ABSTRACT
Structure Proteins are polymers of L-amino acids. Apart from proline, all of the 20 amino acids found in proteins have a common structure in which a carbon atom (the -carbon) is linked to a carboxyl group, a primary amino group, a proton and a side chain (R) which is different in each amino acid (Fig. 1). Except in glycine, the -carbon atom is asymmetric – it has four chemically different groups attached. Thus, amino acids can exist as pairs of optically active stereoisomers (D-and L-). However, only the L-isomers are found in proteins. Amino acids are dipolar ions (zwitterions) in aqueous solution and behave as both acids and bases (they are amphoteric). The side chains differ in size, shape, charge and chemical reactivity, and are responsible for the differences in the properties of different proteins (Fig. 2). A few proteins contain nonstandard amino acids, such as 4-hydroxyproline and 5-hydroxylysine in collagen. These are formed by posttranslational modification of the parent amino acids proline and lysine (see Topic Q4).
