Biosynthesis of Tryptophan in Bacteria and Plants
The aromatic amino acid tryptophan is an essential nutrient, meaning that humans and animals do not themselves have the biosynthetic machinery to synthesize tryptophan but rely on dietary intake from bacteria and plants that do produce it. Tryptophan serves not only as a building block for protein synthesis but a precursor for synthesis of serotonin (see Neurotransmitter biosynthesis pathway). The tryptophan biosynthetic pathway starts with chorismate, also a precursor for the synthesis of the essential aromatic amino acids tyrosine and phenylalanine. Chorismate (see Biosynthesis of Chorismate pathway) is synthesized in several steps starting with phosphoenolpyruvate (see Glycolysis) and the sugar erythrose 4-phosphate. Inhibition of this pathway is a useful target for herbicides since it is only found in plants and not in animals and will therefore have minimal toxicity. The last two steps in the pathway are catalyzed by two subunits of tryptophan synthase joined by a tube-like connection. The intermediate indole diffuses directly from one step to the next, greatly increasing the efficiency and rate of the overall reaction.
Contributor: Glenn Croston, PhD
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