The Organomercurial LyaseMerB

Abstract

Enzyme; lyase; EC 4.99.1.2; an alkylmercury lyase or organomercurial lyase commonly known as MerB.MerB catalyzes the protonolysis of the carbon-mercury bond in organomercurials, yielding ionic mercury and the corresponding hydrocarbon (see Scheme 1). MerB is active on a large variety of substrates ranging from simple alkyl compounds like methylmercury, which is the most naturally abundant organomercurial, to polyaromatic heterocyclic compounds like merbromin. 3,4 MerB also catalyzes the cleavage of the carbon-tin bond in select organostannanes like tetravinyltin, but in a less efficient manner. 5 O C C U R R E N C EThe organomercurial lyase MerB is a cytosolic protein found in gram-positive and gram-negative bacterial strains that display resistance to both ionic mercury and organomecurial compounds. [6][7][8][9] The mercury resistance is due to a dedicated set of genes that are regulated together by the mer operon. 10,11 The mer operon occurs in most cases on transferable genetic elements such as transposons or plasmids, and it is not part of the native genome. The activation of the mer operon occurs only in response to the presence of compounds containing mercury, which bind to the DNAbound MerR and activate expression of the mer operon. 12 The composition of the mer operon can vary dramatically between different bacterial strains, but resistance to organomercurial requires the presence of two enzymes, the lyase MerB and the mercuric reductase MerA. 9 B I O L O G I C A L F U N C T I O NMerB belongs to a group of proteins that confers organomercurials with resistance to specific bacteria containing the mer operon. 9,13 The mercury detoxification 3D Structure Ribbon representation of the mercury-bound form of the organomercurial lyase MerB. PDB code: 3F0O. The core domain of the protein is colored in dark blue, the amino-terminal region of the protein is colored in teal, and the mercury ion is represented as a gray sphere. This representation and all other structural representations were prepared using PyMOL 1 unless noted otherwise.

Structural and Biochemical Characterization of a Copper-Binding Mutant of the Organomercurial Lyase MerB: Insight into the Key Role of the Active Site Aspartic Acid in Hg–Carbon Bond Cleavage and Metal Binding Specificity

“…The MerB D99S Mutant Binds Copper. Four of the known variants of MerB contain a serine as opposed to an aspartic acid in the active site 41 (see Figure S1 of the Supporting Information), and this change alters both the enzymatic activity and the relative substrate specificity for these variants. 42 In an attempt to structurally characterize the basis for these observed differences with the MerB serine variants, we initially prepared a protein in which serine was substituted for aspartic acid in E. coli MerB (MerB D99S).…”

“…(RC607) MerB2, and Clostridium butyricum MerB2], in which the D99 is replaced with a serine residue. 41 Although these four serine-containing MerB variants are derived from different organisms, they have 100% amino acid sequence identity so they are in fact identical proteins. Interestingly, the presence of the serine in the active site appears to alter both the cleavage activity and the relative substrate specificity of these variant proteins.…”

Structural and Biochemical Characterization of Organotin and Organolead Compounds Binding to the Organomercurial Lyase MerB Provide New Insights into Its Mechanism of Carbon–Metal Bond Cleavage

“…The residues corresponding to C96, D99, and C159 in E. coli MerB are conserved in all MerB proteins expressed from a wide range of bacterial strains, except for four cases. 46 In these four strains, the bacteria express two MerB proteins from their mer operon. The first, designated MerB1, contains the three catalytic residues corresponding to C96, D99, and C159 in E. coli MerB, but the second, designated MerB2, contains a serine residue in the position of the aspartic acid residue (D99), in addition to the two cysteine residues.…”

“…The residues corresponding to C96, D99, and C159 in E. coli MerB are conserved in all MerB proteins expressed from a wide range of bacterial strains, except for four cases . In these four strains, the bacteria express two MerB proteins from their mer operon.…”