Chemical Structures

The structure of AAs consists of a central carbon atom that is bound to an amino group (-NH2), an acid group (-COOH), and a side chain (R group) with the following basic structure:

H2N-CH-COOH I

The R group is what gives each AA its identity and determines its specific potential functions in the body. At the pH of body fluids, free AAs exist as dipolar ions (zwitterions) in the following basic form:

The basic zwitterion form has no net electrical charge because the polar charges cancel each other out. However, some AAs have charged R groups that give them a net positive or negative charge. With the exception of glycine (R group = H), AAs can exist as D or L stereoisomers. The human body only uses L-AAs for protein synthesis (PS). The AA names used in this chapter refer to the L forms unless otherwise indicated. Figure 15.1 provides three additional illustrations of the general structure of most AAs.

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