Study of the Active Site Structure of Myeloperoxidase

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Myeloperoxidase (MPO) is a major component of the antimicrobial system of polymorphonuclear neutrophils. Despite many investigations of the spectoscopic and exzymatic properties of this enzyme, the chemical structure of the heme group of this enzyme has not so far been identified (iron porphyrin vs iron chlorin).1. During the course of search for a plausible model for the heme group in MPO, photoproto-Mb (pPPMb) was reconstituted from apomyoglobin and iron photoprotoporphyrin. The chemical and electronic structure of the prosthetic group and the ligand coordination structure of pPPMb derivatives were studied by light absorption, EPR, resonance Raman and magnetic circular dichroism (MCD) spectroscopy. The pPPMb derivatives exhibit visible spectra too far "red-shifted" to make pPPMb a very good MPO model.2. It is important to identify the substrate binding site on the enzyme to further our understanding of the mechanism of the reaction catalyzed by peroxidase. We measured the EPR spectra of a number of aromatic substrate complexes of mammalian peroxidases to determine the effect of aromatic substrate binding upon the electronic structure of the heme iron. A molecular model for the MPO-SHA (salicylhydroxamic acid) complex based on the recently determined three dimensional structure of MPO indicated that the six-membered ring of aromatic substrate molecules could bind to a hydrophobic region at the entrance to the distal heme pocket. The distinct physiological function of MPO could be understood on the basis of its active site structure.