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.