Catayzes the decarboxylation/dehydration of prephenate to phenylpyruvate. H(+) + prephenate = 3-phenylpyruvate + CO2 + H2O Amino-acid biosynthesis; L-phenylalanine biosynthesis; phenylpyruvate from prephenate: step 1/1. By L-phenylalanine. Present with 2020 molecules/cell in log phase SD medium.