Aromatic amino acids
Phenylalanine, tryptophan and histidine are essential amino acids for animals. Since they are not synthesized in the human body, they must be derived from the diet. Tyrosine is semi-essential; it can be synthesized, but only from phenylalanine. A lack of the enzyme phenylalanine hydroxylase, used in tyrosine synthesis, causes phenylketonuria, and concurrently renders tyrosine an essential amino acid.
Aromatic amino acids are able to absorb light due to their conjugated double bonds. This characteristic of aromatic amino acids are used to quantify the concentration of proteins in an unknown sample. These amino acids are able to absorb light which excites its electron to the excited state, when the electron fall back to the ground state it will either emit light or release energy. if the molecule is able to emit light it is known as fluorescent molecule. Tryptophan is widely used as a fluorescent molecule.
Animals get aromatic amino acids from diet, but all plants and micro-organisms must synthesize their aromatic amino acids through the metabolically costly shikimate pathway in order to make proteins. Herbicides and antibiotics work by inhibiting enzymes involved in aromatic acid synthesis, thereby rendering them toxic to plants and micro-organisms but not to animals.
All plants and micro-organisms must synthesize their aromatic amino acids through the shikimate pathway in order to make proteins, unlike animals, which obtain them through their diet. Because of the energy intensive nature of amino acid biosynthesis, animals have lost these costly metabolic pathways, since they can obtain aromatic amino acids by ingesting other organisms. Herbicides and antibiotics take advantage of this by inhibiting enzymes involved in aromatic acid synthesis, thereby rendering them toxic to plants and micro-organisms but not animals.
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