Monomers of proteins link together to make the macromolecule

Each amino acid has both a carboxyl functional group and an amino functional group (see Figure 3.1) attached to the same carbon atom, called the α (alpha) carbon. Also attached to the α carbon atom are a hydrogen atom and a side chain, or R group, designated by the letter R.

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The α carbon in this example is asymmetrical because it is bonded to four different atoms or groups of atoms. Therefore amino acids can exist as optical isomers called D-amino acids and L-amino acids. D and L are abbreviations of the Latin terms for right (dextro) and left (levo). Only L-amino acids (with the configuration shown above) are commonly found in the proteins of most organisms, and their presence is an important chemical “signature” of life.

At the pH levels typically found in cells (usually about pH 7), both the carboxyl and amino groups of amino acids are ionized: the carboxyl group has lost a hydrogen ion:

—COOH → —COO + H+

and the amino group has gained a hydrogen ion:

—NH2 + H+ → —NH3+

Thus amino acids are simultaneously acids and bases.

Activity 3.2 Features of Amino Acids

www.life11e.com/ac3.2

The side chains (or R groups) of amino acids contain functional groups that are important in determining the three-dimensional structure and thus the function of the protein. As Table 3.2 shows, the 20 amino acids found in living organisms are grouped and distinguished by their side chains:

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Three amino acids—cysteine, glycine, and proline—are special cases, although the side chains of the latter two are generally hydrophobic.

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