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-FX core) that also lacks subunits PsaD and PsaE and the two iron-sulfur clusters FA and FB, which constitute an integral part of PsaC. In this P700-FX core, the redox potentials (Em) of the two quinones A1A/B and the iron-sulfur cluster FX 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-FX core, being mainly protonated in the native PSI complex but ionized in the P700-FX core. In the P700-FX core, Em(A1A/B) remains practically unchanged, whereas Em(FX) is upshifted by 42 mV. With these calculated Em values, the electron transfer rate from A1 to FX in the P700-FX core is estimated to be slightly faster on the A1A side than that of the wild type, which is consistent with kinetic measurements.
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