| AralkylamineN-acetyltransferase | |||||||||
|---|---|---|---|---|---|---|---|---|---|
 Crystallographic structure of aralkylamine N-acetyltransferase.[1] | |||||||||
| Identifiers | |||||||||
| EC no. | 2.3.1.87 | ||||||||
| CAS no. | 92941-56-5 | ||||||||
| 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 | ||||||||
| |||||||||
| AralkylamineN-acetyltransferase | |||||||
|---|---|---|---|---|---|---|---|
| Identifiers | |||||||
| Symbol | AANAT | ||||||
| NCBI gene | 15 | ||||||
| HGNC | 19 | ||||||
| OMIM | 600950 | ||||||
| RefSeq | NM_001088 | ||||||
| UniProt | Q16613 | ||||||
| Other data | |||||||
| EC number | 2.3.1.87 | ||||||
| Locus | Chr. 17q25 | ||||||
| |||||||
AralkylamineN-acetyltransferase (AANAT) (EC2.3.1.87), also known asarylalkylamineN-acetyltransferase orserotoninN-acetyltransferase (SNAT), is anenzyme that is involved in theday/night rhythmic production ofmelatonin, by modification ofserotonin. It is in humans encoded by the ~2.5 kbAANAT gene[2] containing fourexons, located onchromosome 17q25.[3] The gene istranslated into a 23 kDa large enzyme. It is well conserved through evolution and the human form of the protein is 80 percent identical to sheep and rat AANAT. It is anacetyl-CoA-dependent enzyme of the GCN5-related family ofN-acetyltransferases (GNATs). It may contribute to multifactorialgenetic diseases such asaltered behavior in sleep/wake cycle[2] and research is on-going with the aim of developing drugs that regulate AANAT function.
Thesystematic name of this enzyme class is acetyl-CoA:2-arylethylamine N-acetyltransferase. Other names in common use include:
The officially accepted name is aralkylamine N-acetyltransferase.[4]
The AANAT mRNA transcript is mainly expressed in thecentral nervous system (CNS). It is detectable at low levels in severalbrain regions including thepituitary gland as well as in theretina. It is most highly abundant in thepineal gland which is the site of melatonin synthesis. Brain and pituitary AANAT may be involved in the modulation of serotonin-dependent aspects of human behavior and pituitary function.[3]
In thepinealocyte cells of thepineal gland, aralkylamine N-acetyltransferase is involved in the conversion ofserotonin tomelatonin. It is the penultimate enzyme in the melatonin synthesis controlling the night/day rhythm in melatonin production in the vertebratepineal gland. Melatonin is essential for seasonal reproduction, modulates the function of thecircadian clock in thesuprachiasmatic nucleus, and influences activity and sleep. Due to its important role in circadian rhythm, AANAT is subjected to extensive regulation that is responsive to light exposure (see Regulation). It may contribute to multifactorial genetic diseases such as altered behavior in sleep/wake cycle and mood disorders.[2]
The primarychemical reaction that iscatalyzed by aralkylamine N-acetyltransferase uses twosubstrates,acetyl-CoA and serotonin. AANAT catalyzes the transfer of the acetyl group of Acetyl-CoA to the primaryamine of serotonin, thereby producingCoA andN-acetylserotonin. In humans, other endogenous substrates of the enzyme include specifictrace amine neuromodulators, namelyphenethylamine,tyramine, andtryptamine, in turn formingN-acetylphenethylamine,N-acetyltyramine, andN-acetyltryptamine.[5]
In thebiosynthesis ofmelatonin,N-acetylserotonin is furthermethylated by another enzyme,N-acetylserotonin O-methyltransferase (ASMT) to generate melatonin. The N-acetyltransferase reaction has been suggested to be therate-determining step, and thus Serotonin N-acetyltransferase has emerged as a target for inhibitor design (see below).[6]
AANAT obeys an orderedternary-complex mechanism. The substrates bind sequentially (ordered) with acetyl-CoA binding to the free enzyme followed by the binding of serotonin to form the ternary complex. After the transfer of the acetyl group has occurred, the products are orderly released with N-acetyl-serotonin first and CoA last.[7]
Arylkylamine N-acetyltransferase is amonomericpolypeptide with a length of 207amino acid residues, and with a molecular weight of 23,344 daltons. Thesecondary structure consists ofalpha helices andbeta sheets. It is 28 percent helical (10 helices; 60 residues) and 23 percent beta sheet (9 strands; 48 residues). This family shares four conservedsequence motifs designated A-D. Motif B serves as the location of the serotonin binding slot. The structure was determined byX-ray diffraction.[1]
Severalstructures have been solved for this class of enzymes, withPDB accession codes1CJW,[8]1B6B,[9]1L0C,[1][10] and1KUV/1KUX/1KUY.[1]
Aralkylamine N-acetyltransferase has also been crystallized in complex with 14-3-3ζ from the14-3-3 protein family, with thePDB accession code1IB1.[11]
Aralkylamine N-acetyltransferase belongs to the GCN5-related N-acetyltransferase (GNAT) superfamily which consists 10,000 acetyltransferases, named so because of their sequence homology to a class of eukaryotictranscription factors, therein the yeast GCN5. Other well-studied members of the superfamily areglucosamine-6-phosphate N-acetyltransferase andhistone acetyltransferases.
All members of this superfamily has a structurally conserved fold consisting of an N-terminal strand followed by two helices, three antiparallel β-strands, followed by a ‘‘signature’’ central helix, a fifth β-strand, a fourth α-helix and a final β-strand. These elements are nearly universally conserved in spite of poor pairwise identity in sequence alignments.[12]
Regulation of AANAT varies between species. In some, AANAT levels oscillate dramatically between light and dark periods, and thus control melatonin synthesis. In others, rhythm is regulated primarily on the protein level.[13] One example is in rodents, where AANAT mRNA levels increase more than 100-fold in dark periods. In other species,cyclic AMP plays an important part in inhibition ofproteolytic degradation of AANAT, elevating protein levels at night. Experiments using human AANAT expressed in a 1E7 cell line show an ~8-fold increase in enzyme activity upon exposure toforskolin.[14]
Dynamic degradation of AANAT mRNA has proven essential to the circadian action of the enzyme. The 3’UTR sequences have importance with regards to the rhythmic degradation of AANAT mRNA in some species. In rodents, varioushnRNPs maintain dynamic degradation of AANAT mRNA. In other species, such as ungulates and primates, the stable AANAT mRNAs with a shorter 3’UTR is suspected not to be under control of the hnRNPs that bind and direct degradation of AANAT mRNA in rodents.[15]
Exposure to light induces signals to travel from retinal cells, ultimately causing a drop innorepinephrine stimulation of the pineal gland. This, in turn, leads to a signaling cascade, resulting inProtein Kinase A phosphorylation of two key Ser and Thr residues of serotonin N-acetyltransferase.Phosphorylation of these residues causes changes in catalytic activity through recruitment and interaction with14-3-3 proteins, specifically 14-3-3ζ.[16]
Another protein which interacts and regulates AANAT activity isprotein kinase C. Protein kinase C acts, likeprotein kinase A, on threonine and serine residues, enhancing the stability and enzymatic activity of AANAT.[17]
Inhibition of theacetyl-CoA-binding to the catalytic site through the formation and cleavage of intramolecular disulfide bonds has been suggested to be a mechanism of regulation. Formation of a disulfide bond between two cystein residues within the protein closes the hydrophobic funnel of the catalytic site, and thus acts as an on/off switch for catalytic activity. It is not yet certain if this mechanism is present inin vivo cells through the regulation of intracellular redox conditions, but it is suggested thatglutathione (GSH) could be anin vivo regulator of the formation and cleavage of these disulfide bonds.[18]
Inhibitors of AANAT may eventually lead to development of a drug that would be useful incircadian biology research and in the treatment of sleep andmood disorders. Synthetic inhibitors of the enzyme have been discovered.[19][20][21] However, no AANAT inhibitor with potentin vivo activity has been reported.[22] Up to now, five classes of AANAT inhibitors have been described in the literature.[6] Below are the five classes:
Since it was reported that melatonin is acompetitive inhibitor of AANAT, thisneurotransmitter seems to exert anautoregulatory control on its own biosynthesis. Thus, loose structural analogues of theindolaminehormone were evaluated on AANAT, and moderate inhibitors were discovered.[23]
Peptide combinatorial libraries of tri-, tetra-, and pentapeptides with various amino acid compositions were screened as potential sources of inhibitors, to see if it serves as either pure or mixed competitive inhibitor for the hAANAT enzyme.Molecular modeling andstructure-activity relationship studies made it possible to pinpoint the amino acid residue of the pentapeptide inhibitor S 34461 that interacts with the cosubstrate-binding site.[24]
It is suggested that AANAT catalyzes the transfer of an acetyl group from acetyl-CoA to serotonin, with the involvement of anintermediateternary complex, to produce N-acetylserotonin. Based on this mechanism, it might be expected that a bisubstrate analog inhibitor, derived from the tethering of indole and CoASH parts, could potentially mimic the ternary complex and exert strong inhibition of AANAT.[25] The first bisubstrate analog (1), which linkstryptamine and CoA via an acetyl bridge, was synthesized by Khalil and Cole, and shown to be a very potent and specific AANAT inhibitor.[26]
AANAT has shown that it also has a secondary alkyltransferase activity as well as acetyltransferase activity.[27] N-Haloacetyltryptamines were developed and serve as substrates of AANAT alkyltransferase and are also potent (low micromolar)in vitro inhibitors against AANAT acetyltransferase activity. AANAT catalyzes reaction between N-bromoacetyltryptamine (BAT) and reduced CoA, resulting a tight-binding bisubstrate analog inhibitor.[27][28] The first synthesized cell-permeable inhibitor of AANAT N-bromoacetyltryptamine was studied further on melatonin secretion from rat and pig pineal glands.[29] New N-halogenoacetyl derivatives leading to a strongin situ inhibition of AANAT. The concept behind the mechanism of action of these precursors was studied by following the biosynthesis of the inhibitor from tritiated-BAT in a living cell.[20]
The first druglike and selective inhibitors of AANAT has been identified. Lawrence M. Szewczuket al. have virtually screened more than a million compounds by 3Dhigh-throughput docking into the active site ofX-ray structure for AANAT, and then tested 241 compounds as inhibitors. One compound class which containing arhodanine scaffold has shown low micromolar competitive inhibition against acetyl-CoA and proved to be effective in blocking melatonin production in pineal cells.[19]
The recent study about inhibitor of AANAT has described the discovery of a new class of nonpeptidic AANAT inhibitors based on a 2,2′-bithienyl scaffold.[22]
This article incorporates text from theUnited States National Library of Medicine, which is in thepublic domain.
