Practice question

Shikimate pathway bridges central metabolism and aromatic compound biosynthesis unique to plants, fungi, and bacteria, absent in animals. Starting substrates phosphoenolpyruvate from glycolysis and erythrose-4-phosphate from pentose phosphate pathway undergo condensation by DAHP synthase initiating seven enzymatic steps producing 3-dehydroquinate, dehydroshikimate, shikimate, shikimate-3-phosphate, EPSP, and chorismate. Chorismate branches to prephenate leading to phenylalanine and tyrosine via arogenate pathway, and to anthranilate leading to tryptophan. Aromatic amino acids contribute to protein synthesis and also serve as precursors for phenylpropanoids, flavonoids, lignin, alkaloids, and indole-3-acetic acid. Up to 30 percent of photosynthetically fixed carbon flows through pathway in actively growing tissues. Plastid localization subjects pathway to light regulation. Absence in animals makes enzymes attractive selective herbicide and antimicrobial targets, explaining intensive study of EPSP synthase and other pathway enzymes in plant biochemistry curriculum. Regulation involves feedback inhibition of DAHP synthase by aromatic amino acids and transcriptional control of pathway genes by light and wounding. Chorismate mutase branching point controlled by tyrosine inhibition. Compartmentalization in plastids plus presence in cytosol of some steps creates complexity. Study of shikimate pathway enzymes supported development of glyphosate herbicide and provided target for biofortification of aromatic amino acid derived nutrients including folate and tocopherols essential