Abstract
The x-ray crystal structure of human myeloperoxidase has been extended to 1.8 A resolution, using x-ray data recorded at -180 degrees C (r = 0.197, free r = 0.239). Results confirm that the heme is covalently attached to the protein via two ester linkages between the carboxyl groups of Glu(242) and Asp(94) and modified methyl groups on pyrrole rings A and C of the heme as well as a sulfonium ion linkage between the sulfur atom of Met(243) and the beta-carbon of the vinyl group on pyrrole ring A. In the native enzyme a bound chloride ion has been identified at the amino terminus of the helix containing the proximal His(336). Determination of the x-ray crystal structure of a myeloperoxidase-bromide complex (r = 0.243, free r = 0.296) has shown that this chloride ion can be replaced by bromide. Bromide is also seen to bind, at partial occupancy, in the distal heme cavity, in close proximity to the distal His(95), where it replaces the water molecule hydrogen bonded to Gln(91). The bromide-binding site in the distal cavity appears to be the halide-binding site responsible for shifts in the Soret band of the absorption spectrum of myeloperoxidase. It is proposed that halide binding to this site inhibits the enzyme by effectively competing with H(2)O(2) for access to the distal histidine, whereas in compound I, the same site may be the halide substrate-binding site.
Highlights
Myeloperoxidase (MPO, EC 1.11.1.7)1 is a heme-containing enzyme found in mammalian neutrophils, where it catalyzes the hydrogen peroxide mediated peroxidation of halide ions [1, 2] and the pseudohalide thiocyanate [3, 4], according to the following reaction
The heme was identified as a derivative of protoporphyrin IX in which the methyl groups on pyrrole rings A and C had been modified to allow formation of ester linkages with Glu242 and Asp94, respectively
It is likely that other mammalian peroxidases such as eosinophil peroxidase, LPO, and thyroid peroxidase have heme-protein ester linkages analogous to those of MPO, but that the sulfonium ion linkage is a feature unique to MPO
Summary
Ions and thiocyanate or single-electron oxidation of a wide variety of aromatic alcohols and amines. Chloride, bromide, and iodide induce red-shifts of the Soret maximum from 428 nm to 435, 434, 432, and 423 nm, respectively, and measurements of these spectral changes have been used to determine the pH dependence of halide binding to the enzyme [19, 23]. These studies have arrived at differing conclusions concerning the number of halide-binding sites on the enzyme and whether there are separate sites for halide binding as a substrate and as an inhibitor. Structure determination of an MPO-bromide complex has revealed a number of halide-binding sites, and their possible relevance to the catalytic mechanism of the enzyme is discussed
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