Threonine ammonia-lyase (biosynthetic)

 

Catalyses the anaerobic formation of alpha-ketobutyrate and ammonia from threonine in a two-step reaction. The first step involved a dehydration of threonine and a production of enamine intermediates (aminocrotonate), which tautomerises to its imine form (iminobutyrate). Both intermediates are unstable and short-lived. The second step is the non-enzymatic hydrolysis of the enamine/imine intermediates to form 2-ketobutyrate and free ammonia.

 

Reference Protein and Structure

Sequence
P04968 UniProt (4.3.1.19) IPR005787 (Sequence Homologues) (PDB Homologues)
Biological species
Escherichia coli K-12 (Bacteria) Uniprot
PDB
1tdj - THREONINE DEAMINASE (BIOSYNTHETIC) FROM E. COLI (2.8 Å) PDBe PDBsum 1tdj
Catalytic CATH Domains
3.40.50.1100 CATHdb (see all for 1tdj)
Cofactors
Pyridoxal 5'-phosphate(2-) (1)
Click To Show Structure

Enzyme Reaction (EC:4.3.1.19)

L-threonine zwitterion
CHEBI:57926ChEBI
→
ammonium
CHEBI:28938ChEBI
+
2-oxobutanoate
CHEBI:16763ChEBI
Alternative enzyme names: L-serine dehydratase, L-threonine deaminase, L-threonine dehydratase, L-threonine hydro-lyase (deaminating), Serine deaminase, Threonine deaminase, Threonine dehydrase, Threonine dehydratase, L-threonine ammonia-lyase,

Enzyme Mechanism

Introduction

Although there is no mechanism currently in the literature, the enzyme is supposed to function in a similar way to other PLP-dependent enzymes. Thus: (1) formation of external aldimine (with amine group of Threonine substrate), (2) isomerisation, (3) elimination of hydroxyl group, forming double bonded carbon atoms, leading to aminoacrylate intermediate, (4) formation of internal aldimine, leading to the elimination of imine intermediate from PLP, and finally (5) non-enzymatic formation of ammonia and oxobutanoate.

Catalytic Residues Roles

UniProt PDB* (1tdj)
Lys62 Lys62A In the ground state of the enzyme, this residue is covalently attached to the PLP-cofactor. During the course of the reaction the residue is replaced by the substrate and then acts as a general acid/base. covalent catalysis, proton shuttle (general acid/base)
Ser315 Ser315A The pyridinium nitrogen is hydrogen bonded to the side chain of Ser315. This residue therefore acts to stabilise the reactive intermediates and facilitates the PLP cofactor to act as an electron sink during the course of the reaction. 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

References

  1. Gallagher DT et al. (1998), Structure, 6, 465-475. Structure and control of pyridoxal phosphate dependent allosteric threonine deaminase. PMID:9562556.

Step 1.

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Catalytic Residues Roles

Residue Roles
Lys62A covalent catalysis
Lys62A proton shuttle (general acid/base)
Ser315A electrostatic stabiliser

Chemical Components

Step 1.

Contributors

Christian Drew, Craig Porter, Gemma L. Holliday