Electrostatic influence of PsaC protein binding to the PsaA/PsaB heterodimer in Photosystem I
Hiroshi Ishikita 1, Dietmar Stehlik 1, John Golbeck 2 and Ernst-Walter Knapp 3*
1 Free University of Berlin
2 Pennstate University
3 Freie Universitat Berlin
* To whom correspondence should be addressed. E-mail: knapp{at}chemie.fu-berlin.de.
Submitted on July 1, 2005
Revised on August 4, 2005
Accepted on 7 October 2005
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Abstract |
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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, 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 (LPB) 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 while Em(FX) is up-shifted 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.
Key Words:
electron transfer, iron-sulfur cluster, phylloquinone, redox potential
