The phenylketonuria-associated substitution R68S converts phenylalanine hydroxylase to a constitutively active enzyme but reduces its stability

The naturally occurring R68S substitution of phenylalanine hydroxylase (PheH) causes phenylketonuria (PKU). However, the molecular basis for how the R68S variant leads to PKU remains unclear. Kinetic characterization of R68S PheH establishes that the enzyme is fully active in the absence of allosteric binding of phenylalanine, in contrast to the WT enzyme. Analytical ultracentrifugation establishes that the isolated regulatory domain of R68S PheH is predominantly monomeric in the absence of phenylalanine and dimerizes in its presence, similar to the regulatory domain of the WT enzyme. Fluorescence and small-angle X-ray scattering analyses establish that the overall conformation of the resting form of R68S PheH is different from that of the WT enzyme. The data are consistent with the substitution disrupting the interface between the catalytic and regulatory domains of the enzyme, shifting the equilibrium between the resting and activated forms ∼200-fold, so that the resting form of R68S PheH is ∼70% in the activated conformation. However, R68S PheH loses activity 2 orders of magnitude more rapidly than the WT enzyme at 37 °C and is significantly more sensitive to proteolysis. We propose that, even though this substitution converts the enzyme to a constitutively active enzyme, it results in PKU because of the decrease in protein stability.

Phosphorylation of Phenylalanine Hydroxylase Increases the Rate Constant for Formation of the Activated Conformation of the Enzyme

Liver phenylalanine hydroxylase (PheH) is an allosteric enzyme that is activated by phenylalanine. The enzyme is also phosphorylated by protein kinase A, but the effects of phosphorylation are unclear. Recent structural studies ( Meisburger et al. ( 2016 ) J. Amer. Chem. Soc. 138 , 6506 - 6516 ) support a model in which activation of the enzyme involves dimerization of the regulatory domains, creating the allosteric site for phenylalanine at the dimer interface. This conformational change also results in a change in the fluorescence of the protein that can be used to monitor activation. The kinetics of activation of PheH are biphasic over a range of phenylalanine concentrations. These data are well-described by a model involving an initial equilibrium between the resting form and the activated conformation, with a value of the equilibrium constant for formation of the activated conformation, L, equal to 0.007, followed by binding of two molecules of phenylalanine. Phosphorylation increases L 10-fold by increasing the rate constant for conversion of the resting form to the activated form. The results provide functional support for the previous structural model, identify the specific effect of phosphorylation on the enzyme, and rationalize the lack of change in the protein structure upon phosphorylation.

Domain Movements upon Activation of Phenylalanine Hydroxylase Characterized by Crystallography and Chromatography-Coupled Small-Angle X-ray Scattering

Mammalian phenylalanine hydroxylase (PheH) is an allosteric enzyme that catalyzes the first step in the catabolism of the amino acid phenylalanine. Following allosteric activation by high phenylalanine levels, the enzyme catalyzes the pterin-dependent conversion of phenylalanine to tyrosine. Inability to control elevated phenylalanine levels in the blood leads to increased risk of mental disabilities commonly associated with the inherited metabolic disorder, phenylketonuria. Although extensively studied, structural changes associated with allosteric activation in mammalian PheH have been elusive. Here, we examine the complex allosteric mechanisms of rat PheH using X-ray crystallography, isothermal titration calorimetry (ITC), and small-angle X-ray scattering (SAXS). We describe crystal structures of the preactivated state of the PheH tetramer depicting the regulatory domains docked against the catalytic domains and preventing substrate binding. Using SAXS, we further describe the domain movements involved in allosteric activation of PheH in solution and present the first demonstration of chromatography-coupled SAXS with Evolving Factor Analysis (EFA), a powerful method for separating scattering components in a model-independent way. Together, these results support a model for allostery in PheH in which phenylalanine stabilizes the dimerization of the regulatory domains and exposes the active site for substrate binding and other structural changes needed for activity.

Activation of phenylalanine hydroxylase by phenylalanine does not require binding in the active site

Phenylalanine hydroxylase (PheH), a liver enzyme that catalyzes the hydroxylation of excess phenylalanine in the diet to tyrosine, is activated by phenylalanine. The lack of activity at low levels of phenylalanine has been attributed to the N-terminus of the protein’s regulatory domain acting as an inhibitory peptide by blocking substrate access to the active site. The location of the site at which phenylalanine binds to activate the enzyme is unknown, and both the active site in the catalytic domain and a separate site in the N-terminal regulatory domain have been proposed. Binding of catecholamines to the active-site iron was used to probe the accessibility of the active site. Removal of the regulatory domain increases the rate constants for association of several catecholamines with the wild-type enzyme by ∼2-fold. Binding of phenylalanine in the active site is effectively abolished by mutating the active-site residue Arg270 to lysine. The k_cat/K_phe value is down 10⁴ for the mutant enzyme, and the K_m value for phenylalanine for the mutant enzyme is >0.5 M. Incubation of the R270K enzyme with phenylalanine also results in a 2-fold increase in the rate constants for catecholamine binding. The change in the tryptophan fluorescence emission spectrum seen in the wild-type enzyme upon activation by phenylalanine is also seen with the R270K mutant enzyme in the presence of phenylalanine. Both results establish that activation of PheH by phenylalanine does not require binding of the amino acid in the active site. This is consistent with a separate allosteric site, likely in the regulatory domain.

Mutagenesis of a specificity-determining residue in tyrosine hydroxylase establishes that the enzyme is a robust phenylalanine hydroxylase but a fragile tyrosine hydroxylase

The aromatic amino acid hydroxylases tyrosine hydroxylase (TyrH) and phenylalanine hydroxylase (PheH) have essentially identical active sites; however, PheH is nearly incapable of hydroxylating tyrosine, while TyrH can readily hydroxylate both tyrosine and phenylalanine. Previous studies have indicated that Asp425 of TyrH is important in determining the substrate specificity of that enzyme [Daubner, S. C., Melendez, J., and Fitzpatrick, P. F. (2000) Biochemistry 39, 9652-9661]. Alanine-scanning mutagenesis of amino acids 423-427, a mobile loop containing Asp425, shows that only mutagenesis of Asp425 alters the activity of the enzyme significantly. Saturation mutagenesis of Asp425 results in large (up to 10(4)) decreases in the V(max) and V(max)/K(tyr) values for tyrosine hydroxylation, but only small decreases or even increases in the V(max) and V(max)/K(phe) values for phenylalanine hydroxylation. The decrease in the tyrosine hydroxylation activity of the mutant proteins is due to an uncoupling of tetrahydropterin oxidation from amino acid hydroxylation with tyrosine as the amino acid substrate. In contrast, with the exception of the D425W mutant, the extent of coupling of tetrahydropterin oxidation and amino acid hydroxylation is unaffected or increases with phenylalanine as the amino acid substrate. The decrease in the V(max) value with tyrosine as the substrate shows a negative correlation with the hydrophobicity of the amino acid residue at position 425. The results are consistent with a critical role of Asp425 being to prevent a hydrophobic interaction that results in a restricted active site in which hydroxylation of tyrosine does not occur.

Molecular analysis and implications of neurovirulent circulating vaccine-derived poliovirus in Jamaica. A case report and review of literature

As the goal to eradicate wild polio virus (WPV) is approached, outbreaks associated with vaccine derived polioviruses (VDPV) with neurovirulent properties have emerged. The relevance for the spread of infection by nonparalytic cVDPV cases, with mutations associated with neurovirulence, is discussed with reference to the molecular analysis of a VDPV isolated from a Jamaican child who presented with aseptic meningitis. Potential risks to the Jamaican community resulting from circulation of cVDPV and critical factors defined by the World Health Organization (WHO) in the global eradication of Polio are analyzed in the context of immunization coverage, and the need to stop all Oral Polio Vaccine (OPV) use once wild polioviruses (WPVs) have been eradicated.