Title

X-ray crystallographic studies of human myeloperoxidase

Date of Award

1996

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Biochemistry and Molecular Biology

First Committee Member

Roger E. Fenna, Committee Chair

Abstract

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.

Keywords

Biology, Molecular; Chemistry, Biochemistry; Biophysics, General

Link to Full Text

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