cystinecysteine

Cystinecysteine refers to the relationship between the amino acid cysteine and its oxidized dimer, cystine. Cystine is formed when two cysteine molecules are linked by a disulfide bond (S-S), yielding a dimer with two amino and two carboxyl groups; the molecular formula is C6H12N2O4S2. This disulfide link reflects a cysteine-to-cysteine connection that is central to redox biology.

In proteins, disulfide bonds between cysteine residues stabilize structure and assist folding. These bonds form in the oxidizing environment of the endoplasmic reticulum and can be reduced in the cytosol or during unfolding.

In physiology, the cystine/cysteine redox couple participates in redox signaling and metabolism. Cystine is transported into cells via the cystine/glutamate antiporter (system Xc-); inside the cell it is reduced to cysteine, supplying the precursor for glutathione and other thiols. The extracellular cystine/cysteine balance influences cellular redox state and oxidative stress.

Clinically, the plasma cysteine/cystine ratio is used as a marker of oxidative stress, with imbalances linked to aging and various diseases. Dietarily, methionine is a precursor to cysteine; cysteine is obtained from dietary proteins, and cystine is present in many protein-rich foods.

Note: cystine and cysteine are standard topics in biochemistry; cystinecysteine is not a separate compound but a reference to the redox relationship between the two forms.