title: Mercury methylation by HgcA: theory supports carbanion transfer to Hg(II).
abstract: Many proteins use corrinoid cofactors to facilitate methyl transfer reactions. Recently, a corrinoid protein, HgcA, has been shown to be required for the production of the neurotoxin methylmercury by anaerobic bacteria. A strictly conserved Cys residue in HgcA was predicted to be a lower-axial ligand to Co(III), which has never been observed in a corrinoid protein. Here, we use density functional theory to study homolytic and heterolytic Co-C bond dissociation and methyl transfer to Hg(II) substrates with model methylcobalamin complexes containing a lower-axial Cys or His ligand to cobalt, the latter of which is commonly found in other corrinoid proteins. We find that Cys thiolate coordination to Co facilitates both methyl radical and methyl carbanion transfer to Hg(II) substrates, but carbanion transfer is more favorable overall in the condensed phase. Thus, our findings are consistent with HgcA representing a new class of corrinoid protein capable of transferring methyl groups to electrophilic substrates.
authors.affiliation: Graduate School of Genome Science and Technology, University of Tennessee , Knoxville, Tennessee 37996, United States.
authors.forename: Jing
authors.initials: J
authors.lastname: Zhou
authors.forename: Demian
authors.initials: D
authors.lastname: Riccardi
authors.forename: Ariana
authors.initials: A
authors.lastname: Beste
authors.forename: Jeremy C
authors.initials: JC
authors.lastname: Smith
authors.forename: Jerry M
authors.initials: JM
authors.lastname: Parks
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D002244:Carbon / Q000737:chemistry; D003035:Cobalt / Q000737:chemistry; D008024:Ligands; D008628:Mercury / Q000737:chemistry*; D008745:Methylation; D008958:Models, Molecular; D008968:Molecular Conformation; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D008024:Ligands; D003035:Cobalt; D002244:Carbon; D008628:Mercury