Any of a class of organic compounds having a carboxyl group (COOH) and an amino group (NH2). Some 22 amino acids are commonly found in animals, and more than 100 less common forms are found in nature, chiefly in plants. When the carboxyl carbon atom of one amino acid binds to the nitrogen of another with the release of a water molecule, a linkage called a peptide bond is formed. Chains of amino acids, joined head-to-tail in this manner, are synthesized by living systems and are called polypeptides (up to about 50 amino acids) and proteins (over 50 amino acids).