Choline O-acetyltransferase

 

Choline acetyltransferase synthesises the neurotransmitter acetylcholine from choline in neurones and other cell types. It catalyses the reversible transfer of an acetyl group between acetyl CoA and choline, and belongs to the choline/carnitine acyltranserase family which also includes enzymes involved in fatty acid metabolism.

 

Reference Protein and Structure

Sequence
P32738 UniProt (2.3.1.6) IPR000542 (Sequence Homologues) (PDB Homologues)
Biological species
Rattus norvegicus (Norway rat) Uniprot
PDB
1q6x - Crystal structure of rat choline acetyltransferase (2.5 Å) PDBe PDBsum 1q6x
Catalytic CATH Domains
3.30.559.40 CATHdb 3.30.559.70 CATHdb (see all for 1q6x)
Click To Show Structure

Enzyme Reaction (EC:2.3.1.6)

acetyl-CoA(4-)
CHEBI:57288ChEBI
+
choline
CHEBI:15354ChEBI
→
acetylcholine
CHEBI:15355ChEBI
+
coenzyme A(4-)
CHEBI:57287ChEBI
Alternative enzyme names: Choline acetylase, Choline acetyltransferase, CHOACTase,

Enzyme Mechanism

Introduction

His 334 acts as a general base to remove the proton from the choline OH group as the oxygen attacks the carbonyl of acetyl CoA. The resulting tetrahedral oxyanion intermediate is stabilised by Ser 550. Collapse of the tetrahedral intermediate releases CoA which is protonated by the His 334. Tyr 95 and Pro 108 function to stabilise the unprotonated form of His 334 N-epsilon so that it can act as a general base in the first step of the reaction.

Catalytic Residues Roles

UniProt PDB* (1q6x)
Tyr95, Pro108 Tyr95A, Pro108A Steric packing with His 334 forces the His 334 to adopt a strained conformation with an intraresidue hydrogen bond between N-delta and its carbonyl oxygen. This is proposed to position N-epsilon and stabilise its non-protonated, nucleophilic state.
His334 His334A Acts as a general base, extracting a proton from the attacking hydroxyl group of choline. Later protonates the departing sulphydryl group of CoA.
Ser550 Ser550A Hydrogen bonding from side chain OH stabilises the tetrahedral intermediate/transition state.
*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

References

  1. Jogl G et al. (2003), Cell, 112, 113-122. Crystal Structure of Carnitine Acetyltransferase and Implications for the Catalytic Mechanism and Fatty Acid Transport. DOI:10.1016/s0092-8674(02)01228-x. PMID:12526798.
  2. Kim AR et al. (2006), Biochemistry, 45, 14621-14631. Substrate binding and catalytic mechanism of human choline acetyltransferase. DOI:10.1021/bi061536l. PMID:17144655.
  3. Cai Y et al. (2004), EMBO J, 23, 2047-2058. Choline acetyltransferase structure reveals distribution of mutations that cause motor disorders. DOI:10.1038/sj.emboj.7600221. PMID:15131697.
  4. Wu D et al. (2003), J Biol Chem, 278, 13159-13165. Structure of Human Carnitine Acetyltransferase: MOLECULAR BASIS FOR FATTY ACYL TRANSFER. DOI:10.1074/jbc.m212356200. PMID:12562770.

Contributors

Steven Smith, Gemma L. Holliday