The Organomercurial LyaseMerB

Abstract

AbstractMercury is introduced into the environment either from natural occurrences or from human activities, and consumption of mercury‐contaminated fish poses a serious human health issue. The three inorganic forms of mercury are elemental mercury, mercurous compounds, and mercuric compounds, while the most abundant organic form is methylmercury. Owing to its ability to permeate membranes and accumulate in organisms, methylmercury is more toxic than ionic mercury. Mercury‐resistant bacteria have developed a two enzyme system to convert both Hg (II) and methylmercury to the less toxic elemental mercury. The first enzyme is an organomercurial lyase (MerB) and the second enzyme is a mercuric ion reductase (MerA). MerB catalyzes the protonolysis of the carbon–mercury bond on a wide range of organomercurials, including methylmercury, resulting in a reduced carbon compound and ionic mercury. The cleavage of the carbon–mercury bond and the formation of the electrophile‐carbon bond are concerted (SE2). Structural studies demonstrated that MerB contains a unique fold and that significant conformational changes occur on binding of organomercurial substrates. On the basis of mutagenesis, structural, and computational studies, two cysteines and an aspartic acid residue in the active site are known to play key roles in the cleavage of the carbon–mercury bond.