Changes in the Electron Transfer Symmetry in the Photosystem I Reaction Centers upon Removal of Iron–Sulfur Clusters

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Andrey A. Sukhanov¹, Mahir D. Mamedov², Georgy E. Milanovsky², Kev M. Salikhov¹, and Alexey Yu. Semenov^2,a*

¹Zavoisky Physical-Technical Institute, FRC Kazan Scientific Center, Russian Academy of Sciences, 420029 Kazan, Russia

²Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, 119234 Moscow, Russia

^* To whom correspondence should be addressed.

Received July 4, 2022; Revised August 9, 2022; Accepted August 16, 2022

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In photosynthetic reaction centers of intact photosystem I (PSI) complexes from cyanobacteria, electron transfer at room temperature occurs along two symmetrical branches of redox cofactors A and B at a ratio of ~3 : 1 in favor of branch A. Previously, this has been indirectly demonstrated using pulsed absorption spectroscopy and more directly by measuring the decay modulation frequencies of electron spin echo signals (electron spin echo envelope modulation, ESEEM), which allows to determine the distance between the separated charges of the primary electron donor P₇₀₀⁺ and phylloquinone acceptors A_1A^– and A_1B^– in the symmetric redox cofactors branches A and B. In the present work, these distances were determined using ESEEM in PSI complexes lacking three 4Fe–4S clusters, F_X, F_A, and F_B, and the PsaC protein subunit (the so-called P₇₀₀–A₁ core), in which phylloquinone molecules A_1A and A_1B serve as the terminal electron acceptors. It was shown that in the P₇₀₀–A₁ core preparations, the average distance between the centers of the P₇₀₀⁺A₁^– ion-radical pair at a temperature of 150 K in an aqueous glycerol solution and in a dried trehalose matrix, as well as in a trehalose matrix at 280 K, is ~25.5 Å, which corresponds to the symmetrical electron transfer along the A and B branches of redox cofactors at a ratio of 1 : 1. Possible reasons for the change in the electron transfer symmetry in PSI upon removal of the PsaC subunit and 4Fe–4S clusters F_X, F_A, and F_B are discussed.

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KEY WORDS: photosystem I, electron transfer, electron spin echo envelope modulation (ESEEM), trehalose matrix, iron–sulfur clusters

DOI: 10.1134/S0006297922100042

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