Enzymatic Transformation of Metals

Microorganisms can carry out chemical transformations of heavy metals, such as oxidation and reduction, methylation and demethylation, and these are important to not only biogeochemical cycling but also many metal resistance mechanisms. Most research into metal transformation has concentrated on the involvement of bacteria in the mercury cycle, which can be represented as:

+ Demethylation 2+ Redu

3 Methylation Oxid

Mercury resistance is a common property of Gram-positive and Gram-negative bacteria, and the determinants are usually plasmid encoded, particularly in Gramnegatives. The most common mercury resistance mechanism employed by bacteria is the enzymatic reduction of Hg2+ by cytoplasmic mercuric reductase to metallic HgO, which is less toxic than Hg2+, volatile, and is rapidly lost from the environment. This enzyme has also been found in certain fungi and yeast. Organomercury compounds are enzymatically detoxified by organomercurial lyase, which cleaves the Hg-C bonds of methyl-, ethyl-, and phenylmercury (for example) to form Hg2+ and methane, ethane, and benzene, respectively. The Hg2+ can be volatilized by mercuric reductase. FAD-containing mercuric reductase is a flavoprotein and is the best-studied metal detoxification enzyme. This enzyme and organomercurial lyase both require an excess of thiol for activity. NADPH is the preferred reducing cofac-tor for mercuric reductase as well as organomercurial lyase, but in some bacteria NADH is effective. The thiols prevent the formation of NADPH-Hg2+ complexes and ensure that Hg2+ is present as a dimercaptide. The reaction catalyzed by the mercuric reductase in vitro can be represented as:

Mercuric reductase is structurally and mechanically related to glutathione reductase and lipoamide dehydrogenase. The mechanism of mercuric reductase probable involves electrons being transferred from NADPH via FAD to reduce the active site cystine, converting it into two cysteine residues with titratable SH groups. One cysteine residue forms a charge transfer complex with FAD. The active site cysteines then reduce Hg2+ (which is bound to the C-terminal cysteines), forming HgO.

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