title: One-state downhill versus conventional protein folding.
abstract: Classical protein folding invokes a cooperative transition between distinct thermodynamic states that are individually populated at equilibrium and separated by an energy barrier. It has been proposed, however, that the small protein, BBL, undergoes one-step downhill folding whereby it folds non-cooperatively to its native state without encountering an appreciable energy barrier. Only a single conformational ensemble is populated under given conditions, and so the denatured state ensemble progressively changes into the native structure. A wide dispersion of thermal denaturation midpoints that was observed for an extrinsically labelled fragment of BBL is proposed to be evidence for its one-state, downhill folding, a phenomenon that is also suggested to be functionally important for BBL and its homologues. We found, however, that thermal denaturation of unlabelled wild-type BBL was highly cooperative, with very similar transition midpoints for the melting of secondary and tertiary interactions, as well as for individual residues when monitored by NMR. Similar results were also observed for two other homologues, E3BD and POB. Further, the extrinsic fluorophores perturbed the unfolding energetics of labelled BBL, and complicated its equilibrium behaviour. One-step downhill folding may well occur for some proteins that do not have distinct folded states but not for BBL and its well-folded homologues.
authors.affiliation: MRC Centre for Protein Engineering, MRC Centre, Hills Road, Cambridge CB2 2QH, UK.
authors.forename: Neil
authors.initials: N
authors.lastname: Ferguson
authors.forename: Pamela J
authors.initials: PJ
authors.lastname: Schartau
authors.forename: Timothy D
authors.initials: TD
authors.lastname: Sharpe
authors.forename: Satoshi
authors.initials: S
authors.lastname: Sato
authors.forename: Alan R
authors.initials: AR
authors.lastname: Fersht
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D002152:Calorimetry, Differential Scanning; D002247:Carbon Isotopes; D002942:Circular Dichroism; D031541:Fluorescence Resonance Energy Transfer; D008958:Models, Molecular; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D010455:Peptides / Q000737:chemistry / Q000235:genetics; D011487:Protein Conformation; D011489:Protein Denaturation; D015202:Protein Engineering; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011522:Protons; D017386:Sequence Homology, Amino Acid; D012996:Solutions; D013696:Temperature; D013816:Thermodynamics; D014364:Tryptophan / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D002247:Carbon Isotopes; D010455:Peptides; D011522:Protons; D012996:Solutions; D014364:Tryptophan