| Sulfur-containing amino acids
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Cysteine and its oxidized form, cystine, are sulfur-containing amino acids characterized by low polarity. Cysteine plays an important role in stabilization of protein structure, since it can participate in formation of a disulfide bond with other cysteine residues to form cystine residues, crosslinking protein chains and stabilizing protein structure. Two regions of a single polypeptide chain, remote from each other in the sequence, may be covalently linked through a disulfide bond (intrachain disulfide bond). Disulfide bonds are also formed between two polypeptide chains (interchain disulfide bond), forming covalent protein dimers. These bonds can be reduced by enzymes, or by reducing agents such as 2-mercaptoethanol or dithiothreitol to form cysteine residues. Methionine is the third sulfur-containing amino acid and contains a nonpolar methyl thioether group in its side chain.
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