1. Crystal structures of the organomercurial lyase MerB in its free and mercury-bound forms: insights into the mechanism of methylmercury degradation.
- Author
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Lafrance-Vanasse J, Lefebvre M, Di Lello P, Sygusch J, and Omichinski JG
- Subjects
- Bacterial Proteins genetics, Catalytic Domain, Crystallography, X-Ray, Cysteine genetics, Cysteine metabolism, Escherichia coli enzymology, Escherichia coli genetics, Lyases genetics, Models, Molecular, Mutation genetics, Nuclear Magnetic Resonance, Biomolecular, Protein Multimerization, Protein Structure, Quaternary, Protein Structure, Tertiary, Bacterial Proteins chemistry, Bacterial Proteins metabolism, Lyases chemistry, Lyases metabolism, Methylmercury Compounds chemistry, Methylmercury Compounds metabolism
- Abstract
Bacteria resistant to methylmercury utilize two enzymes (MerA and MerB) to degrade methylmercury to the less toxic elemental mercury. The crucial step is the cleavage of the carbon-mercury bond of methylmercury by the organomercurial lyase (MerB). In this study, we determined high resolution crystal structures of MerB in both the free (1.76-A resolution) and mercury-bound (1.64-A resolution) states. The crystal structure of free MerB is very similar to the NMR structure, but important differences are observed when comparing the two structures. In the crystal structure, an amino-terminal alpha-helix that is not present in the NMR structure makes contact with the core region adjacent to the catalytic site. This interaction between the amino-terminal helix and the core serves to bury the active site of MerB. The crystal structures also provide detailed insights into the mechanism of carbon-mercury bond cleavage by MerB. The structures demonstrate that two conserved cysteines (Cys-96 and Cys-159) play a role in substrate binding, carbon-mercury bond cleavage, and controlled product (ionic mercury) release. In addition, the structures establish that an aspartic acid (Asp-99) in the active site plays a crucial role in the proton transfer step required for the cleavage of the carbon-mercury bond. These findings are an important step in understanding the mechanism of carbon-mercury bond cleavage by MerB.
- Published
- 2009
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