Publication Date

2009-08-07

Availability

Open access

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Biochemistry and Molecular Biology (Medicine)

Date of Defense

2009-07-21

First Committee Member

Arun Malhotra - Committee Chair

Second Committee Member

Thomas K. Harris - Committee Member

Third Committee Member

Gregory Conner - Committee Member

Fourth Committee Member

Roger E. Fenna - Mentor

Abstract

Compound I of myeloperoxidase is capable of both one-electron oxidation and two-electron oxidation reactions. Halides and pseudohalides are the substrates for the two-electron oxidation and other compounds including a large variety of aromatic alcohols and amines can be oxidized via the single electron oxidation pathway. To investigate the catalytic mechanism of myeloperoxidase four structures of complexes of myeloperoxidase were solved. Two of them are complexes with hydroxamic acids and the other two are complexes with nitrite. Hydroxamic acids (salicylhydroxamic acid and benzylhydroxamic acid) can function as structural analogues for the aromatic alcohol and amine substrates of myeloperoxidase. The crystal structures of complexes of MPO with both hydroxamic acids have been solved at 1.85 Å resolution and their binding to myeloperoxidase is compared. The models show similar binding of their hydroxamic acid moieties but different orientations of their aromatic rings. The absence of the hydroxyl group covalently bound to the benzyl group in benzylhydroxamic acid creates an environment that does not permit the same favorable interactions with MPO when compared to salicylhydroxamic acid. These findings could explain the three orders of magnitude difference in the value of the dissociation constants of the two complexes. Nitrite has been shown to bind myeloperoxidase and also to reduce Compound I and Compound II. Crystal structures of the complex between myeloperoxidase and nitrite confirmed the binding of nitrite to the native enzyme both in the distal cavity and the chloride-binding site. The binding in the distal cavity occurred to the heme iron in the nitro mode. In the MPO-cyanide-nitrite ternary complex, nitrite had been shown to bind only at the chloride-binding site. No secondary site for nitrite binding had been seen in the distal cavity when cyanide was liganded to the iron. Overall, this study is the first to show from a crystallographic point of view a comparison in the mode of binding of the two hydroxamic acids to a mammalian peroxidase and also the binding of nitrite to a heme peroxidase.

Keywords

X-ray Crystallography; Peroxidase; Hydroxamic Acids; Nitrite

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