Mercury(II) reductase
| mercury (II) reductase | |||||||||
|---|---|---|---|---|---|---|---|---|---|
ExPASy NiceZyme view | | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDB PDBe PDBsum | ||||||||
| Gene Ontology | AmiGO / QuickGO | ||||||||
| |||||||||
Mercury(II) reductase (
Gene
Mercury(II) reductase, commonly known as MerA, is encoded in a
Function
 |
Free mercury ions can bind to
Mechanism
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
Mercury(II) reductase cannot completely reduce
Structure
The active form of mercury(II) reductase is found as a homodimer.[4] It has a quaternary conformation and the monomer is composed of two domains.[4]
NmerA
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 catalytic active site of MerA.[7] Very few mercuric(II) reductases have been found to lack the NmerA domain.[6]
Active site
The active site of MerA consists of four cysteine residues, a FAD, and a tyrosine 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, a
Mercury transport
Various proteins assist in transporting mercury to mercury(II) reductase. MerP, a
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 its
In the case of organomercury compounds, MerB breaks the Hg-C bonds and transports the Hg to mercuric(II) reductase.[5]
Regulation
When not bound to Hg2+, mercury(II) reductase acts as an oxidase creating toxic hydrogen 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 an
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 act antagonistically to Hg2+ bound MerR.[1] MerD is produced when MerR is active with Hg2+ since MerD is encoded in the structural mer genes.
Applications and uses
In waste-water treatment procedures, mercury is sometimes removed from the water by making the water flow through a biofilm rich with mercury(II) reductase-containing bacteria.[1]
