Cysteinecystine

Cysteinecystine refers to the redox pair formed by the amino acid cysteine and its oxidized form, cystine. Cystine is the disulfide-linked dimer of two cysteine residues connected by a covalent S–S bond; the reduced counterpart is two free cysteine molecules.

In solution, the thiol group of cysteine can be oxidized to form cystine, while cystine can be reduced back to two cysteines. The Cys/Cystine couple is used in biochemistry to describe the redox state of thiol-disulfide groups within and between molecules and to mediate thiol-disulfide exchange reactions that influence protein structure and signaling.

Biologically, cellular compartments differ in redox environment. The cytosol is generally reducing, favoring cysteine, whereas extracellular spaces and the endoplasmic reticulum are more oxidizing, promoting cystine formation and disulfide bond formation in secreted and membrane proteins. The ratio of cysteine to cystine acts as a redox buffer and a marker of oxidative stress, with a shift toward cystine indicating more oxidizing conditions.

Measurement and relevance: the Cys/Cystine redox couple is studied to assess redox status in research and clinical settings. Techniques such as chromatography and spectrophotometry are used to quantify cysteine and cystine in biological samples. Dysregulation of this redox pair has been linked to aging, inflammatory diseases, cancer, and metabolic disorders, making it a useful, albeit indirect, indicator of cellular redox balance.