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.