AMINO ACID

any of a group of organic molecules that consist of a basic amino group (-NH2), an acidic carboxyl group (-COOH), and an organic R group (or side chain), which is unique to each amino acid. The formula of a general amino acid is The only exceptions to this basic structure are proline and hydroxyproline, in which the primary amino group is replaced by a secondary amino group present in a cyclic structure. Although more than 100 amino acids occur naturally, only 20 are commonly used in protein synthesis; these are the same in all living organisms, from protozoa to plants and animals. In humans, the body is unable to synthesize approximately 10 of these amino acids (essential amino acids), making them a requirement in the diet. The remaining 10 (nonessential amino acids) are synthesized by an oxidation-reduction reaction called transamination. Amino acids are joined covalently by peptide bonds to yield proteins (including enzymes and hormones; structural, transport, and contractile elements; and molecules of special biological activity). A peptide bond is formed by a condensation (water-loss) reaction between the carboxyl group of one amino acid and the amino group of the next amino acid occurring in a protein. Thus, proteins are formed by the linear arrangement of amino acids in a particular order. Most of the common proteins contain more than 100 amino acids. DNA (deoxyribonucleic acid) contains the genetic information that dictates the specific sequence of amino acids found in all proteins. The properties common to all amino acids are due to the relative special arrangements of the carboxyl and amino groups. The physical and chemical properties unique to each amino acid are the result of the structure and chemical properties of the R group. Amino acids are generally grouped according to the polarity (the tendency to interact with water at a neutral pH) and charge of the R group. In an aqueous solution at a neutral pH, amino acids act as both weak acids and weak bases, because the carboxyl group loses a hydrogen ion and becomes negative, and the amino group picks up a hydrogen ion and becomes positive. When individual amino acids are combined to form proteins, their carboxyl and amino groups are no longer able to act as acids or bases because they have been used to form the peptide bonds. The acid-base properties of proteins are therefore dependent upon the overall ionization characteristics of the individual R groups of the component amino acids.