1.4.4 Covalent cross-linkages stabilise protein structure
Proteins that are secreted by the cell, or are attached to the extracellular surface of the plasma membrane, can be subject to more extreme conditions than those experienced by intracellular proteins. Often, covalent cross-linkages stabilise these proteins by connecting specific amino acids within a polypeptide or between polypeptide chains in multisubunit proteins (see below). Typically such a linkage will be a covalent sulfur–sulfur bond which forms between the –SH groups of two cysteine residues that are in close proximity in the folded protein (Figure 18). Called disulfide bonds, these covalent linkages do not affect the conformation of the protein, and are only formed when the folding is complete. They act, therefore, to secure the conformation and increase the stability of the protein.