Cysteine synthase

 

O-Acetylserine sulfhydrylase (cystenie synthase, OASS) is a pyridoxal 5′-phosphate (PLP)-dependent enzyme that catalyses the conversion of O-acetylserine and bisulfide to L-cysteine and acetate in bacteria and higher plants. Enteric bacteria have two isozymes of OASS, A and B, produced under aerobic and anaerobic growth conditions, respectively, with different substrate specificity. This entry represents CysK (the A isoenzyme), which can use sulphide instead of thiosulphate, to produce cysteine instead of cysteine thiosulphonate.

 

Reference Protein and Structure

Sequence
P0A1E3 UniProt (2.5.1.47) IPR005859 (Sequence Homologues) (PDB Homologues)
Biological species
Salmonella enterica subsp. enterica serovar Typhimurium str. LT2 (Bacteria) Uniprot
PDB
1oas - O-ACETYLSERINE SULFHYDRYLASE FROM SALMONELLA TYPHIMURIUM (2.2 Å) PDBe PDBsum 1oas
Catalytic CATH Domains
3.40.50.1100 CATHdb (see all for 1oas)
Cofactors
Pyridoxal 5'-phosphate(2-) (1)
Click To Show Structure

Enzyme Reaction (EC:2.5.1.47)

O-acetyl-L-serine zwitterion
CHEBI:58340ChEBI
+
hydrosulfide
CHEBI:29919ChEBI
→
L-cysteine zwitterion
CHEBI:35235ChEBI
+
acetate
CHEBI:30089ChEBI
Alternative enzyme names: O-acetyl-L-serine sulfhydrylase, O-acetyl-L-serine sulfohydrolase, O-acetylserine (thiol)-lyase, O-acetylserine (thiol)-lyase A, O-acetylserine sulfhydrylase, O(3)-acetyl-L-serine acetate-lyase (adding hydrogen-sulfide), Acetylserine sulfhydrylase, Cysteine synthetase, OAS sulfhydrylase, 3-O-acetyl-L-serine:hydrogen-sulfide 2-amino-2-carboxyethyltransferase, O(3)-acetyl-L-serine:hydrogen-sulfide 2-amino-2-carboxyethyltransferase,

Enzyme Mechanism

Introduction

O-Acetylserine sulfhydrylase (OASS)1 catalyses the PLP-dependent replacement of the beta-acetoxy of OAS by inorganic bisulfide to generate l-cysteine. Steady-state kinetic studies indicate that the kinetic mechanism of OASS-A is ping-pong or double displacement. The first half of the reaction results in the conversion of the internal Schiff base and OAS to acetate and an external Schiff base with alpha-aminoacrylate. The second half of the reaction involves the reaction of the alpha-aminoacrylate intermediate with inorganic sulfide to regenerate the internal Schiff base and give the second product, l-cysteine. It has been suggested that the central alpha,beta-elimination proceeds via an anti-E2 mechanism

Catalytic Residues Roles

UniProt PDB* (1oas)
Lys42 Lys41A The PLP cofactor in the active site is in Schiff base linkage with the eta-amino group of K41, covalent catalysis, proton shuttle (general acid/base)
Ser273 Ser272A Forms a hydrogen bond to N1 of PLP, stabilising the reactive PLP intermediates. 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. Tai CH et al. (2001), Acc Chem Res, 34, 49-59. Pyridoxal 5‘-Phosphate-Dependent α,β-Elimination Reactions:  Mechanism ofO-Acetylserine Sulfhydrylase. DOI:10.1021/ar990169l. PMID:11170356.
  2. Tian H et al. (2010), Biochemistry, 49, 6093-6103. Identification of the structural determinants for the stability of substrate and aminoacrylate external Schiff bases in O-acetylserine sulfhydrylase-A. DOI:10.1021/bi100473v. PMID:20550197.
  3. Ozaki S et al. (2008), 3, 351-358. Mutagenesis of Arginine-186 Located on a Helix Interacting with Pyridoxal 5`-Phosphate in O-Acetylserine Sulfhydrylase. DOI:10.3923/ajb.2008.351.358.
  4. Chattopadhyay A et al. (2007), Biochemistry, 46, 8315-8330. Structure, mechanism, and conformational dynamics of O-acetylserine sulfhydrylase from Salmonella typhimurium: comparison of A and B isozymes. DOI:10.1021/bi602603c. PMID:17583914.
  5. Rabeh WM et al. (2005), Biochemistry, 44, 5541-5550. Mechanism of the addition half of the O-acetylserine sulfhydrylase-A reaction. DOI:10.1021/bi047479i. PMID:15807548.
  6. Daum S et al. (2003), Biochemistry, 42, 106-113. Characterization of the S272A,D Site-Directed Mutations ofO-Acetylserine Sulfhydrylase:  Involvement of the Pyridine Ring in the α,β-Elimination Reaction†. DOI:10.1021/bi0268044. PMID:12515544.
  7. Bettati S et al. (2000), J Biol Chem, 275, 40244-40251. Role of pyridoxal 5'-phosphate in the structural stabilization of O-acetylserine sulfhydrylase. DOI:10.1074/jbc.M007015200. PMID:10995767.
  8. Burkhard P et al. (1998), J Mol Biol, 283, 121-133. Three-dimensional structure of O-acetylserine sulfhydrylase from Salmonella typhimurium. PMID:9761678.

Step 1.

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Catalytic Residues Roles

Residue Roles
Lys41A proton shuttle (general acid/base), covalent catalysis
Ser272A electrostatic stabiliser

Chemical Components

Step 1.

Contributors

Alex Gutteridge, Craig Porter, Gemma L. Holliday