Redox potential of cytochrome c550 in the cyanobacterium Thermosynechococcus elongates

Abstract 

Cytochrome c550 (cyt c550) from photosystem II (PSII) exists in the PSII-bound form but can be released from PSII by treatment with divalent cations or Tris, yielding the isolated form. We calculated heme redox potentials (E_m) based on the crystal structures of cyt c550 by solving the Poisson–Boltzmann equation. In the isolated form, the calculated E_m are −240mV at pH 6.0 and −352mV at pH 9.0. This pH-dependence is predominantly due to deprotonation of the heme-propionic group near Asn-49. In the PSII-bound form, the calculated E_m was up-shifted by 160mV versus the isolated form due to a conformational change of protein backbone, yielding E_m=−84mV.

Abbreviations: A. maxima, Arthrospira maxima, cyt c550, cytochrome c550, E_m, midpoint redox potential, LPB equation, linearized Poisson–Boltzmann equation, Mn-cluster, oxygen-evolving complex with Mn ions, prop-A/prop-D, heme-propionic group A/D, PSII, photosystem II, S. 6803, Synechocystis PCC 6803, T. elongatus, Thermosynechococcus elongatus

Keywords: Cytochrome c550, Photosystem II, PsbV, Heme redox potential, Electrostatic energy computation, CP43-Arg320