Aralkylamine N-acetyltransferase

 

Aralkylamine N-acetyltransferase (AANAT) catalyses the conversion of serotonin to N-acetylserotonin, the precursor to the circadian neurohormone melatonin, important in coordinating daily and seasonal rhythms with light levels in the surrounding environment. Levels of AANAT (and levels of melatonin) are controlled by the proteolysis of AANAT.

 

Reference Protein and Structure

Sequence
Q29495 UniProt (2.3.1.87) IPR016181 (Sequence Homologues) (PDB Homologues)
Biological species
Ovis aries (sheep) Uniprot
PDB
1b6b - MELATONIN BIOSYNTHESIS: THE STRUCTURE OF SEROTONIN N-ACETYLTRANSFERASE AT 2.5 A RESOLUTION SUGGESTS A CATALYTIC MECHANISM (2.5 Å) PDBe PDBsum 1b6b
Catalytic CATH Domains
3.40.630.30 CATHdb (see all for 1b6b)
Click To Show Structure

Enzyme Reaction (EC:2.3.1.87)

serotonin(1+)
CHEBI:350546ChEBI
+
acetyl-CoA(4-)
CHEBI:57288ChEBI
→
coenzyme A(4-)
CHEBI:57287ChEBI
+
N-acetylserotonin
CHEBI:17697ChEBI
+
hydron
CHEBI:15378ChEBI
Alternative enzyme names: AANAT, Arylalkylamine N-acetyltransferase, Serotonin acetylase, Serotonin acetyltransferase, Serotonin N-acetyltransferase, Melatonin rhythm enzyme,

Enzyme Mechanism

Introduction

His122 initiates the reaction by deprotonating the indol substrate, which adds to the carbonyl carbon of acyl-CoA. The CoA is eliminated and reprotonated from Tyr168, which returns to its native state by deprotonating the product secondary amine.

It remains uncertain whether Tyr168 or His120 acts as the proton donor to the coenzyme-A anion, although crystallographic data would suggest His120 to be situated too far from the acyl-CoA binding site to be directly involved. The structural element Loop1 appears to undergo significant changes in organisation during substrate binding. Pro64, which is positioned within this flexible region, has been shown to be crucial for efficient substrate recognition.

Catalytic Residues Roles

UniProt PDB* (1b6b)
Tyr168, His122 Tyr168(141)A, His122(95)A Acts as a general acid/base. hydrogen bond acceptor, hydrogen bond donor, proton acceptor, proton donor, electrostatic stabiliser
Ser97, Leu111 (main-C), Leu124 (main-N) Ser97(70)A, Leu111(84)A (main-C), Leu124(97)A (main-N) Activate the catalytic general acid/base residues. activator, hydrogen bond acceptor, hydrogen bond donor, electrostatic stabiliser
*PDB label guide - RESx(y)B(C) - RES: Residue Name; x: Residue ID in PDB file; y: Residue ID in PDB sequence if different from PDB file; B: PDB Chain; C: Biological Assembly Chain if different from PDB. If label is "Not Found" it means this residue is not found in the reference PDB.

Chemical Components

proton transfer, bimolecular nucleophilic addition, overall reactant used, intermediate formation, unimolecular elimination by the conjugate base, overall product formed, intermediate collapse, intermediate terminated, native state of enzyme regenerated, inferred reaction step

References

  1. Hickman AB et al. (1999), Mol Cell, 3, 23-32. Melatonin biosynthesis: the structure of serotonin N-acetyltransferase at 2.5 A resolution suggests a catalytic mechanism. DOI:10.2210/pdb1b6b/pdb. PMID:10024876.
  2. Klein DC (2007), J Biol Chem, 282, 4233-4237. Arylalkylamine N-acetyltransferase: "the Timezyme". DOI:10.1074/jbc.R600036200. PMID:17164235.
  3. Scheibner KA et al. (2002), J Biol Chem, 277, 18118-18126. Investigation of the Roles of Catalytic Residues in SerotoninN-Acetyltransferase. DOI:10.1074/jbc.m200595200. PMID:11884405.
  4. Zheng W et al. (2001), Chem Biol, 8, 379-389. Mechanistic studies on the alkyltransferase activity of serotonin N-acetyltransferase. DOI:10.1016/s1074-5521(01)00020-5.

Step 1. His122 deprotonates the amine of 3-(2-aminoethyl)-indol-5-ol, which initiates a nucleophilic attack on the carbonyl carbon of acyl-CoA in an addition reaction

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
Leu111(84)A (main-C) hydrogen bond acceptor, steric role
Leu124(97)A (main-N) hydrogen bond donor
Ser97(70)A hydrogen bond donor, hydrogen bond acceptor, activator
His122(95)A hydrogen bond acceptor, hydrogen bond donor
Tyr168(141)A hydrogen bond donor
His122(95)A proton acceptor

Chemical Components

proton transfer, ingold: bimolecular nucleophilic addition, overall reactant used, intermediate formation, proton transfer

Step 2. The oxyanion collapses, eliminating CoA, which deprotonated Tyr168

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
Tyr168(141)A hydrogen bond donor, electrostatic stabiliser
Leu111(84)A (main-C) hydrogen bond acceptor, steric role
Ser97(70)A hydrogen bond donor, hydrogen bond acceptor, electrostatic stabiliser
His122(95)A hydrogen bond donor
Leu124(97)A (main-N) hydrogen bond donor, electrostatic stabiliser
Tyr168(141)A proton donor

Chemical Components

ingold: unimolecular elimination by the conjugate base, overall product formed, intermediate collapse, intermediate formation, proton transfer

Step 3. Tyr168 deprotonates the secondary amine of the product.

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
Tyr168(141)A hydrogen bond acceptor
Leu111(84)A (main-C) hydrogen bond acceptor, steric role
Ser97(70)A hydrogen bond donor, hydrogen bond acceptor, electrostatic stabiliser
His122(95)A hydrogen bond donor
Leu124(97)A (main-N) hydrogen bond donor
Tyr168(141)A proton acceptor

Chemical Components

overall product formed, intermediate terminated, proton transfer

Step 4. Water deprotonates His122 in an inferred return step.

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
His122(95)A hydrogen bond donor
Ser97(70)A hydrogen bond acceptor, hydrogen bond donor
Leu111(84)A (main-C) hydrogen bond acceptor, steric role
His122(95)A proton donor

Chemical Components

proton transfer, native state of enzyme regenerated, inferred reaction step
Download: Image, Marvin File

Step 1. His122 deprotonates the amine of 3-(2-aminoethyl)-indol-5-ol, which initiates a nucleophilic attack on the carbonyl carbon of acyl-CoA in an addition reaction

Step 2. The oxyanion collapses, eliminating CoA, which deprotonated Tyr168

Step 3. Tyr168 deprotonates the secondary amine of the product.

Step 4. Water deprotonates His122 in an inferred return step.

Products.

Contributors

Gemma L. Holliday, Daniel E. Almonacid, Gail J. Bartlett, Sophie T. Williams, Alex Gutteridge, Craig Porter, Katherine Ferris, Morwenna Hall