N-acetyltransferase

N-acetyltransferase (NAT) is an enzyme that catalyzes the transfer of acetyl groups from acetyl-CoA to arylamines, arylhydroxylamines and arylhydrazines.[1][2][3] They have wide specificity for aromatic amines, particularly serotonin, and can also catalyze acetyl transfer between arylamines without CoA. N-acetyltransferases are cytosolic enzymes found in the liver and many tissues of most mammalian species, except the dog and fox, which cannot acetylate xenobiotics.[4]

Arylamine N-acetyltransferase 2
A 3d cartoon depiction of human N-acetyltransferase 2
Identifiers
EC number2.3.1.5
Databases
IntEnzIntEnz view
BRENDABRENDA entry
ExPASyNiceZyme view
KEGGKEGG entry
MetaCycmetabolic pathway
PRIAMprofile
PDB structuresRCSB PDB PDBe PDBsum

Acetyl groups are important in the conjugation of metabolites from the liver, to allow excretion of the byproducts (phase II metabolism). This is especially important in the metabolism and excretion of drug products (drug metabolism).

Enzyme Mechanism

NAT enzymes are differentiated by the presence of a conserved catalytic triad that favors aromatic amine and hydrazine substrates.[5][6] NATs catalyze the acetylation of small molecules through a double displacement reaction called the ping pong bi bi reaction.[5] The mechanism consists of two sequential reactions.[5] In reaction one acetyl-CoA initially binds to the enzyme and acetylates Cys68.[5] In reaction two, after acetyl-CoA is released, the acetyl acceptor interacts with the acetylated enzyme to form product.[5] This second reaction is independent of the acetyl donor since it leaves the enzyme before the acetyl acceptor binds.[5] However, like with many ping pong bi bi reactions, its possible there is competition between the acetyl donor and acetyl acceptor for the unacetylated enzyme.[5] This leads to substrate-dependent inhibition at high concentrations.[5]

Depiction of the N-acetyltransfersase enzyme mechanism.[7]

Enzyme Structure
3D depiction of NAT2 active site and catalytic triad.[8]

The two NAT enzymes in humans are NAT1 and NAT2.[4] Mice and rats express three enzymes, NAT1, NAT2, and NAT3.[4] NAT1 and NAT2 have been found to be closely related in species examined thus far, since the two enzymes share 75-95% of their amino acid sequence.[9][10] Both also have an active site cysteine residue (Cys68) in the N-terminal region.[9][10] Further, all functional NAT enzymes contain a triad of catalytically essential residues made up of this cysteine, histidine, and asparagine.[7] It has been hypothesized that the catalytic effects of the breast cancer drug Cisplatin are related to Cys68.[11] The inactivation of NAT1 by Cisplatin is caused by an irreversible formation of a Cisplatin adduct with the active-site cysteine residue.[11] The C-terminus helps bind acetyl CoA and differs among NATs including prokaryotic homologues.[12]

NAT1 and NAT2 have different but overlapping substrate specificities.[4] Human NAT1 preferentially acetylates 4-aminobenzoic acid (PABA), 4 amino salicylic acid, sulfamethoxazole, and sulfanilamide.[4] Human NAT2 preferentially acetylates isoniazid (treatment for tuberculosis), hydralazine, procainamide, dapsone, aminoglutethimide, and sulfamethazine.[4]

Biological Significance

NAT2 is involved in the metabolism of xenobiotics, which can lead to both the inactivation of drugs and formation of toxic metabolites that can be carcinogenic.[13] The biotransformation of xenobiotics may occur in three phases.[13] In phase I, reactive and polar groups are introduced into the substrates. In phase II, conjugation of xenobiotics with charged species occurs, and in phase III additional modifications are made, with efflux mechanisms leading to excretion by transporters.[13] A genome-wide association study (GWAS) identified human NAT2 as the top signal for insulin resistance, a key marker of diabetes and a major cardiovascular risk factor[13] and has been shown to be associated with whole-body insulin resistance in NAT1 knockout mice.[14] NAT1 is thought to have an endogenous role, likely linked to fundamental cellular metabolism.[13] This may be related to why NAT1 is more widely distributed among tissues than NAT2.[13]

Importance in Humans

Each individual metabolizes xenobiotics at different rates, resulting from polymorphisms of the xenobiotic metabolism genes.[13] Both NAT1 and NAT2 are encoded by two highly polymorphic genes located on chromosome 8.[4] NAT2 polymorphisms were one of the first variations to explain this inter-individual variability for drug metabolism.[15] These polymorphisms modify the stability and/ or catalytic activity of enzymes that alter acetylation rates for drugs and xenobiotics, a trait called acetylator phenotype.[16] For NAT2, the acetylator phenotype is described as either slow, intermediate, or rapid.[17] Beyond modifying enzymatic activity, epidemiological studies have found an association of NAT2 polymorphisms with various cancers, likely from varying environmental carcinogens.[13]

Indeed, NAT2 is highly polymorphic in several human populations.[18] Polymorphisms of NAT2 include the single amino acid substitutions R64Q, I114T, D122N, L137F, Q145P, R197Q, and G286E.[18] These are classified as slow acetylators, while the wild-type NAT2 is classified as a fast acetylator.[18] Slow acetylators tend to be associated with drug toxicity and cancer susceptibility.[18] For instance, the NAT2 slow acetylator genotype is associated with an increased risk of bladder cancer, especially among cigarette smokers.[19] Single nucleotide polymorphisms (SNPs) of NAT1 include R64W, V149I, R187Q, M205V, S214A, D251V, E26K, and I263V, and are related to genetic predisposition to cancer, birth defects, and other diseases.[20] The effect of the slow acetylator SNPs in the coding region predominantly act through creating an unstable protein that aggregates intracellularly prior to ubiquitination and degradation.[3]

50% of the British population are deficient in hepatic N-acetyltransferase. This is known as a negative acetylator status. Drugs affected by this are:

  • isoniazid
  • procainamide
  • hydralazine
  • dapsone
  • sulfasalazine

Adverse events from this deficiency include peripheral neuropathy and hepatoxicity.[21] The slowest acetylator haplotype, NAT2*5B (strongest association with bladder cancer), seems to have been selected for in the last 6,500 years in western and central Eurasian people, suggesting slow acetylation gave an evolutionary advantage to this population, despite the recent unfavorable epidemiological health outcomes data.[22]

Examples

The following is a list of human genes that encode N-acetyltransferase enzymes:

Symbol Name
AANAT aralkylamine N-acetyltransferase
ARD1A ARD1 homolog A, N-acetyltransferase (S. cerevisiae)
GNPNAT1 glucosamine-phosphate N-acetyltransferase 1
HGSNAT heparan-alpha-glucosaminide N-acetyltransferase
MAK10 MAK10 homolog, amino-acid N-acetyltransferase subunit (S. cerevisiae)
NAT1 N-acetyltransferase 1 (arylamine N-acetyltransferase)
NAT2 N-acetyltransferase 2 (arylamine N-acetyltransferase)
NAT5 N-acetyltransferase 5 (GCN5-related, putative)
NAT6 N-acetyltransferase 6 (GCN5-related)
NAT8 N-acetyltransferase 8 (GCN5-related, putative)
NAT8L N-acetyltransferase 8-like (GCN5-related, putative)
NAT9 N-acetyltransferase 9 (GCN5-related, putative)
NAT10 N-acetyltransferase 10 (GCN5-related)
NAT11 N-acetyltransferase 11 (GCN5-related, putative)
NAT12 N-acetyltransferase 12 (GCN5-related, putative)
NAT13 N-acetyltransferase 13 (GCN5-related)
NAT14 N-acetyltransferase 14 (GCN5-related, putative)
NAT15 N-acetyltransferase 15 (GCN5-related, putative)
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References
  1. Evans DA (1989). "N-acetyltransferase". Pharmacology & Therapeutics. 42 (2): 157–234. doi:10.1016/0163-7258(89)90036-3. PMID 2664821.
  2. Ma Y, Ghoshdastider U, Wang J, Ye W, Dötsch V, Filipek S, Bernhard F, Wang X (2012). "Cell-free expression of human glucosamine 6-phosphate N-acetyltransferase (HsGNA1) for inhibitor screening". Protein Expr. Purif. 86 (2): 120–6. doi:10.1016/j.pep.2012.09.011. PMID 23036358.
  3. Sim, Edith; Lack, Nathan; Wang, Chan-Ju; et al. (May 2008). "Arylamine N-acetyltransferases: Structural and functional implications of polymorphisms". Toxicology. 254 (3): 170–183. doi:10.1016/j.tox.2008.08.022. PMID 18852012.
  4. Klaassen, Curtis D. (2008). Casarett and Doull's Toxicology: The Basic Science of Poisons 7th Ed. McGraw-Hill. ISBN 978-0071470513.
  5. Minchin, Rodney F.; Neville, Butcher J. (April 2015). "The role of lysine100 in the binding of acetylcoenzyme A to human arylamine N-acetyltransferase 1: Implications for other acetyltransferases" (PDF). Biochemical Pharmacology. 94 (3): 195–202. doi:10.1016/j.bcp.2015.01.015. PMID 25660616.
  6. Weber, W.W.; Cohen, S.N.; Steinberg, M.S. (1968). "Purification and properties of N-acetyltransferase from mammalian liver". Ann N Y Acad Sci. 151: 734–741. doi:10.1111/j.1749-6632.1968.tb11934.x.
  7. Westwood, I.M.; Kawamura, A.; Fullam, E.; et al. (2006). "Structure and Mechanism of Arylamine N-Acetyltransferases". Current Topics in Medicinal Chemistry. 6 (15): 1641–1654. doi:10.2174/156802606778108979.
  8. Sinclair, J.C.; Sandy, J.; Delgoda, R.; Sim, E.; Noble, M.E. (2000). "Structure of arylamine N-acetyltransferase reveals a catalytic triad". Nature Structural Biology. 7 (7): 560–564. doi:10.1038/76783. PMID 10876241.
  9. Grant, D.M.; Blum, M.; Meyer, U.A. (1992). "Polymorphisms of N-acetyltransferase genes". Xenobiotica. 22 (9–10): 1073–1081. doi:10.3109/00498259209051861.
  10. Vatsis, K.P.; Weber, W.W.; Bell, D.A. (1995). "Nomenclature for N-acetyltransferases". Pharmacogenetics. 5 (1): 1–17. doi:10.1097/00008571-199502000-00001. PMID 7773298.
  11. Ragunathan, Nilusha; Dairou, Julien; Pulvinage, Benjamin; et al. (June 2008). "Identification of the Xenobiotic-Metabolizing Enzyme Arylamine N-Acetyltransferase 1 as a New Target of Cisplatin in Breast Cancer Cells: Molecular and Cellular Mechanisms of Inhibition". Molecular Pharmacology. 73 (6): 1761–1768. doi:10.1124/mol.108.045328. PMID 18310302.
  12. Sim, E.; Abuhammad, A.; Ryan, A. (May 2014). "Arylamine N-acetyltransferases: from drug metabolism and pharmacogenetics to drug discovery". Br J Pharmacol. 171 (11): 2705–2725. doi:10.1111/bph.12598. PMC 4158862. PMID 24467436.
  13. Laureri, Nicola; Sim, Edith (2018). Arylamine N-Acetyltransferases in Health and Disease: From Pharmacogenetics to Drug Discovery and Diagnostics. World Scientific. ISBN 9789813232006.
  14. Camporez, João Paulo; Wang, Yongliang; Faarkrog, Kasper; et al. (Dec 2017). "Mechanism by which arylamine N-acetyltransferase 1 ablation causes insulin resistance in mice". PNAS. 114 (52): E11285–E11292. doi:10.1073/pnas.1716990115. PMC 5748223. PMID 29237750.
  15. McDonagh, E.M.; et al. (2014). "PharmGKB summary: very important pharmacogene infor- mation for N-acetyltransferase 2". Pharmacogenet. Genomics. 24: 409–425.
  16. Evans, D.A.; White, T.A. (1964). "Human acetylation polymorphism". J. Lab. Clin. Med. 63: 394–403. PMID 14164493.
  17. Hein, D.W.; Doll, M.A. (2012). "Accuracy of various human NAT2 SNP genotyping panels to infer rapid, intermediate and slow acetylator phenotypes". Pharmacogenomics. 13 (1): 31–41. doi:10.2217/pgs.11.122. PMC 3285565. PMID 22092036.
  18. Rajasekaran, M.; Abirami, Santhanam; Chen, Chinpan (2011). "Effects of Single Nucleotide Polymorphisms on Human N-Acetyltransferase 2 Structure and Dynamics by Molecular Dynamics Simulation". PLoS ONE. 6 (9): 1–12. doi:10.1371/journal.pone.0025801.
  19. Hein, D.W. (2000). "Molecular genetics and epidemiology of the NAT1 and NAT2 acetylation polymorphisms". Cancer Epidemiol. Biomarkers Prev. 9 (1): 29–42. PMID 10667461.
  20. Walraven, Jason M.; Trent, John O.; Hein, David W. (2008). "Structure-Function Analysis of Single Nucleotide Polymorphisms in Human N-Acetyltransferase 1". Drug Metabolism Reviews. 40 (1): 169–184. doi:10.1080/03602530701852917. PMC 2265210. PMID 18259988 – via Informa Healthcare.
  21. Unissa, Ameeruddin Nusrath; Subbian, Selvakumar; Hanna, Luke Elizabeth; Selvakumar, Nagamiah (2016). "Overview on mechanisms of isoniazid action and resistance in Mycobacterium tuberculosis". Infection, Genetics and Evolution. 45: 474–492. doi:10.1016/j.meegid.2016.09.004. PMID 27612406.
  22. Patin, E.; Barreiro, L.B.; Sabeti, P.C.; et al. (2006). "Deciphering the ancient and complex evolutionary history of human arylamine N-acetyltransferase genes". Am J Hum Genet. 78 (3): 423–436. doi:10.1086/500614. PMC 1380286. PMID 16416399.
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