amino acid

amino acid , any one of a class of simple organic compounds containing carbon, hydrogen, oxygen, nitrogen, and in certain cases sulfur. These compounds are the building blocks of proteins. They are characterized by the presence of a carboxyl group (COOH) and an amino group (NH ₂ ) attached to the same carbon at the end of the compound. The 20 amino acids commonly found in animals are alanine , arginine , asparagine , aspartic acid , cysteine , glutamic acid , glutamine , glycine , histidine , isoleucine , leucine , lysine , methionine , phenylalanine , proline , serine , threonine , tryptophan , tyrosine , and valine . More than 100 less common amino acids also occur in biological systems, particularly in plants. Every amino acid except glycine can occur as either of two optically active stereoisomers, d or l ; the more common isomer in nature is the l -form. When the carboxyl carbon atom of one amino acid covalently binds to the amino nitrogen atom of another amino acid with the release of a water molecule, a peptide bond is formed. Amino acids are released in the intestinal tract by the digestion of food proteins and are then carried in the bloodstream to the body cells, where they are used for growth, maintenance, and repair. Cellular catabolism breaks amino acids down into smaller fragments. Many of the amino acids necessary in metabolism can be synthesized in the human or animal body when needed; these are called nonessential. Others cannot be synthesized in sufficient quantities; these are termed essential and must be provided in the diet.

amino acid Organic acid containing at least one carboxyl group (COOH) and at least one amino group (NH₂). Amino acids are of great biological importance because they combine together to form protein. Amino acids form peptides by the reaction of adjacent amino and carboxyl groups. Proteins are polypeptide chains consisting of hundreds of amino acids. About 20 amino acids occur in proteins; not all organisms are able to synthesize all of them. Essential amino acids are those that an organism has to obtain ready-made from its environment. There are ten essential amino acids for humans: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.

amino acid (ă-mee-noh) n. an organic compound that contains an amino group (–NH₂) and a carboxyl group (–COOH). Amino acids are fundamental constituents of all proteins (see table). Some can be synthesized by the body; others (see essential amino acid) must be obtained from protein in the diet.