Characterizing the effects of the protein environment on the reduction potentials of metalloproteins
The reduction potentials of electron transfer proteins are critically determined by the degree of burial of the redox site within the protein and the degree of permanent polarization of the polypeptide around the redox site. Although continuum electrostatics calculations of protein structures can predict the net effect of these factors, quantifying each individual contribution is a difficult task. Here, the burial of the redox site is characterized by a dielectric radius Rp (a Born-type radius for the protein), the polarization of the polypeptide is characterized by an electret potential ϕp (the average electrostatic potential at the metal atoms), and an electret-dielectric spheres (EDS) model of the entire protein is then defined in terms of Rp and ϕp. The EDS model shows that for a protein with a redox site of charge Q, the dielectric response free energy is a function of Q2, while the electret energy is a function of Q. In addition, Rp and ϕp are shown to be characteristics of the fold of a protein and are predictive of the most likely redox couple for redox sites that undergo different redox couples.