BIOCHEMISTRY is the long-established first-and best resource for the essentials of biochemistry
Proteins are the most a bundant a nd functionally divers e molecules in living systems. Virtually every life process depends on this class of macromolecules . For example , enzymes and polype ptide hormones direct and regulate metabolism in the body, whereas contractile proteins in muscle permit movement. In bone, the protein collagen forms a framework for the deposition of calcium phosphate crystals, acting like the steel cables in reinforced concrete. In the bloodstream, proteins, such as hemoglobin and plasma albumin, shuttle molecules essential to life, whereas immunoglobulins fight infectious bacteria and viruses. In short, proteins display an incredible dive rs ity of functions , yet all share the common structural feature of being linear polymers of amino acids. This chapter describes the properties of amino acids. Chapter 2 explores how these simple building blocks are joined to form proteins that have unique three-dimensional structures, making them capable of performing specific biologic functions.
Although more tha n 300 different a mino a cids have bee n de scribe d in nature, only 20 are commonly found as constituents of mammalian proteins. [Note: These are the only amino acids that are coded for by DNA, the genetic material in the cell (see p. 395).] Each amino acid has a carboxyl group, a primary amino group (except for proline, which has a secondary amino group), and a distinctive side chain (“R group”) bonded to the a -carbon atom (Figure 1.1A). At physiologic pH (approximately 7.4), the carboxyl group is dissociated, forming the negatively charged carboxylate ion (– COO– ), and the amino group is protonated (– NH3 +). In proteins, almost all of these carboxyl and amino groups are combined through peptide linkage and, in general, are not available for chemical reaction except for hydrogen bond formation (Figure 1.1B). Thus, it is the nature of the side chains that ultimately dictates.