| mercury (II) reductase | |||||||||
|---|---|---|---|---|---|---|---|---|---|
 | |||||||||
| Identifiers | |||||||||
| EC no. | 1.16.1.1 | ||||||||
| CAS no. | 67880-93-7 | ||||||||
| Databases | |||||||||
| IntEnz | IntEnz view | ||||||||
| BRENDA | BRENDA entry | ||||||||
| ExPASy | NiceZyme view | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDBPDBePDBsum | ||||||||
| Gene Ontology | AmiGO /QuickGO | ||||||||
| |||||||||
Mercury(II) reductase (EC1.16.1.1), commonly known as MerA, is anoxidoreductaseenzyme andflavoprotein thatcatalyzes thereduction ofHg2+ to Hg0. Mercury(II) reductase is found in the cytoplasm of manyeubacteria[1] in both aerobic and anaerobic environments[2] and serves to convert toxic mercury ions into relatively inertelemental mercury.
Mercury(II) reductase, commonly known as MerA, is encoded in astructural gene found on the merloci or astransposon 501 (Tn501).[3] It shares the samepromoter region as mercury transport classproteins, such as MerP and MerT, and regulatory factor MerD.[1] MerAtranscription is regulated by both MerR and MerD.[1]
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Free mercury ions can bind tometalloproteins, particularly those withcysteine 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−
Thesubstrates used in mercuric(II) reductase, as shown above, are Hg2+ andNADPH. In the catalyticactive site of the enzyme, Hg2+ is held as acomplex with twocysteinethiolates in alinear geometry.[4] NADPH from thecytoplasm of the cell undergo ahydride transfer with an embeddedFAD forming FADH−.[4] The resulting FADH− then reduces Hg2+ into Hg0, in turn beingoxidized back into FAD.[4] After reduction, the mercury is then released from the enzyme as a volatile vapour.
Mercury(II) reductase cannot completely reduceorganomercury compounds such asmethyl 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.[5]
The active form of mercury(II) reductase is found as ahomodimer.[4] It has aquaternary conformation and the monomer is composed of twodomains.[4]
One of the domains of mercuric reductase, NmerA, has a structural fold of βαββαβ.[6] It is attached to the active site through linkers made of around 30 amino acids.[6] NmerA contains two cysteine residues which function in the acquisition of Hg2+ from other proteins or inorganic ligands such as MerT and direct transport to the catalyticactive site of MerA.[7] Very few mercuric(II) reductases have been found to lack the NmerA domain.[6]
The active site of MerA consists of four cysteine residues, a FAD, and atyrosine residue. When bound to a Hg2+, a complex is formed with at least two cysteine thiolates at any time until release.[4] Two cysteine residues (Cys-136 and Cys-141) are buried within the protein and the other two cysteine residues (Cys-558' and Cys-559') are found near the surface near the C terminus.[4] The buried cysteine residues function as the site of catalysis whereas the surface cysteine residues function as transport to the site of catalysis.[4]
During Hg2+ transfer to the catalytic active site from the C terminus cysteine residues, atrigonal planarintermediate is formed stabilized byhydrogen bonding of a water molecule to the thiolates.[4] The water molecule is held in place by hydrogen bonding from the hydroxyl group of a nearby tyrosine residue (Tyr-194).[4]
Various proteins assist in transporting mercury to mercury(II) reductase. MerP, aperiplasmic mercury transport protein found ingram negative bacteria, transports mercury through the outer membrane into the inner membrane where it holds the mercury for another protein to bind to it and transport it to mercury(II) reductase.[1] MerT, a membrane bound protein found in both gram negative andgram positive bacteria, binds to free floating mercury.[1] Mercury(II) reductase can directly take mercury from MerT and MerP.[1]
When mercury enters the cell and is not bound to a membrane protein, mercury(II) reductase can transport it to its active site on its own depending on the size of itsligands.[8] If the ligands attached to mercury are large, mercury(II) reductase uses the C-terminus cysteine residues to transport the mercury to its active site.[8] If the ligands are small, mercury can go directly the active site for reduction.[8] The ligands can be removed by the NmerA domain.[8]
In the case of organomercury compounds, MerB breaks the Hg-C bonds and transports the Hg to mercuric(II) reductase.[5]
When not bound to Hg2+, mercury(II) reductase acts as anoxidase creating toxichydrogen peroxide.[1] Thus, excess of the enzyme can result in bacterial death. Bacteria developed two regulatory proteins, MerR and MerD, for mercury(II) reductase.[1] There are two promoter regions on the mer loci: The first region encodes regulator protein MerR, and the second region encodes the structural mer genes and the gene for the regulatory protein MerD. Both promoter regions overlap.[1]
MerR binds to anoperator in the structural mer gene promoter called MerO.[1] This binding causes the DNA of the mer loci to bend to whereRNA polymerase can not read the region. However, Hg2+ can bind to MerR andallosterically change the shape of the DNA, so that RNA polymerase can read the promoter region of the structural genes.[1] Since both promoter regions overlap when apoMerR is bound to MerO, the change in DNA conformation causes neither the structural genes nor the regulatory genes to be read. This makes MerR a negativeautoregulator.[1]
MerR forms a stable trigonal planar complex with Hg2+, which causes it to be released much later than when mercury(II) reductase has reduced all free Hg2+ in the cytoplasm.[1] Thus, it causes an excess in production of mercuric(II) reductase. To circumvent this problem, MerD also binds to MerO in order to actantagonistically to Hg2+ bound MerR.[1] MerD is produced when MerR is active with Hg2+ since MerD is encoded in the structural mer genes.
Inwaste-water treatment procedures, mercury is sometimes removed from the water by making the water flow through abiofilm rich with mercury(II) reductase-containing bacteria.[1]
