Cysteine (Cys) is a sulfur containing amino acid which plays important structural and functional roles in proteins and enzymes. Cysteine often participates in the enzymatic reactions as a nucleophile through its thiol side chain. Cysteine also plays a major role in the antioxidant defense mechanism in the human parasite, Entamoeba histolytica; and acts a precursor of methionine, glutathione, phytochelatins, iron-sulfur clusters, vitamin cofactors and multiple secondary metabolites. The biosynthesis of cysteine in mammals is quite different to that of in plants and microorganisms. In plants and microorganisms, the cysteine biosynthesis is a two-step process and requires two enzymes: (1) Serine Acetyltransferase (SAT/ CysE) and (2) O-AcetylserineSulfhydrylase (OASS/ CysK). CysE catalyzes the first half of the reaction to produce O-acetylserine (OAS) and CysK participates in the second half of the reaction with OAS and HS (Hydrogen Sulphide) to produce L-cysteine. The focal point of this paper will however be, CysK, regarding its structure, mechanism and regulation.

CysK is a pyridoxal-dependent, ?-replacement enzyme which performs the second half reaction of the cysteine biosynthesis by catalyzing substitution of acetate in the side-chain of OAS by sulfide to produce L-cysteine. There are many facets of CysK, some of which will be discussed in this paper. The successfully solved structures of CysK are found in Salmonella typhimurium, Entamoeba histolytica and Arabidopsis thaliana with the resolution of 2.2Å which reveals that it’s a 34.5kDa homodimer protein. Apart from that, many mutant CysK structures have also been developed in order to study the functions of significant amino acids in the enzyme. The kinetic mechanism of OASS is Ping Pong Bi Bi as shown by initial velocity studies in the absence and presence of products and dead end inhibitors, isotope exchange at equilibrium, and equilibrium spectral studies. O-acetyl-L-serine binds to the internal aldimine form of the enzyme, and acetate is released as the first product. Bisulfide then adds as the second substrate to the ?-aminoacrylate intermediate form of the enzyme, and L-cysteine is released as the final product. The regulatory aspect of CysK comes with its complex formation with the preceding enzyme of the pathway i.e. CysE. This bi-enzyme complex is known as Cysteine Synthase Complex (CSC). Upon depletion of the substrate for CysK i.e. OAS, CysK forms a complex with CysE which leads to its inhibition. The inherent inhibition of CysK comes from its binding to the ?-aminoacrylate intermediate as seen in M. tuberculosis. But apart from this, there has been a multitude of researches going on to develop novel inhibitors of CysK for as therapeutics. For instance, designing the inhibitor peptides that mimic the terminal peptide sequence of CysE (which directly interacts with CysK active site), synthetic inhibitors (UPAR40), thiazolidine inhibitors.