What Makes an Amino Acid Essential?
Not all the organisms are capable of synthesizing all the amino acids, and many are synthesized by pathways that are present only in certain plants and bacteria. Mammals, for example, must obtain eight of twenty amino acids from their diets. This requirement leads to a convention that divides amino acids into two categories: essential and nonessential (given a certain metabolism). Because of particular structural features, essential amino acids cannot be synthesized by mammalian enzymes ADDIN REFMGR.CITE
