amino acids, peptides, and proteins

Amino acids 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 only exceptions to this basic structure are proline and hydroxyproline, in which a secondary amino group present in a cyclic structure replaces the primary amino group. 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. An oxidation-reduction reaction called transamination synthesizes the remaining 10 (nonessential amino acids). 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 class of peptides, the molecules of which are structurally like those of proteins, but smaller, includes many hormones, antibiotics, and other compounds that participate in the metabolic functions of living organisms. Peptide molecules are composed of two or more amino acids joined through amide formation involving the carboxyl group of each amino acid and the amino group of the next. Proteins, present in all living organisms, are of great nutritional value and are directly involved in the chemical processes essential for life. Chemists in the early 19th century coined the name for these substances from the Greek proteios, meaning "holding first place." Proteins are species-specific; in other words, the proteins of one species differ from those of another species. They are also organ-specific; e.g., within a single organism, muscle proteins differ from those of the brain and liver.