title: Apparent radii of the native, stable intermediates and unfolded conformers of the alpha-subunit of tryptophan synthase from E. coli, a TIM barrel protein.
abstract: The urea-induced equilibrium unfolding of the alpha-subunit of tryptophan synthase (alphaTS) from Escherichia coli can be described by a four-state model, N right harpoon over left harpoon I1 right harpoon over left harpoon I2 right harpoon over left harpoon U, involving two highly populated intermediates, I1 and I2 [Gualfetti, P. J., Bilsel, O., and Matthews, C. R. (1999) Protein Sci. 8, 1623-1635]. To extend the physical characterization of these stable forms, the apparent radius was measured by several techniques. Size-exclusion chromatography (SEC), analytical ultracentrifugation (UC), and dynamic light scattering (DLS) experiments yield an apparent Stokes radius, R(s), of approximately 24 A for the native state of alphaTS. The small-angle X-ray scattering (SAXS) experiment yields a radius of gyration, R(g), of 19.1 A, consistent with the value predicted from the X-ray structure and the Stokes radius. As the equilibrium is shifted to favor I1 at approximately 3.2 M and I2 at 5.0 M urea, SEC and UC show that R(s) increases from approximately 38 to approximately 52 A. Measurements of the radius by DLS and SAXS between 2 and 4.5 M urea were complicated by the self-association of the I1 species at the relatively high concentrations required by those techniques. Above 6 M urea, SEC and UC reveal that R(s) increases linearly with increasing urea concentration to approximately 54 A at 8 M urea. The measurements of R(s) by DLS and R(g) by SAXS are sufficiently imprecise that both values appear to be identical for the I2 and U states and, considering the errors, are in good agreement with the results from SEC and UC. Thermodynamic parameters extracted from the SEC data for the N right harpoon over left harpoon I1 and I1 right harpoon over left harpoon I2 transitions agree with those from the optical data, showing that this technique accurately monitors a part of the equilibrium model. The lack of sensitivity to the I2 right harpoon over left harpoon U transition, beyond a simple swelling of both species with increasing urea concentration, implies that the Stokes radii for the I2 and U states are not distinguishable. Surprisingly, the hydrophobic core known to stabilize I2 at 5.0 M urea [Saab-Rincón, G., Gualfetti, P. J., and Matthews, C. R. (1996) Biochemistry 35, 1988-1994] develops without a significant contraction of the polypeptide, i.e., beyond that experienced by the unfolded form at decreasing urea concentrations. Kratky plots of the SAXS data, however, reveal that I2, similar to N and I1, has a globular structure while U has a more random coil-like form. By contrast, the formation of substantial secondary structure and the burial of aromatic side chains in I1 and, eventually, N are accompanied by substantial decreases in their Stokes radii and, presumably, the size of their respective conformational ensembles.
authors.affiliation: Department of Chemistry, Center for Biomolecular Structure and Function, The Pennsylvania State University, University Park 16802, USA.
authors.forename: P J
authors.initials: PJ
authors.lastname: Gualfetti
authors.forename: M
authors.initials: M
authors.lastname: Iwakura
authors.forename: J C
authors.initials: JC
authors.lastname: Lee
authors.forename: H
authors.initials: H
authors.lastname: Kihara
authors.forename: O
authors.initials: O
authors.lastname: Bilsel
authors.forename: J A
authors.initials: JA
authors.lastname: Zitzewitz
authors.forename: C R
authors.initials: CR
authors.lastname: Matthews
mesh_terms: D002850:Chromatography, Gel; D004926:Escherichia coli / Q000201:enzymology*; D008027:Light; D008958:Models, Molecular; D010446:Peptide Fragments / Q000737:chemistry*; D011487:Protein Conformation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012542:Scattering, Radiation; D013816:Thermodynamics; D014367:Tryptophan Synthase / Q000737:chemistry*; D014461:Ultracentrifugation; D014965:X-Rays
chemical_list: D010446:Peptide Fragments; D014367:Tryptophan Synthase