disulfide bond between 2 cysteines
the double amino acid cystine
Figure 9.11. Oxidation of adjoining cysteine residues produces a disulfide bond. Proteolysis of the polypeptide releases cystine.
There are many other nonprotein amino acids. Very early in the evolution of life, the palette of amino acids used to make polypeptides became fixed at the 20 that we have described. Almost the entire substance of all living things on earth is either polypeptide, composed of these 20 monomers, or other molecules synthesized by enzymes that are themselves proteins made of these 20 monomers. Natural selection has directed evolution within the constraints imposed by the palette.
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