X-ray crystallographic studies of human myeloperoxidase

The peroxidases are a class of enzyme which catalyze the oxidation of a variety of organic and inorganic compounds by reducing peroxides. Previously, the crystal structure of canine myeloperoxidase was solved to 3.0 A resolution. However, the resolution was insufficient to establish the chemical nature of the heme, the solvent structure, and details of the protein structure. Herein, the structure of human myeloperoxidase to 2.28 A resolution is reported. The heme prosthetic group of myeloperoxidase is a modified protoporphyrin IX, in which three pyrrole ring substituents form covalent bonds with amino acid side chains of the protein: Asp94 and Glu242 form ester linkages with the methyl carbons of heme rings C and A, respectively, while Met243 forms a sulfonium ion linkage with the terminal vinyl carbon of heme ring A. Amino acid sequence comparisons of myeloperoxidase with the closely evolutionarily-related enzymes eosinophil peroxidase, lactoperoxidase, and thyroid peroxidase reveal that the two heme ester linkages may be a common feature of all four of these mammalian peroxidases, whereas the sulfonium ion linkage is a unique feature of myeloperoxidase. The heme-protein linkages of myeloperoxidase are likely to account for the unusual spectroscopic properties and oxidizing capabilities of this enzyme.The crystal structure of human myeloperoxidase complexed with the bi-substrate analogue salicylhydroxamic acid was solved to 2.3 A resolution. Salicylhydroxamic acid binds between the distal arginine and heme ring D of myeloperoxidase, displacing three water molecules from the distal heme cavity. The three oxygen atoms of salicylhydroxamic acid occupy similar positions as the displaced water molecules. The hydroxamic acid-hydroxyl group donates a proton to the unprotonated NE2-nitrogen of the distal histidine. This interaction reveals that the first step in the reaction of myeloperoxidase with $\rm H\sb2O\sb2$ entails the binding of the peroxide hydroxyl-hydrogen to the distal histidine NE2, as opposed to peroxide-oxygen binding to the heme iron. Furthermore, the near parallel orientation of the aromatic ring of salicylhydroxamic acid with respect to heme ring D, in addition to the 4 A proximity of the two rings, suggests a direct $\pi$-$\pi$ electron transfer pathway between pyrrole ring D and substrate, for the one-electron oxidations of aromatic compounds