Mercuric reductase bacterial

Another flavoprotein constructed on the glutathione reductase pattern is the bacterial plasmid-encoded mercuric reductase which reduces the highly toxic Hg2+ to volatile elemental mercury, Hg°. A reducible... 

Bacterial mercuric reductase is a unique metal-detoxification biocatalyst, reducing mercury(II) salts to the metal. The enzyme contains flavin adenine dinucleotide, a reducible active site disulfide (Cys 135, Cys i4o), and a C-terminal pair of cysteines (Cys 553, Cys 559). Mutagenesis studies have shown that all four cysteines are required for efficient mercury(II) reduction. Mercury Lm-EXAFS studies for mercury(II) bound to both the wild-type enzyme and a very low-activity C-terminal double-alanine mutant (Cys 135, Cys uo, Ala 553, Ala 559) suggest the formation of an Hg(Cys)2 complex in each case (39). The Hg—S distances obtained were 2.31 A and are consistent with the correlation of bond length with coordination number presented above. Thus, no evidence was obtained for coordination of mercury(II) by all four active-site cysteines in the wild-type mercuric reductase. However, these studies do not define the full extent of the catalytic mechanism for mercury(II) reduction, and it is possible that a three- or four-coordinate Hg(Cys) complex is a key intermediate in the process. 

Microbial demethylations, or dealkylations, of organometallic forms are important reactions in detoxification mechanisms for some metals, such as mercury and tin. Bacterial detoxification of organomercurials is well established and involves the enzyme organomercurial lyase (OL)—a product of the merB gene—that enzymatically cleaves the Hg-C bond to form Hg, which is then reduced by the enzyme mercuric reductase (MR) to the less toxic Hg(0) (Equations (4) and (5)). The high volatility of Hg(0) results in it being rapidly removed to the atmosphere. 

Enzymatic detoxification was determined to be the major resistance mechanism in all species of mercury-resistant bacteria. For example, mercuric reductase was essential for volatilization of Hg from Hg + and various organomercurial hydrolases were responsible for volatilization of methane (CH4) from methylmercury, for ethane (C2H4) from ethylmercury, and for benzene from phenylmercury. Minamata Bay bacterial isolates can also volatilize Hg from added inorganic and organic mercurials. Genes which govern the chemistry of mercury detoxification were abundant in bacteria found in Minamata Bay and other mercury-polluted sites these genetic strains of mercury-resistant bacteria show promise for bioremediation of mercury pollution. 

HSAB theory is important in bioinorganic chemistry, as well. For instance, mercuric reductase (MerA) is a bacterial enzyme that can reduce toxic ions to Hg°. The active site of MerA contains two cysteine residues (Cys-207 and Cys-628). The Cys amino acid residues bind to the soft Hg ions through their soft HS side chains. Only a few other metals can bind in the active site of MerA and inhibit this reduction. These are the soft metals Ag+, Au +, and Cd " ", as well as the intermediate metals Cu " " and Co. No hard metals are capable of binding to the soft Cys amino acid residues in the MerA enzyme. 

In all bacterial systems studied, resistance to mercury requires a group of about six genes. Their functions include (a) regulation so that the system will function only when needed, (b) uptake of extracellular Hg " in a controlled and bound form to (c) mercuric reductase for conversion of toxic... 

Bacteria can unstick it by reversing what oxygen did to release It. .. T. Barkay et al. A thermophilic bacterial origin and subsequent constraints by redox, light and salinity on the evolution of the microbial mercuric reductase. 2010. Environ Microbiol. 12(11), p. 2904. DOl 10.1111/j.l462-2920.2010.02260.x. 

---