Electrostatic influence of PsaC protein binding to the PsaA/PsaB heterodimer in photosystem I

The absence of the PsaC subunit in the photosystem I (PSI) complex (native PSI complex) by mutagenesis or chemical manipulation yields a PSI core (P700-F_X core) that also lacks subunits PsaD and PsaE and the two iron-sulfur clusters F_A and F_B, which constitute an integral part of PsaC. In this P700-F_X core, the redox potentials (E_m) of the two quinones A_1A/B and the iron-sulfur cluster F_X as well as the corresponding protonation patterns are investigated by evaluating the electrostatic energies from the solution of the linearized Poisson-Boltzmann equation. The B-side specific Asp-B558 changes its protonation state significantly upon isolating the P700-F_X core, being mainly protonated in the native PSI complex but ionized in the P700-F_X core. In the P700-F_X core, E_m(A_1A/B) remains practically unchanged, whereas E_m(F_X) is upshifted by 42 mV. With these calculated E_m values, the electron transfer rate from A₁ to F_X in the P700-F_X core is estimated to be slightly faster on the A_1A side than that of the wild type, which is consistent with kinetic measurements.