title: Early hydrogen-bonding events in the folding reaction of ubiquitin.
abstract: The formation of hydrogen-bonded structure in the folding reaction of ubiquitin, a small cytoplasmic protein with an extended beta-sheet and an alpha-helix surrounding a pronounced hydrophobic core, has been investigated by hydrogen-deuterium exchange labeling in conjunction with rapid mixing methods and two-dimensional NMR analysis. The time course of protection from exchange has been measured for 26 back-bone amide protons that form stable hydrogen bonds upon refolding and exchange slowly under native conditions. Amide protons in the beta-sheet and the alpha-helix, as well as protons involved in hydrogen bonds at the helix/sheet interface, become 80% protected in an initial 8-ms folding phase, indicating that the two elements of secondary structure form and associate in a common cooperative folding event. Somewhat slower protection rates for residues 59, 61, and 69 provide evidence for the subsequent stabilization of a surface loop. Most probes also exhibit two minor phases with time constants of about 100 ms and 10 s. Only two of the observed residues, Gln-41 and Arg-42, display significant slow folding phases, with amplitudes of 37% and 22%, respectively, which can be attributed to native-like folding intermediates containing cis peptide bonds for Pro-37 and/or Pro-38. Compared with other proteins studied by pulse labeling, including cytochrome c, ribonuclease, and barnase, the initial formation of hydrogen-bonded structure in ubiquitin occurs at a more rapid rate and slow-folding species are less prominent.
authors.affiliation: Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia 19104-6059.
authors.forename: M S
authors.initials: MS
authors.lastname: Briggs
authors.forename: H
authors.initials: H
authors.lastname: Roder
mesh_terms: D000595:Amino Acid Sequence; D002151:Calorimetry; D019791:Guanidine; D006146:Guanidines; D006860:Hydrogen Bonding; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008958:Models, Molecular; D011392:Proline; D011487:Protein Conformation; D011489:Protein Denaturation; D014452:Ubiquitins / Q000737:chemistry* / Q000378:metabolism
chemical_list: D006146:Guanidines; D014452:Ubiquitins; D011392:Proline; D019791:Guanidine