UNDERSTANDING THE CELLULAR BIOCHEMISTRY OF MERCURY METHYLATING MICROORGANISMS

Mercury is a globally dispersed contaminant. Its most toxic form, methyl mercury (MeHg), is a potent neurotoxin that bioaccumulates in the food web, and humans are primarily exposed to MeHg by eating MeHg-contaminated fish and wildlife. The focus of our research is to understand the cellular biochemistry of mercury methylating microorganisms using biophysical, biochemical, and computational approaches. MeHg is formed by the action of several anaerobic bacteria and archaea that contain the two-gene cluster hgcAB, which encodes two proteins; (1) HgcA - a corrinoid-dependent membrane protein that is predicted to transfer a methyl group to Hg(II), and (2) HgcB - a 2[4Fe-4S] ferredoxin that is hypothesized to be bind Hg(II) and is required for the delivery of electrons needed for the reaction. To date, more than 300 microorganisms, bacteria and archaea, are predicted to methylate mercury. Other than hgcAB, the cellular machinery important for MeHg formation in these microorganisms is not well understood. To address this knowledge gap, we performed a bioinformatics-based analysis to develop a predictive understanding of the cellular machinery associated with MeHg formation exploring biochemical pathways, transport mechanisms, and the evolutionary origins of mercury methylation. Furthermore, we conducted a series of methylation assays to probe the molecular mechanisms governing MeHg formation in cell extracts of mercury methylating bacteria. We demonstrate that mercury methylation is an enzymatic process facilitated by HgcA and HgcB, and calculated the enzymatic rate constants for MeHg formation. Current efforts are centered on a combination of experimental and computational approaches to identify and characterize the chemistry and interactions of the corrinoid cofactor with HgcA. A comprehensive understanding of the various biochemical factors culminating in the production of MeHg will facilitate the development of effective strategies to reduce exposure to this pervasive neurotoxin.