Extracellular proteins often have disulfide bonds between specific cysteine residues. These are strong covalent bonds, and they tend to lock the molecule into its conformation. Although relatively few proteins contain disulfide bonds, those that do are more stable and are therefore easy to purify and study. For this reason many of the first proteins studied in detail, such as the digestive enzymes chymotrypsin and ribonuclease and the bacterial cell wall degrading enzyme lysozyme, have disulfide bonds.
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