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"Amino Acids. Organic compounds that contain an amino group, -NH2, and a carboxyl group, -COOH, and that constitute the building blocks of the larger protein molecules (q.v.). Of the more than 150 known amino acids, only about 25 are commonly found in natural proteins some of whose molecules consist of many hundreds of amino acids. Of these common amino acids, six are designated as essential amino acids (q.v.) because they cannot be synthesized in the human body and must, therefore, be supplied by man's diet, and the proteins that contain them are correspondingly rated as ""complete"" proteins (q.v.)."