The effect of ligands of phenylalanine 4-monooxygenase on the cAMP-dependent phosphorylation of the enzyme.

The rate of phosphorylation of phenylalanine 4-monooxygenase by the cAMP-dependent protein kinase was found to be under substrate-directed regulation. Thus, L-phenylalanine made the hydroxylase a better substrate for the kinase, whereas the cofactor l-erythro-5,6,7,8-tetrahydrobiopterin (BH4) was a negative effector. The dephosphorylation of the enzyme by the kinase in the presence of high concentrations of MgADP was also stimulated by phenylalanine and inhibited by BH4. A kinetic analysis indicated that the effects of phenylalanine and BH4 were mediated by distinct sites coupled by a free energy of 3.2 kJ X mol-1. Among the ligands tested, only phenylalanine and BH4 affected the phosphorylation of the hydroxylase at physiologically relevant concentrations. Whereas higher concentrations of several naturally occurring or synthetic amino acids acted like phenylalanine, the widely used synthetic cofactor 6,7-dimethyltetrahydropterin did not mimic the effect of BH4. Less phenylalanine was required to activate the phosphorylated hydroxylase (0.9 mol of phosphate/subunit) than the dephosphorylated enzyme (0.07 mol of phosphate/subunit). This was true whether BH4 was present or not. In conclusion, the substrate phenylalanine makes the hydroxylase more prone to cAMP-dependent phosphorylation, which in turn sensitizes the enzyme towards allosteric activation by phenylalanine. The joint operation of these mechanisms in vivo would increase the efficiency with which phenylalanine controls the activity of the enzyme.