Cystine

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(CC) Image: David E. Volk
Comparison of cysteine and cystine structures, which can be interconverted by oxidation or reduction.

Cystine refers to the oxidative linkage between two molecules of cysteine, one of the common amino acids, in the form of a disulfide bond. It may also refer to such a bond that occurs between two cysteine amino acids in a protein. Typically, intracellular proteins have few, if any, such disulfide bonds, while extracellular proteins tend to have several of them.

Such disulfide bonds are integral to the structural stability of many proteins.