Resonance Raman characterization of the heme prosthetic group in eosinophil peroxidase

Abstract

The resonance‐enhanced Raman spectrum of eosinophil peroxidase (EPO) from horse and human eosinophils is reported. Based upon the spectral energies, distribution and depolarization ratios of the high‐frequency skeletal modes and upon the presence of weak bands assignable to vinyl substituent groups, we conclude that the heme prosthetic group is high‐spin, 6 coordinate protoporphyrin. The Raman spectrum reveals clear differences from lactoperoxidase (LPO), an enzyme which appears nearly structurally isomorphous by other physical techniques; the data indicate a stronger axial 6th ligand in EPO. Mechanistic implications are discussed in relation to LPO and myeloperoxidase, an enzyme present in neutrophils and monocytes which contains a unique functional active‐site chlorin.