Branched-chain-amino-acid transaminase

 

Branched-chain amino acid aminotransferase is involved in the biosynthesis of hydrophobic amino acids ie leucine, isoleucine, valine. The enzyme reversibly catalyses the transfer of the amino group of a hydrophobic amino acide to 2-oxoglutarate to form a 2-oxo acid and a glutamate. This enzyme is a pyridoxal 5'-phosphate-dependent enzyme.

 

Reference Protein and Structure

Sequence
P0AB80 UniProt (2.6.1.42) IPR005785 (Sequence Homologues) (PDB Homologues)
Biological species
Escherichia coli K-12 (Bacteria) Uniprot
PDB
1iyd - CRYSTAL STRUCTURE OF ESCHELICHIA COLI BRANCHED-CHAIN AMINO ACID AMINOTRANSFERASE (2.15 Å) PDBe PDBsum 1iyd
Catalytic CATH Domains
3.20.10.10 CATHdb (see all for 1iyd)
Cofactors
Pyridoxal 5'-phosphate(2-) (1)
Click To Show Structure

Enzyme Reaction (EC:2.6.1.42)

L-leucine zwitterion
CHEBI:57427ChEBI
+
2-oxoglutarate(2-)
CHEBI:16810ChEBI
→
4-methyl-2-oxopentanoate
CHEBI:17865ChEBI
+
L-glutamate(1-)
CHEBI:29985ChEBI
Alternative enzyme names: L-branched chain amino acid aminotransferase, Branched-chain amino acid aminotransferase, Branched-chain amino acid--glutamate transaminase, Branched-chain aminotransferase, Glutamate--branched-chain amino acid transaminase, Transaminase B,

Enzyme Mechanism

Introduction

The mechanism of branched-chain amino acid aminotransferase has a double substrate recognition component for the reversibility of this enzyme. The substrate approaches, and the C4' of PLP, activated by protonation of the Schiff base, is nucleophilically attacked by the substrate alpha-amino group and through the tetrahedral intermediate a new Schiff base is formed, with release of Lys 159 by protonation with the substrate. Lys 159 makes a hydrogen bond to the O3' of PLP to lower the free energy of the transition states. This Lys also facilitates proton transfer to the C-alpha atom of the substrate and the C4' of the cofactor. The deprotonated Lys 159 abstracts a proton from the substrate to yield the quinonoid intermediate. After this alpha-proton elimination, Lys 159 adds a proton to the C4' atom of the quinonoid intermediate to yield the ketimine intermediate. The ketimine intermediate is hydrolysed using Lys 159 to activate the water as a general base catalyst to yield 2-oxoglutarate and PMP.

Catalytic Residues Roles

UniProt PDB* (1iyd)
Lys160 Lys159(160)A Involved in proton transfer and general acid/base catalysis. proton shuttle (general acid/base)
*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. Goto M et al. (2003), Biochemistry, 42, 3725-3733. Crystal Structures of Branched-Chain Amino Acid Aminotransferase Complexed with Glutamate and Glutarate:  True Reaction Intermediate and Double Substrate Recognition of the Enzyme†,‡. DOI:10.1021/bi026722f. PMID:12667063.

Step 1.

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Catalytic Residues Roles

Residue Roles
Lys159(160)A proton shuttle (general acid/base)

Chemical Components

Step 1.

Contributors

Gary McDowell, Gemma L. Holliday