Mechanisms underlying responsiveness to tetrahydrobiopterin in mild phenylketonuria mutations.

Abstract

A subtype of phenylalanine hydroxylase (PAH) deficiency that responds to cofactor (tetrahydrobiopterin, BH4) supplementation has been associated with phenylketonuria (PKU) mutations. The underlying molecular mechanism of this responsiveness is as yet unknown and requires a detailed in vitro expression analysis of the associated mutations. With this aim, we optimized the analysis of the kinetic and cofactor binding properties in recombinant human PAH and in seven mild PKU mutations, i.e., c.194T>C (p.I65T), c.204A>T (p.R68S), c.731C>T (p.P244L), c.782G>A (p.R261Q), c.926C>T (p.A309V), c.1162G>A (p.V388M), and c.1162G>A (p.Y414C) expressed in E. coli. For p.I65T, p.R68S, and p.R261Q, we could in addition study the equilibrium binding of BH4 to the tetrameric forms by isothermal titration calorimetry (ITC). All the mutations resulted in catalytic defects, and p.I65T, p.R68S, p.P244L, and most probably p.A309V, showed reduced binding affinity for BH4. The possible stabilizing effect of the cofactor was explored using a cell-free in vitro synthesis assay combined with pulse-chase methodology. BH4 prevents the degradation of the proteins of folding variants p.A309V, p.V388M, and p.Y414C, acting as a chemical chaperone. In addition, for wild-type PAH and all mild PKU mutants analyzed in this study, BH4 increases the PAH activity of the synthesized protein and protects from the rapid inactivation observed in vitro. Catalase and superoxide dismutase partially mimic this protection. All together, our results indicate that the response to BH4 substitution therapy by PKU mutations may have a multifactorial basis. Both effects of BH4 on PAH, i.e., the chemical chaperone effect preventing protein misfolding and the protection from inactivation, may be relevant mechanisms of the responsive phenotype.

• Authors: Angel L Pey, Belén Pérez, Lourdes R Desviat, Maria Angeles Martínez, Cristina Aguado, Heidi Erlandsen, Alejandra Gámez, Raymond C Stevens, Matthías Thórólfsson, Magdalena Ugarte, Aurora Martínez
• Journal: Human mutation (Hum Mutat) Vol. 24 Issue 5 Pg. 388-99 (Nov 2004) ISSN: 1098-1004 [Electronic] United States
• PMID: 15459954 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)
• Copyright: Copyright 2004 Wiley-Liss, Inc.

Chemical References

• Recombinant Proteins
• Biopterins
• Phenylalanine Hydroxylase
• sapropterin

Topics

• Biopterins (analogs & derivatives, metabolism, pharmacology)
• Calorimetry
• Catalysis (drug effects)
• Cell-Free System
• Escherichia coli (genetics)
• Half-Life
• Humans
• Kinetics
• Models, Molecular
• Molecular Sequence Data
• Mutation (genetics)
• Phenylalanine Hydroxylase (chemistry, deficiency, genetics, metabolism)
• Phenylketonurias (enzymology, genetics)
• Protein Binding
• Protein Biosynthesis
• Protein Conformation
• Recombinant Proteins (genetics, metabolism)
• Thermodynamics
• Transcription, Genetic