| - | Heme oxygenases catalyze the oxidation of heme to biliverdin, CO, and free, iron. For pathogenic microorganisms, heme uptake and degradation are, critical mechanisms for iron acquisition that enable multiplication and, survival within hosts they invade. Here we report the first crystal, structure of the pathogenic Escherichia coli O157:H7 heme oxygenase ChuS, in complex with heme at 1.45 A resolution. When compared with other heme, oxygenases, ChuS has a unique fold, including structural repeats and a, beta-sheet core. Not surprisingly, the mode of heme coordination by ChuS, is also distinct, whereby heme is largely stabilized by residues from the, C-terminal domain, assisted by a distant arginine from the N-terminal, domain. Upon heme binding, there is no large conformational change beyond, the fine tuning of a key histidine (His-193) residue. Most intriguingly, in contrast to other heme oxygenases, the propionic side chains of heme, are orientated toward the protein core, exposing the alpha-meso carbon, position where O(2) is added during heme degradation. This unique, orientation may facilitate presentation to an electron donor, explaining, the significantly reduced concentration of ascorbic acid needed for the, reaction. Based on the ChuS-heme structure, we converted the histidine, residue responsible for axial coordination of the heme group to an, asparagine residue (H193N), as well as converting a second histidine to an, alanine residue (H73A) for comparison purposes. We employed spectral, analysis and CO measurement by gas chromatography to analyze catalysis by, ChuS, H193N, and H73A, demonstrating that His-193 is the key residue for, the heme-degrading activity of ChuS.
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| - | Structure of the Escherichia coli O157:H7 heme oxygenase ChuS in complex with heme and enzymatic inactivation by mutation of the heme coordinating residue His-193., Suits MD, Jaffer N, Jia Z, J Biol Chem. 2006 Dec 1;281(48):36776-82. Epub 2006 Oct 5. PMID:[http://ispc.weizmann.ac.il//pmbin/getpm?pmid=17023414 17023414]
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