Conserved Structure, Altered Energetics: Iron-Sulfur Clusters in Visible and Far-Red Light Acclimated Photosystem I

Photosystem I (PSI) drives light-induced electron transfer through a conserved chain of cofactors that terminates at the acceptor-side iron-sulfur clusters F_X, F_A, and F_B. Far-red light photoacclimation enables some cyanobacteria to use lower-energy photons, but how the protein environment influences the redox energetics of these [4Fe-4S] clusters remain unclear. Here, we used Multi Conformation Continuum Electrostatics (MCCE), combined with mean-field energy decomposition, to calculate the midpoint potentials of F_X, F_A, and F_B in PSI from Thermosynechococcus elongatus, Halomicronema hongdechloris, and Fischerella thermalis. The spatial arrangement of the [4Fe-4S] clusters is highly conserved across all structures, and the ordering of calculated reduction midpoint potentials F_X < F_A < F_B is maintained. However, the electrostatic contributions differ among species and light-acclimation states. In the direct comparison between white-light and far-red light H. hongdechloris PSI, F_X shifts from -770 to -822 mV, whereas F_A and F_B shift from -610 to -572 mV and from -589 to -514 mV, respectively. Mean-field analysis indicates that backbone and reaction-field terms are comparatively conserved, while residue-level electrostatics provide the major source of cluster-specific variation. Overall, the calculations show that PSI preserves the [4Fe-4S] structural framework while displaying local electrostatic differences around the acceptor-side cofactors.