Mercury is introduced into the environment through both natural occurrences and as a result of numerous human activities. Once introduced into the environment, mercury can exist as elemental mercury (Hg0), ionic mercury (HgI and HgII) or methylmercury (MeHg), and there is a constant flux between these three forms as part of the natural biogeochemical cycle. Select bacterial strains survive in mercury-contaminated environments due to the presence of a transferable genetic element (the mer operon), which enables them to convert both HgII and MeHg to the less toxic Hg0 using two enzymes, the organomercurial lyase MerB and the mercuric ion reductase MerA. To identity features responsible for its unique catalytic activity at the atomic level, structural studies were performed with MerB in the presence of organomercurial, organotin or organolead compounds. The studies with organomercurial compounds identified three key catalytic residues (C96, D99 and C159) required for carbon-Hg bond cleavage and demonstrated that the HgII product remains bound in the active site. Studies with dimethyltin (DMT), triethyltin (TET) and trimethyllead (TML) revealed that these compound inhibit MerB using distinct mechanisms, but in all cases the initial binding event requires D99 (Fig. 1). In contrast, diethyltin (DET) and diethyllead (DEL) are substrates and the resulting metal ion products remain bound in the active site of MerB. Together, these studies demonstrate that D99 of MerB plays a key role in organometal substrate selectivity, carbon-metal bond cleavage as well as in transferring the metal ion product to MerA. Due to its unique activity, MerB represents an ideal model system for creating green chemistry to remediate MeHg-contaminated sites and these results provide important mechanistic insights into identifying new ways to exploit MerB for such purposes.

Understanding the mechanism of carbon-metal bond cleavage by the organomercurial lyase MerB

256th National Meeting (2018)

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