title: Detection of an intermediate in the folding of the (beta alpha)8-barrel N-(5'-phosphoribosyl)anthranilate isomerase from Escherichia coli.
abstract: We have used thermodynamic and kinetic techniques to monitor the guanidinium chloride induced (GdmCl-induced) denaturation of N-(5'-phosphoribosyl)anthranilate isomerase from Escherichia coli (ePRAI). Although CD-monitored equilibrium denaturation curves are consistent with cooperative unfolding of the protein centered at 1.45 M GdmCl, fluorescence readings drop by over 25% in the region preceding the CD-monitored transition, suggesting non-two-state behavior. Kinetics experiments measure a slow relaxation rate with negative fluorescence amplitude when protein is diluted from 0 to 0.5 M GdmCl, corroborating results from equilibrium conditions. Detection of several unfolding and refolding rates in final GdmCl concentrations from 0 to 5.0 M indicates the presence of at least one intermediate along unfolding and refolding pathways. GdmCl dependence of the relaxation rates can be explained most easily by a nonsequential mechanism for ePRAI unfolding, though a sequential mechanism cannot be ruled out. The data corroborate the fragment complementation studies of Eder and Kirschner [Eder, J., & Kischner, K. (1992) Biochemistry 31, 3617-3625], which are consistent with unfolding of the C-terminal portion of a yeast-derived PRAI in its folding intermediate. In ePRAI, such partial unfolding would expose W391 to quenching by solvent molecules; W356, ePRAI's other tryptophan, is buried in the hydrophobic core and is unlikely to be affected by local changes in structure. A C-terminally unfolded folding intermediate has been demonstrated in the folding of tryptophan synthase (alpha-subunit), a related beta alpha-barrel enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: MRC Unit for Protein Function and Design, Department of Chemistry, Cambridge, U.K.
authors.forename: A
authors.initials: A
authors.lastname: Jasanoff
authors.forename: B
authors.initials: B
authors.lastname: Davis
authors.forename: A R
authors.initials: AR
authors.lastname: Fersht
mesh_terms: D019747:Aldose-Ketose Isomerases; D002238:Carbohydrate Epimerases / Q000737:chemistry*; D002942:Circular Dichroism; D004926:Escherichia coli / Q000201:enzymology*; D019791:Guanidine; D006146:Guanidines / Q000494:pharmacology; D006358:Hot Temperature; D007700:Kinetics; D011489:Protein Denaturation; D017510:Protein Folding; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics
chemical_list: D006146:Guanidines; D002238:Carbohydrate Epimerases; D019747:Aldose-Ketose Isomerases; C029865:phosphoribosylanthranilate isomerase; D019791:Guanidine