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amino acid
any of a class of organic compounds containing the amino (NH2) and the carboxyl (COOH) groups, occurring naturally in plant and animal tissues and forming the chief constituents of protein. Twenty amino acids are necessary for protein synthesis. Eleven (the nonessential amino acids) can be synthesized by the human body and thus are not specifically required in the diet: alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine. Nine (the essential amino acids) cannot be synthesized by humans and thus are required in the diet: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

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