The Effects of Protein Conformation on the Heme Symmetry in High Spin Ferric Heme Proteins as Studied by Electron Paramagnetic Resonance

All heme proteins containing mononuclear high spin ferric heme, when examined at low temperatures (near 1°K), exhibit X-band electron paramagnetic resonance (EPR) absorptions extending from near g = 6 to g = 2 which arise from transitions of the lowest Kramers doublet. Meaningful absolute quantitation of high spin ferric EPR spectra cannot be made other than from experiments at very low temperature or from a knowledge of the zero field splitting. The characteristics of the EPR spectrum may be used to describe the symmetry of the heme. The incorporation of hemin into a protein constrains the heme in such a manner that there is a departure from tetragonal symmetry toward rhombic (gx ≠ gy). In these cases, the resonance absorption derivative near g = 6 is either broadened or split into two resolvable g values dependent upon the interaction of the protein with the heme. The greater the constraint on the heme imposed by the protein, the greater will be the departure from tetragonality. Thus, the EPR of high spin heme proteins can be used as a protein conformational probe.