Amino acids are organic compounds that are the building blocks of proteins. In most animal metabolisms, a number of amino acids play an essential role. The genetic code, which determines the assembly of amino acids into body proteins, is mediated by the nucleic acids.
In terms of structure, each amino acid has at least one carboxyl (COOH) group, which is acidic, and one amino (NH(2)) group, which is basic. (The name of the acids comes from the stem word amine, meaning "derived from ammonia.") Amino acids join together in long chains, the amino group of one amino acid linking with the carboxyl group of another. The linkage is known as a peptide bond, and a chain of amino acids is known as a polypeptide. Proteins are large, naturally occurring polypeptides. Many different amino acids are found, about 20 of which are the main constituents of proteins; only about half of these are classified as essential nutrients--that is, necessary in the human diet.
Although all amino acids have amino and carboxyl groups, they differ in the rest of the molecule. Some have an additional amino or carboxyl group, and others have water-repelling (hydrophobic) groups. The shape and the overall properties of a protein are dependent upon its constituent amino acids. In some proteins, a change in just one amino acid in the polymer chain, out of a total of perhaps 250 amino acids, or even a change in its position, can cause the protein to become nonfunctional, if it is an enzyme, or to perform its function differently, if it is not an enzyme. Chemists are currently trying to relate the role of each of these amino acids to the way in which the protein works.
Proteins are an essential substance in the diets of humans and most animals because of their constituent amino acids. Nutritionally, complete proteins are those which contain the right concentrations of the amino acids that humans cannot synthesize from other amino acids or nitrogenous sources. An adequate diet, however, may be achieved by consuming a correct mixture of proteins, some of which might be deficient in one amino acid but rich in another.
A first step in the digestion of proteins is their cleavage by proteolytic (protein-splitting) enzymes into smaller chains of amino acids, or peptides. These initial peptides are then cleaved into smaller and smaller peptides until free amino acids are available. The amino acids are absorbed from the intestine by a complete biochemical process and are circulated by the blood to the tissues that utilize them. Some of the amino acids are used directly as the building blocks in the synthesis of new proteins unique to the species. Other amino acids may be used to supply energy, and still others, particularly when large amounts of proteins are consumed, may be excreted in the urine. Most amino acids and small peptides frequently have a bad or even a bitter taste, although the same amino acids in a small protein would most likely be without taste.
In addition to the 20 common amino acids, others occur in proteins. Analysis of many proteins from plant and animal sources has shown that all are made up of about 25 to 30 amino acids. In addition, more than 50 amino acids, not combined in proteins, have been found in plants. Some of these are simple derivatives of the common amino acids; others, usually found in plants and microorganisms, have more complicated structures.