Proteins

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:

Figure 75.

It should be noted that there are actually 22 amino acids in nature; however, the last two have been recently found and are only found in minimal amounts in living things. These two are selenocysteine and pyrrolysine.

Amino acids are usually written as neutral molecules, which they are but not in the way you’d think. Most exist as zwitterions, which are negatively charged on one end and positively charged at the other. This explains the extreme reactivity of amino acids and the fact that they tend to connect to one another. This is shown in figure 76:

Figure 76.

The R chain is what makes an amino acid unique from other amino acids. There are simple side chains and very complex side chains. The side chain confers the ability of an