Catalyzes the non-oxidative deamination of L-phenylalanine and L-tyrosine to form trans-cinnamic acid and p-coumaric acid respectively with similar efficiencies. Facilitates the commitment step in phenylpropanoid pathways that produce secondary metabolites such as lignins, coumarins and flavonoids. L-phenylalanine = (E)-cinnamate + NH4(+) L-tyrosine = (E)-4-coumarate + NH4(+) Optimum pH is 8.5. Phenylpropanoid metabolism; trans-cinnamate biosynthesis; trans-cinnamate from L-phenylalanine: step 1/1. Homotetramer. Dimer of dimers. Contains an active site 4-methylidene-imidazol-5-one (MIO), which is formed autocatalytically by cyclization and dehydration of residues Ala-Ser-Gly. Belongs to the PAL/histidase family.