Catalyzes the Claisen rearrangement of chorismate to prephenate and the decarboxylation/dehydration of prephenate to phenylpyruvate. chorismate = prephenate H(+) + prephenate = 3-phenylpyruvate + CO2 + H2O Amino-acid biosynthesis; L-phenylalanine biosynthesis; phenylpyruvate from prephenate: step 1/1. Metabolic intermediate biosynthesis; prephenate biosynthesis; prephenate from chorismate: step 1/1.