Mercuric reductase

Mercury(II) reductase (EC 1.16.1.1), commonly known as MerA, is an oxidoreductase enzyme and flavoprotein that catalyzes the reduction of Hg2+ to Hg0. Mercury(II) reductase is found in the cytoplasm of many eubacteria in both aerobic and anaerobic environments and serves to convert toxic mercury ions into relatively inert elemental mercury. Mercury(II) reductase (EC 1.16.1.1), commonly known as MerA, is an oxidoreductase enzyme and flavoprotein that catalyzes the reduction of Hg2+ to Hg0. Mercury(II) reductase is found in the cytoplasm of many eubacteria in both aerobic and anaerobic environments and serves to convert toxic mercury ions into relatively inert elemental mercury. Mercury(II) reductase, commonly known as MerA, is encoded in a structural gene found on the mer loci or as transposon 501 (Tn501). It shares the same promoter region as mercury transport class proteins, such as MerP and MerT, and regulatory factor MerD. MerA transcription is regulated by both MerR and MerD. Free mercury ions can bind to metalloproteins, particularly those with cysteine residues, and can cause incorrect conformations resulting in function loss. This can cause death in many bacteria, as can many other heavy metals, and thus, needs to be removed from the cell or transformed into a chemically inert form. Mercury(II) reductase takes Hg2+ and catalyzes its reduction into Hg0 which is then released from the cell as a vapour. Mercury in its elemental form does not have the ability to form stable complexes with amino acid residues in proteins so is less dangerous than its ionic form. Hg2+ + NADPH → Hg0 + H+ + NADP+ 1. Hg2+ + 2Cys-S− → Cys-S-Hg-S-Cys 2. FAD + NADPH → FADH− + NADP+ 3. Cys-S-Hg-S-Cys + FADH− → H+ + Hg0 + FAD + 2Cys-S− The substrates used in mercuric(II) reductase, as shown above, are Hg2+ and NADPH. In the catalytic active site of the enzyme, Hg2+ is held as a complex with two cysteine thiolates in a linear geometry. NADPH from the cytoplasm of the cell undergo a hydride transfer with an embedded FAD forming FADH−. The resulting FADH− then reduces Hg2+ into Hg0, in turn being oxidized back into FAD. After reduction, the mercury is then released from the enzyme as a volatile vapour. Mercury(II) reductase cannot completely reduce organomercury compounds such as methyl mercury. Thus, MerB cleaves the carbon-mercury bonds via protonolysis and forms a mercury dithiolate complex, upon which MerB transports the mercury directly to MerA for reduction.