ИСТИНА

Photosystem I (PS I) from the mutant menB of the cyanobacteria Synechocystis sp. PCC 6803 binds plastoquinone (PQ) instead of phylloquinone (PhQ) in A1-site. One of the most important features of the menB is that due to weak binding PQ can be easily replaced by other soluble quinones in water solution. In this work we turned to the investigation of the electron transfer from quinone to the iron-sulfur cluster FX, and an interaction of quinone acceptor with artificial electron acceptor methylviologen (MV). In the PS I from menB mutant (menB-PQ), as it was previously shown [1], the forward electron transfer from PQ to the iron-sulfur clusters is slowed down by a factor of ~1000 in contrast to the wild type, where these reactions proceed in submicrosecond time range. This allowed us to resolve the kinetic of the membrane potential generation (Δψ) due to forward electron transfer from the PhQ in the A1-site to the iron-sulfur clusters by direct electrometrical technique. Our experiments showed, that besides unresolvable fast phase of Δψ generation, additional slower electrogenic phases in the 10 – 100 ms time range were observed. These phases accounted for ~40% from the overall Δψ amplitude, which is in line with [1]. These phase were not observed, when PQ in the A1-site was replaced by 2,3-dichloro-1,4-naphtoquinone (Cl2NQ). This finding corroborates the inability of forward electron transfer from Cl2NQ to the iron-sulfur clusters [2]. The time-resolved spectroscopy at 820 nm showed the effective interaction of menB-PQ PS I with methylviologen (MV) at 10 μM concentration, while no such interaction was observed in the case of menB-Cl2NQ. However at concentration of MV ~1 mM, the kinetics of P700+ reduction was slowed down due to interaction of menB-Cl2NQ with MV. This finding was confirmed by monitoring the P700+ with X-band EPR under steady state illumination. [1] Semenov AYu, Vassiliev IR, van der Est A, Mamedov MD, Zybailov B, Shen GZ, Stehlik D, Diner BA, Chitnis PR, Golbeck JH (2000) J. Biol. Chem. 275:23429–23438 doi: 10.1074/jbc.M000508200 [2] Mula S, Savitsky A, Möbius K, Lubitz W, Golbeck JH, Mamedov MD, SemenovA Yu, van der Est A (2012) Photochemical & Photobiological Sciences doi: 10.1039/C2PP05340C