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Catalytic Site Atlas Version 2.2.12
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CSA entry for 1xvt
Original Entry
Title:
Transferase
Compound:
Crotonobetainyl-coa:carnitine coa-transferase
Mutant:
No
UniProt/Swiss-Prot:
-
P31572-CAIB_ECOLI
EC Class:
2.8.3.-
Other CSA Entries:
Overview of all sites for 1xvt
Homologues of 1xvt
Entries for UniProt/Swiss-Prot: 
Entries for UniProt/Swiss-Prot: P31572
Entries for EC: 2.8.3.-
Other Databases:
PDB entry: 1xvt
PDBsum entry: 1xvt
UniProt/Swiss-Prot: 
UniProt/Swiss-Prot: P31572
IntEnz entry: 2.8.3.-
Literature Report:
Introduction:
The enzymes required for the metabolism of L-carnitine in E. Coli is encoded by the cai operon. CaiB, a class III CoA transferase, catalyses the transfer of the CoA group from crotonobetainyl-CoA to L-carnitine.

Mechanism:
Asp 169 is nucleophilic and attacks the carbonyl carbon of the crotonobetainyl-CoA thioester group. This displaces the coenzyme A thiolate, although it is not certain if this is by an addition-elimination mechanism with tetrahedral intermediate, or by an SN2 mechanism with a tetrahedral transition state (this applies to all the steps in the mechanism).

The carboxylate group of carnitine is nucleophilic and attacks the gamma-carbon of the Asp 169 side chain (now forming an anhydride with the crotonobetaine group). This displaces the crotonobetaine product and forms a new anhydride intermediate.

The CoA thiolate attacks the carbonyl group of the carnitine moiety, displacing and regenerating Asp 169 and forming the carnitine-CoA product.
Sites:

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Found by:
Literature reference 

ResidueChainNumberUniProt numberFunctional part FunctionTargetDescription
ASPA 169 169Sidechain
NucleophileSubstrate
Asp 169 acts as a nucleophilic catalyst. It displaces the CoA thiolate from crotonobetainyl-CoA and remains acylated in the anhydride intermediates.
Evidence from paper Evidence concerns Evidence type
PubMed ID 15823031 Current protein Structural similarity to homologue of known mechanism
PubMed ID 15823031 Current protein Residue is positioned appropriately (ligand position known)
PubMed ID 15823031 Current protein Conservation of residue
Notes:

References:
1
Crystal structure of Escherichia coli crotonobetainyl-CoA: carnitine CoA-transferase (CaiB) and its complexes with CoA and carnitinyl-CoA.
E. S. Rangarajan and Y. Li and P. Iannuzzi and M. Cygler and A. Matte
Biochemistry 44, (15) 5728-38, (2005).
15823031
2
Kinetic and mechanistic characterization of the formyl-CoA transferase from Oxalobacter formigenes.
S. Jonsson and S. Ricagno and Y. Lindqvist and N. G. Richards
J Biol Chem 279, (34) 36003-12, (2004).
15213226
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