Each gene is repressed by the same amino acid , except that the Phe sensitive enzyme is repressed by both Trp and Phe. Although prephenate is the true branchpoint, chorismate mutase exists in two forms, complexed with prephenate dehydrogenase or prephenate dehydratase respectively. Prephenate is never released as a product, so regulation must control the chorismate mutase as well. Chorismate is the last common intermediate released to the surroundings. The tyrosine branch is regulated by phe as well, since tyrosine can also be obtained by phenylalanine hydroxylase.
Decarboxylation requires electron shift away from the carboxylate.
The electron sinking action of PLP weakens bonds on carbons in odd number positions from the the Schiff N . This allows decarboxylation of the alpha-carboxylate but not decarboxylation of the beta carboxylate. The gamma carboxylate of glu is not susceptible, because the connecting carbon chain is not double bonded so does not extend the electon shift to gamma carboxylate of glutamate.