Role of the second coordination sphere residue tyrosine 179 in substrate affinity and catalytic activity of phenylalanine hydroxylase

Phenylalanine hydroxylase converts phenylalanine to tyrosine utilizing molecular oxygen and tetrahydropterin as a cofactor, and belongs to the aromatic amino acid hydroxylases family. The catalytic domains of these enzymes are structurally similar. According to recent crystallographic studies, residue Tyr179 in Chromobacterium violaceum phenylalanine hydroxylase is located in the active site and its hydroxyl oxygen is 5.1 A from the iron, where it has been suggested to play a role in positioning the pterin cofactor. To determine the catalytic role of this residue, the point mutants Y179F and Y179A of phenylalanine hydroxylase were prepared and characterized. Both mutants displayed comparable stability and metal binding to the native enzyme, as determined by their melting temperatures in the presence and absence of iron. The catalytic activity (kcat) of the Y179F and Y179A proteins was lower than wild-type phenylalanine hydroxylase by an order of magnitude, suggesting that the hydroxyl group of Tyr179 plays a role in the rate -determining step in catalysis. The KM values for different tetrahydropterin cofactors and phenylalanine were decreased by a factor of 3–4 in the Y179F mutant. However, the KM values for different pterin cofactors were slightly higher in the Y179A mutant than those measured for the wild-type enzyme, and, more significantly, the KM value for phenylalanine was increased by 10-fold in the Y179A mutant. By the criterion of Kcat/kPhe, the Y179F and Y179A mutants display 10% and 1%, respectively, of the activity of wild-type phenylalanine hydroxylase. These results are consistent with Tyr179 having a pronounced role in binding phenylalanine but a secondary effect in the formation of the hydroxylating species. In conjunction with recent crystallographic analyses of a ternary complex of phenylalanine hydroxylase, the reported findings establish that Tyr179 is essential in maintaining the catalytic integrity and phenylalanine binding of the enzyme via indirect interactions with the substrate, phenylalanine. A model that accounts for the role of Tyr179 in binding phenylalanine is proposed.