Different stabilities and denaturation pathways for structurally related aromatic amino acid hydroxylases.
We have compared the urea stability of the human aromatic amino acid hydroxylases (AAAHs), key enzymes involved in neurotransmitter biosynthesis and amino acid homeostasis. Tyrosine-, tryptophan- and phenylalanine hydroxylase (TH, TPH and PAH, respectively) were transiently activated at low urea concentrations and rapidly inactivated in >3 M urea. The denaturation of TH occurred through two cooperative transitions, with denaturation midpoints of 1.41+/-0.06 and 5.13+/-0.05 M urea, respectively. Partially denatured human TH (hTH) retained more of its secondary structure than human PAH (hPAH), and was found to exist as tetramers, whereas hPAH dissociated into dimers. Furthermore, the urea-induced aggregation of hPAH was 100-fold higher than for hTH. These results suggest that the denatured state properties of the AAAHs contribute significantly to the stability of these enzymes and their tolerance towards missense mutations.
Kleppe
Rune
R
Department of Biomedicine, Division for Anatomy and Cell Biology, University of Bergen, Jonas Lies vei 91, N-5009 Bergen, Norway.
Haavik
Jan
J
D002942:Circular Dichroism; D019281:Dimerization; D004305:Dose-Response Relationship, Drug; D006801:Humans; D008027:Light; D020125:Mutation, Missense; D010651:Phenylalanine Hydroxylase / Q000737:chemistry*; D011485:Protein Binding; D011489:Protein Denaturation; D020033:Protein Isoforms; D017433:Protein Structure, Secondary; D012542:Scattering, Radiation; D013050:Spectrometry, Fluorescence; D013997:Time Factors; D014365:Tryptophan Hydroxylase / Q000737:chemistry*; D014446:Tyrosine 3-Monooxygenase / Q000737:chemistry*; D014508:Urea / Q000494:pharmacology
D020033:Protein Isoforms; D014508:Urea; D010651:Phenylalanine Hydroxylase; D014446:Tyrosine 3-Monooxygenase; D014365:Tryptophan Hydroxylase