Enzyme - Threonine synthase

Alternative Name(s)

There are no alternative names for this Enzyme

Catalytic Activity

H2O + O-phospho-L-homoserine = L-threonine + phosphate


Pyridoxal 5'-phosphate.

Reaction Mechanism

    Threonine synthase (TS) from Saccharomyces cerevisiae catalyses the conversion of O-phospho-L-homoserine (OPHS) into threonine and phosphate. This is the final step in threonine biosynthesis. The enzyme requires a pyridoxal 5'-phosphate (PLP) cofactor which binds at the interface of all three of the protein's domains.

    The catalytic cycle starts with a transaldimination reaction. Lys124 which is initially bound to the PLP cofactor is replaced by OPHS forming the external aldimine. Lys124 catalyses the abstraction of the C-alpha proton of the substrate and its transfer to the PLP C4' position. Next, Lys124 stereospecifically abstracts the beta-pro-S hydrogen leading to the non-hydrolytic elimination of the gamma-phosphate. The gamma-methylene group left after the elimination is reprotonated producing the PLP-derivative of E-aminocrotonate. Next, water is added at C-beta. Finally, reverse transaldimination yields L-threonine.
    Catalytic Residues
    AA Uniprot Uniprot Resid PDB PDB Resid
    Lys P16120 124 1kl7 124
    Step Components

    overall reactant used, proton transfer, overall product formed, intramolecular elimination, bimolecular nucleophilic addition, assisted tautomerisation (not keto-enol), native state of cofactor regenerated, intermediate formation, schiff base formed, intermediate terminated, native state of enzyme regenerated, cofactor used, proton relay, intermediate collapse

    Step 1.

    The amino group of the serine performs a nucleophilic attack on the imine carbon of the lys-bound PLP forming a tetrahedral intermediate.

    Step 2.

    Lys124 is cleaved from PLP leaving the substrate bound.

    Step 3.

    Lys124 abstracts a proton from the alpha carbon of the substrate causing tautomerization of the imine.

    Step 4.

    Lys124 abstracts a proton from the beta carbon causing a second tautomerization from imine to enamine.

    Step 5.

    Phosphate is eliminated from the homoserine and another imine is formed.

    Step 6.

    Lys124 acts as a proton relay causing another tautomerization reaction to occur.

    Step 7.

    Water performs a nucleophilic attack on the C=C bond of the substrate, forming a carboanion. The water is activated by Lys124.

    Step 8.

    The carboanion abstracts a proton from Lys124.

    Step 9.

    The amino group of Lys124 performs a nucleophilic attack on the imine carbon of the thr-bound PLP forming a tetrahedral intermediate.

    Step 10.

    Threonine is cleaved from PLP leaving Lys124 bound.


    The products of the reaction.

Reaction Parameters

There are no kinetic parameters information for this Enzyme

Associated Proteins

Protein name Organism
Threonine synthase 2, chloroplastic Mouse-ear cress
Threonine synthase Escherichia coli (strain K12)
Threonine synthase, chloroplastic Potato
Threonine synthase 1, chloroplastic Mouse-ear cress
Probable threonine synthase protein Ralstonia solanacearum (strain GMI1000)