Enzyme - Cystathionine gamma-synthase

Alternative Name(s)
  • O-succinylhomoserine synthetase.
  • O-succinylhomoserine (thiol)-lyase.
  • O-succinyl-L-homoserine succinate-lyase (adding cysteine).
  • Homoserine transsuccinylase.
  • Cystathionine synthase.
  • O-succinylhomoserine synthase.
  • Cystathionine synthetase.

Catalytic Activity

L-cysteine + O-succinyl-L-homoserine = H(+) + L,L-cystathionine + succinate


Pyridoxal 5'-phosphate.

Reaction Mechanism

    Cystathionine gamma-synthase (CGS) is a pyrodxial phosphate-dependent enzyme that catalyses a gamma-replacement reaction, in which the succinyl group of an O-succinyl-L-homoserine (L-OSHS) is displaced by the thiol of L-cysteine to form L-cystathionine, in the first step of the bacterial transsulphuration pathway. CGS is of interest as a potential target for antibiotics and herbicides.

    The reaction proceeds via a series of steps, as is thought to follow a ping-pong mechanism, commonly encountered in PLP-dependent enzymes.
    1. Transaldimination: (i) OSHS binds through Arg 48*, Tyr 101 and Arg 361. (ii) The alpha-amino group of the substrate must be deprotonated for nucleophilic attack on C4' of the internal aldimine. Tyr 101 exists as phenolate due to two neighbouring positive charges (Arg 48* and NH of the internal aldimine). Therefore, Tyr 101 abstracts a proton from the incoming substrate and initiates transaldimination.
    2. Generation of the ketimine intermediate: (i) Lys 198 is responsible for proton transfer from the alpha-C to C4' of the PLP cofactor. The protonated amino group of Lys 198 is guided into a favourable position near C4' by Tyr 46*. After alpha-C deprotonation, a quinonoid intermediate is formed, which is stabilised by stacking interactions with Tyr 101. (ii) The Lys 198 e-amino group is positively charged and is therefore attracted to the negatively charged phosphate group of the PLP cofactor, orientating it into a favourable position for bond cleavage. (iii) Due to the new positioning of Lys 198, this residue is able to abstract a proton from the beta-C to initiate gamma-cleavage.
    3. Release of succinate Tyr 101 facilitates the release of succinate by and acid/base mechanism. The resulting beta-gamma unsaturated ketimine exhibits pronounced electron deficiency, caused by the protonated Schiff base, leading to activation of gamma-C towards Michael nucleophilic addition by L-cysteinate.
    4. Transaldimination: the reverse steps of 1-3 occur (beta-C protonation, C4' deprotonation, alpha-C protonation.)
    Catalytic Residues
    AA Uniprot Uniprot Resid PDB PDB Resid
    Tyr P00935 101 1cs1 101
    Arg P00935 48 1cs1 48
    Lys P00935 198 1cs1 198
    Asp P00935 173 1cs1 173
    Step Components

    overall reactant used, proton transfer, unimolecular elimination by the conjugate base, overall product formed, enzyme-substrate complex formation, bimolecular nucleophilic addition, michael addition, native state of cofactor regenerated, intermediate formation, intermediate terminated, native state of enzyme regenerated, bimolecular electrophilic addition, cofactor used, intermediate collapse, enzyme-substrate complex cleavage

    Step 1.

    Tyr101 initiates transaldimination by abstracting a proton from the substrate.

    Step 2.

    The substrate amine nucleophilically attacks the C4' on the internal aldimine.

    Step 3.

    The secondary aminethaat results from the initial attack initiates an elimination of Lys198.

    Step 4.

    Lys198 deprotonates the allylic Cα atom of intermediate which results in the rearrangement of double bonds and PLP acting as an electron sink.

    Step 5.

    PLP feeds the electron back so that the C4' can accept a proton from Lys198.

    Step 6.

    Lys198 then deprotonates the beta carbon.

    Step 7.

    Tyr101 facilitates the release of succinate by protonating the oxygen which results in the cleavage of succinate from the substrate and the rearrangement of double bonds.

    Step 8.

    The gamma carbon now has pronounced electron deficiency and so cysteine can attack it by Michael nucleophilic addition.

    Step 9.

    Beta carbon is protonated by Lys198

    Step 10.

    Lys198 deprotonates the C4' carbon which results in the arrangement of double bonds as PLP acts as an electron sink.

    Step 11.

    PLP feeds the electrons back which results in the protonation of the alpha carbon.

    Step 12.

    The amine of Lys198 attacks the PLP in a nucleophilic addition reaction, the secondary amine of the attached substrate reprotonates from the bound Lys198.

    Step 13.

    The secondary amine that results from the initial attack initiates an elimination of the covalently bound product, resulting in cystathionine and the regenerated PLP cofactor.


    The products of the reaction.

Reaction Parameters

There are no kinetic parameters information for this Enzyme

Associated Proteins

Protein name Organism
Putative cystathionine gamma-synthase YML082W Baker's yeast
Cystathionine gamma-synthase Escherichia coli (strain K12)
Putative cystathionine gamma-synthase YLL058W Baker's yeast
Cystathionine gamma-synthase 1, chloroplastic Mouse-ear cress
Probable cystathionine gamma-synthase 2 Mouse-ear cress