The structure and properties of some natural organic compounds

Abstract

The structural units of proteins are the α-amino acids. The amino acids are joined together by peptide bonds in a linear manner (Fig. 7.1) and the constituent amino acids are released when the protein is hydrolysed. The importance of amino acids, however, is not limited to their being components of proteins; for, during amino-acid metabolism, reactions occur in which they serve as precursors for many biologically important compounds. Although well over a hundred α-amino acids have been isolated and identified, only about twenty occur in a typical protein hydrolysate. These common amino acids have been grouped together in Table 7.1, on the basis of the nature of the side chain (R). These amino acids are usually denoted by a three-letter abbreviation. It will be noticed that proline has a quite different structure to the other amino acids, and is in fact an amino acid. However, as it is a usual constituent of proteins it is conveniently considered here. It must be remembered that the less common amino acids may be structurally and biologically very important in particular proteins; for example, hydroxyproline and hydroxylysine in the collagens; or they may be essential for certain metabolic processes, such as ornithine in the urea cycle, but otherwise they are not usually found in proteins.