Cofactors

Pyridoxal 5'-phosphate.

Reaction Mechanism

cystathionine gamma-synthase By Reference

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.




• Summary
• Step 1
• Step 2
• Step 3
• Step 4
• Step 5
• Step 6
• Step 7
• Step 8
• Step 9
• Step 10
• Step 11
• Step 12
• Step 13
• Products

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.



Products.

The products of the reaction.

View Reaction Mechanisms in M-CSA

Reaction Parameters

There are no kinetic parameters information for this Enzyme

Associated Proteins

Citations

• Discovery and characterization of small molecule inhibitors of cystathionine gamma-synthase with in planta activity.
• CYSTATHIONINE GAMMA-SYNTHASE activity in rice is developmentally regulated and strongly correlated with sulfate.
• Metabolic control analysis of the transsulfuration pathway and the compensatory role of the cysteine transport in Trypanosoma cruzi.
• Naphthyl-Substituted Indole and Pyrrole Carboxylic Acids as Effective Antibiotic Potentiators-Inhibitors of Bacterial Cystathionine γ-Lyase.
• A metal-organic framework-derived ruthenium-nitrogen-carbon nanozyme for versatile hydrogen sulfide and cystathionine γ-lyase activity assay.
• The role of the circadian clock system in mitochondrial trans-sulfuration pathway and tissue remodeling.
• Inhibition of cystathionine-gamma lyase dampens vasoconstriction in mouse and human intracerebral arterioles.
• Cystathionine gamma lyase is regulated by flow and controls smooth muscle migration in human saphenous vein
• Hydrogen sulfide promotes migration of trophoblast cells by a Rho GTPase mediated actin cytoskeleton reorganization.
• Sulfur-Element containing metabolic pathways in human health and crosstalk with the microbiome.
• Cystathionine gamma-synthase is essential for methionine biosynthesis in Fusarium graminearum.