of Synechocystis sp. PCC 6803, with quinone in the A1 binding site substituted with high-potential 2,3-dichloro-naphthoquinone (Cl2NQ), was studied in the presence of varying concentration of external electron acceptors [1]. Under these conditions electron transfer to the iron-sulfur clusters did not occur. Kinetic modelling of P700+ reduction revealed the existence of electron escape from the secondary electron acceptor A1 even in the absence of artificial acceptors, which was interpreted as A1 reaction with molecular oxygen in the surrounding medium. Analysis of the PS I crystallographic structure revealed the presence of two water-filled cavities within 1 nm of A1 binding sites connected with the external water, which may be engaged by molecular oxygen during its reaction with A1. Molecular dynamics modelling of PS I was used to analyze the oxygen binding to the intraprotein water pocket. Free energy profile of the interaction between molecular oxygen and the protein complex (potential of mean force) was calculated by molecular dynamics simulation of oxygen transfer along 1.5 nm channel from inner-most part of the water pocket to the surrounding water solution. Total molecular dynamics simulation time exceeded 100 ns. The potential of mean force revealed a deep (>20 kJ) minimum in the proximal to A1 hydrophobic part of the channel, where the nonpolar oxygen molecule can be captured owing to substitution of polar water molecules (hydrophobic effect). Interaction of water-dissolved oxygen with quinone in the A1-binding site can lead to the side production of superoxide radical by PS I (Mehler reaction), comprising over 0.3% of the total electron flow in the intact PS I complexes. The existence of highly efficient electron transfer to the iron-sulfur clusters in PS I may serve as an evolutionary implementation against such bypassing. References: 1. Milanovsky et.al. (2017) Photosynth Res 133, pp. 185–199