Essential amino acid
An
essential amino acid or
indispensable amino acid is an amino acid that cannot be synthesized
de novo by the organism being considered, and therefore must be supplied in its diet. The amino acids regarded as essential for humans are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine. Additionally, cysteine, tyrosine, and arginine are always required by infants and growing children. Essential amino acids are "essential" not because they are more important for protein synthesis or general homeostasis than the others, but because the body does not synthesize them. If they are not uptaken through diet, they will not be available for protein synthesis. In addition, the amino acids arginine, cysteine, glycine, glutamine, proline, serine and tyrosine are considered
conditionally essential, meaning they are not normally required in the diet, but must be supplied exogenously to specific populations that do not synthesize them in adequate amounts. An example would be with the disease phenylketonuria.