Mercuric ion reductase

Schiering, N., Kabsch, W., Moore, M. J., Distefano, M. D., Walsh, C. T., and Pai, E. F., 1991, Structure of the detoxification catalyst mercuric ion reductase from Bacillus sp. strain RC607. Nature 3S2 168nl72. 

Williams, C. H. J., 1992, Lipoamide dehydrogenase, glutathione reductase, diioredoxin reductase and mercuric ion reductase family of flavoenzyme transhy(hogenases, in Chemistry and Biochemistry of Elavoenzymes, volume III (F. Muller, ed.), CRC Press, Boca Raton,... 

Mercuric ion reductase, the FAD-containing merA gene product, has several pairs of conserved cysteines. From site-specific mutagenesis studies, cysteine residues in the sequence 134-Thr-Cys-Val-Asn-Val-Gly-Cys-140 are known to comprise a redox-active disulfide group in addition, a redox-inactive pair of cysteines near the carboxyl terminus is also required for the selective reduction of Hg(II). Exactly how the enzyme achieves the chemistry shown in Equation... 

The crucial two-electron reduction step in this pathway is catalyzed by the flavoprotein mercuric ion reductase. The high vapor pressure of elemental mercury results in the volatilization of mercury from aqueous media (Chang et al., 1993 Ogunseitan 1997 Ogunseitan 1998). 

Stefan E. and Miller S.M. (1999) Alternative routes for entry of HgX2 into the active site of mercuric ion reductase depend on the nature of the X ligands. Biochemestry 38 ... 

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. 

We have reviewed the best-known prokaryotic plasmid-based resistance systems from the view of metal ion homeostasis. Plasmid-based systems are, with few exceptions, inducible and are regulated by activators (mercury and copper) or repressors (cadmium) in different cases. The metal-binding motifs used by these systems are frequently localized (e.g., Cys-Xaa-Xaa-Cys, Glu-His-His, and His-Xaa-His), but occasionally coordinating cysteine residues on different subunits interact (as in the MerR regulatory protein and mercuric reductase enzyme). 

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