Crystal Structures of the Organomercurial Lyase MerB in Its Free and Mercury-bound Forms: INSIGHTS INTO THE MECHANISM OF METHYLMERCURY DEGRADATION

Julien Lafrance-Vanasse, Maryse Lefebvre + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m807143200

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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.

Highlights

In the environment, mercury can exist in three different forms: elemental mercury, ionic mercury, and methylmercury
Buried Active Site—One important observation from the methylmercury by the organomercurial lyase (MerB) crystal structures is the occurrence of the active site buried within the hydrophobic core
Burying the MerB active site could serve to inhibit the release of the product ionic mercury

Summary

Introduction

Mercury can exist in three different forms: elemental mercury, ionic mercury, and methylmercury. In the 1960s, bacteria were isolated from soils and sediments contaminated with high levels of mercury (4 – 6) Analysis of these bacteria demonstrated that they had acquired a series of plasmid-encoded genes collectively referred to as the mer operon that imparts resistance to mercury. Crystal Structure of MerB thiol groups, and their unique properties have been proposed to mimic the mechanism of MerB and Cys-96 and Cys-159 [18]. Despite these efforts, the precise details of the mechanism by which MerB cleaves methylmercury are still poorly understood. The structures support earlier studies indicating that Cys-96 and Cys-159 are important for the binding to organomercurials and suggest that Asp-99 is an active site residue that participates in the protonolysis of the carbon-mercury bond

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