Amino Acids Differing Only in Their Side Chains Compose Proteins

The monomeric building blocks of proteins are 20 amino acids, all of which have a characteristic structure consisting of a central a carbon atom (Ca) bonded to four different chemical groups: an amino (NH2) group, a carboxyl (COOH) group, a hydrogen (H) atom, and one variable group, called a side chain, or R group. Because the a carbon in all amino acids except glycine is asymmetric, these molecules can exist in two mirror-image forms called by convention the D (dextro) and the L (levo) isomers (Figure 2-12). The two isomers cannot be interconverted (one made identical with the other) without breaking and then re-forming a chemical bond in one of them. With rare exceptions, only the L forms of amino acids are found in proteins. We discuss the properties of the covalent peptide bond that links amino acids into long chains in Chapter 3.

To understand the structures and functions of proteins, you must be familiar with some of the distinctive properties of the amino acids, which are determined by their side chains. The side chains of different amino acids vary in size, shape, charge, hydrophobicity, and reactivity. Amino acids can be classified into several broad categories based primarily on their solubility in water, which is influenced by the polarity of their side chains (Figure 2-13). Amino acids with polar side chains are hydrophilic and tend to be on the surfaces of proteins; by interacting with water, they make proteins soluble in aqueous solutions and can form noncovalent interactions with other water-soluble molecules. In contrast, amino acids with nonpolar side chains are hydrophobic; they avoid water and often aggregate to help form the water-insoluble cores of many proteins. The polarity of amino acid side chains thus is responsible for shaping the final three-dimensional structure of proteins.

A subset of the hydrophilic amino acids are charged (ionized) at the pH («7) typical of physiological conditions (see Section 2.3). Arginine and lysine are positively charged; as-partic acid and glutamic acid are negatively charged (their charged forms are called aspartate and glutamate). These four amino acids are the prime contributors to the overall charge of a protein. A fifth amino acid, histidine, has an im-

idazole side chain, which can shift from being positively charged to uncharged with small changes in the acidity of its environment:

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