Overview of cysteine metabolism

In mammals, cysteine can be obtained from the diet or via recycling of protein and glutathione (GSH). In addition, cysteine can be synthesized from serine and methionine-derived homocysteine through the transsulfuration pathway. This pathway intersects with the methionine cycle, which supports transmethylation reactions and forms homocysteine as a key intermediate. The transsulfuration pathway leads to the irreversible commitment of the sulfur group of homocysteine to cysteine synthesis by the action of cystathionine β-synthase (CBS) and γ-cystathionase (CSE). Cysteine is catabolized either by desulfuration or oxidation. The products of cysteine catabolism are taurine, pyruvate, H2S, sulfate, and thiosulfate. In addition, cysteine is used in protein synthesis and serves as a precursor and sulfur source for metabolites including GSH and coenzyme A (CoA). In proteins, reversible oxidation or covalent posttranslational modification of cysteines including sulfenylation, thiolation, disulfide formation, nitrosation, persulfidation, prenylation, and acylation, are postulated to be involved in redox-dependent signaling and/or regulation of protein function.