  Photosystem I is a light-driven electron transfer device. Available X-ray
crystal structure from Thermosynechococcus elongatus, showed that electron
transfer pathways consist of two nearly symmetric branches of cofactors
converging at the first iron sulfur cluster FX, which is followed by two
terminal iron sulfur clusters FA and FB. Experiments have shown that Fx has
lower oxidation potential than FA and FB, which facilitate the electron
transfer reaction. Here, we use Density Functional Theory and Multi-Conformer
Continuum Electrostatics to explain the differences in the midpoint Em
potentials of the Fx, FA and FB clusters. Our calculations show that Fx has the
lowest oxidation potential compared to FA and FB due strong pair-wise
electrostatic interactions with surrounding residues. These interactions are
shown to dominated by the bridging sulfurs and cysteine ligands, which may be
attributed to the shorter average bond distances between the oxidized Fe ion
and ligating sulfurs for FX compared to FA and FB. Moreover, the electrostatic
repulsion between the 4Fe-4S clusters and the positive potential of the
backbone atoms is least for FX compared to both of FA and FB. These results
agree with the experimental measurements from the redox titrations of
low-temperature EPR signals and of room temperature recombination kinetics.
