title: Crystal structures of the organomercurial lyase MerB in its free and mercury-bound forms: insights into the mechanism of methylmercury degradation.
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.
authors.affiliation: Department of Biochemistry, Université de Montréal, Montréal, Québec H3C 3J7, Canada.
authors.forename: Julien
authors.initials: J
authors.lastname: Lafrance-Vanasse
authors.forename: Maryse
authors.initials: M
authors.lastname: Lefebvre
authors.forename: Paola
authors.initials: P
authors.lastname: Di Lello
authors.forename: Jurgen
authors.initials: J
authors.lastname: Sygusch
authors.forename: James G
authors.initials: JG
authors.lastname: Omichinski
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D003545:Cysteine / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D008190:Lyases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008767:Methylmercury Compounds / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D009154:Mutation / Q000235:genetics; D019906:Nuclear Magnetic Resonance, Biomolecular; D055503:Protein Multimerization; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary
chemical_list: D001426:Bacterial Proteins; D008767:Methylmercury Compounds; D008190:Lyases; C097247:MerB protein, Bacteria; D003545:Cysteine