PROTEINS Proteins can be small (called oligopeptides) or large (called polypeptides or proteins). These are all based on the same set of 20 different amino acids that are connected to form peptides. Amino acids are compounds that have an amino side chain and a carboxylic acid side chain. Figure 74 illustrates the shape of an amino acid:
Figure 74.
The amino acids in proteins are referred to as alpha-amino acids because the amino group is attached to the alpha carbon of the carboxylic acid compound. About half of these amino acids in humans can be synthesized while 9 of them are essential amino acids, which must be gotten from the diet. Figure 75 shows the 20 amino acids seen in nature:
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