Omega-amidase
Template:Infobox enzyme In enzymology, an omega-amidase (Template:EC number) is an enzyme that catalyzes the chemical reaction
- a monoamide of a dicarboxylic acid + H2O a dicarboxylate + NH3
Thus, the two substrates of this enzyme are monoamide of a dicarboxylic acid and H2O, whereas its two products are dicarboxylate and NH3.
This enzyme belongs to the family of hydrolases, those acting on carbon-nitrogen bonds other than peptide bonds, specifically in linear amides. The systematic name of this enzyme class is omega-amidodicarboxylate amidohydrolase. This enzyme is also called alpha-keto acid-omega-amidase. This enzyme participates in glutamate metabolism and alanine and aspartate metabolism. This enzyme can be found in mammals, plants, and bacteria.[1]
Structure and active site
Omega-amidase has two independent monomers that have structure organizations similar to other nitrilase enzymes found in bacteria.[2] Each monomer has a four layered alpha/beta/beta/alpha conformation.[2] The enzyme is asymmetrical and contains a carbon-nitrogen hydrolase fold.[2]
Just as omega-amidase shares a general structure organization as other nitrilases, omega-amidase also contains the same catalytic triad within the active site. This triad of residues includes a nucleophilic cysteine, a glutamate base, and a lysine, all of which are conserved within the structure.[2] In addition to the catalytic triad, omega-amidase also contains a second glutamate that assists in substrate positioning.[3] This second glutamate is why omega-amidase has no activity with glutamine or asparagine, even though they are sized similarly to typical substrates.[4]
Mechanism
Omega amidase catalyzes the deamidation of several different alpha-keto acids into ammonia and metabolically useful carboxylic acids[5] The general mechanism is the same as for other nitrilases: binding of the substrate to the active site, followed by release of ammonia, formation of a thioester intermediate at the cysteine, binding of water and then release of the carboxylic acid product.[3] Specifically, the active site cysteine acts as a nucleophile and binds to the substrate.[6] The catalytic triad glutamate transfers a proton to the amide group to create and release ammonia.[7] The remaining thioester intermediate is stabilized by the lysine and the backbone amino group following the cysteine.[6] This intermediate is attacked by water to form a stable tetrahedral intermediate.[7] This intermediate breaks down to release the carboxylic acid and restore the enzyme.[7]
Biology
Omega-amidase operates in coordination with glutamine transaminase to finish off the methionine salvage cycle in bacteria and plants.[1] In the last step to obtain methionine from α-ketomethylthiobutyrate(KMTB), glutamine transaminase K(GTK) converts glutamine to α-ketoglutaramate(KGM).[1] KGM is the main substrate for omega amidase, but KGM exists mainly in the ring form at physiological conditions.[4] Omega-amidase has a higher affinity for the open linear form of KGM that forms more readily at pH 8.5.[8] GTK catalyzes a reversible reaction, but coupling it with omega-amidase makes the transamination reaction irreversible at physiological conditions.[8]
Due to omega-amidase's ability to convert toxic substrates like KGM into components that can be used by other processes, this enzyme can be considered a repair enzyme.[9] Some such substrates are linked to diseases or conditions such as hyperammonemia.[10] A list of some of the substrates that omega-amidase catalyzes may be found in Table 1.
| Substrate | Product |
|---|---|
| α-Ketoglutaramate | α-Ketoglutarate |
| α-Ketosuccinamate | Oxaloacetate |
| L-2-Hydroxysuccinamate | L-Malate |
| Succinamic Acid | Succinylmonohydroxamic Acid[11] |
| Glutaramic Acid | Glutarylmonohyoxamic Acid[11] |
Medical relevance
The NIT2 gene in humans has been found to be identical to omega-amidase.[9] The gene has the highest expression in the liver and kidney, but is also expressed in almost every human tissue.[5] Overexpression of the NIT2 gene results in decreasing cell proliferation and growth in HeLa cells, which indicates that the gene may have a role in tumor suppression.[9] However further studies are necessary to determine the effect on specific cancers, as a study done with colon cancer cells showed that downregulation of NIT2 induced cell cycle arrest.[12] In addition to tumor suppression, NIT2/omega-amidase may be useful for detection and conversion of oncometabolites.[13] Because omega-amidase is able to control concentration of toxic substrates such as KGM, it is likely that NIT2 can serve the same purpose.[13]
References
Further reading
Template:Carbon-nitrogen non-peptide hydrolases Template:Enzymes Template:Portal bar
- ↑ 1.0 1.1 1.2 Template:Cite journal
- ↑ 2.0 2.1 2.2 2.3 Template:Cite journal
- ↑ 3.0 3.1 3.2 Template:Cite journal
- ↑ 4.0 4.1 Template:Cite journal
- ↑ 5.0 5.1 Template:Cite journal
- ↑ 6.0 6.1 Template:Cite journal
- ↑ 7.0 7.1 7.2 Template:Cite journal
- ↑ 8.0 8.1 Template:Cite journal
- ↑ 9.0 9.1 9.2 Template:Cite journal
- ↑ Template:Cite journal
- ↑ 11.0 11.1 Template:Cite journal
- ↑ Template:Cite journal
- ↑ 13.0 13.1 Template:Cite journal