title: Structure is lost incrementally during the unfolding of barstar.
abstract: Coincidental equilibrium unfolding transitions observed by multiple structural probes are taken to justify the modeling of protein unfolding as a two-state, N <==> U, cooperative process. However, for many of the large number of proteins that undergo apparently two-state equilibrium unfolding reactions, folding intermediates are detected in kinetic experiments. The small protein barstar is one such protein. Here the two-state model for equilibrium unfolding has been critically evaluated in barstar by estimating the intramolecular distance distribution by time-resolved fluorescence resonance energy transfer (TR-FRET) methods, in which fluorescence decay kinetics are analyzed by the maximum entropy method (MEM). Using a mutant form of barstar containing only Trp 53 as the fluorescence donor and a thionitrobenzoic acid moiety attached to Cys 82 as the fluorescence acceptor, the distance between the donor and acceptor has been shown to increase incrementally with increasing denaturant concentration. Although other probes, such as circular dichroism and fluorescence intensity, suggest that the labeled protein undergoes two-state equilibrium unfolding, the TR-FRET probe clearly indicates multistate equilibrium unfolding. Native protein expands progressively through a continuum of native-like forms that achieve the dimensions of a molten globule, whose heterogeneity increases with increasing denaturant concentration and which appears to be separated from the unfolded ensemble by a free energy barrier.
authors.affiliation: Department of Chemical Sciences, Tata Institute of Fundamental Research Mumbai 400005, India.
authors.forename: G S
authors.initials: GS
authors.lastname: Lakshmikanth
authors.forename: K
authors.initials: K
authors.lastname: Sridevi
authors.forename: G
authors.initials: G
authors.lastname: Krishnamoorthy
authors.forename: J B
authors.initials: JB
authors.lastname: Udgaonkar
mesh_terms: D019943:Amino Acid Substitution / Q000235:genetics; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D002942:Circular Dichroism; D004735:Energy Transfer; D004791:Enzyme Inhibitors / Q000737:chemistry / Q000378:metabolism; D005453:Fluorescence; D011487:Protein Conformation; D011489:Protein Denaturation; D017510:Protein Folding; D013816:Thermodynamics; D014364:Tryptophan / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D004791:Enzyme Inhibitors; C057438:barstar protein, Bacillus amyloliquefaciens; D014364:Tryptophan