Aspartokinases are present only in microorganisms and plants, but not in animals, which must obtain aspartate-family amino acids from their diet. Consequently, methionine, lysine and threonine are essential amino acids in animals.
The presence of aspartykinases is limited to certain organisms like bacteria and plants, while animals lack this enzyme. This necessitates the consumption of essential amino acids such as methionine, lysine, and threonine from their diet.