title: Molecular dissection of the folding mechanism of the alpha subunit of tryptophan synthase: an amino-terminal autonomous folding unit controls several rate-limiting steps in the folding of a single domain protein.
abstract: The alpha subunit of tryptophan synthase (alphaTS) from Escherichia coli is a 268-residue 8-stranded beta/alpha barrel protein. Two autonomous folding units, comprising the first six strands (residues 1-188) and the last two strands (residues 189-268), have been previously identified in this single structural domain protein by tryptic digestion [Higgins, W., Fairwell, T., and Miles, E. W. (1979) Biochemistry 18, 4827-4835]. The larger, amino-terminal fragment, alphaTS(1-188), was overexpressed and independently purified, and its equilibrium and kinetic folding properties were studied by absorbance, fluorescence, and near- and far-UV circular dichroism spectroscopies. The native state of the fragment unfolds cooperatively in an apparent two-state transition with a stability of 3.98 +/- 0.19 kcal mol(-1) in the absence of denaturant and a corresponding m value of 1.07 +/- 0.05 kcal mol(-1) M(-1). Similar to the full-length protein, the unfolding of the fragment shows two kinetic phases which arise from the presence of two discrete native state populations. Additionally, the fragment exhibits a significant burst phase in unfolding, indicating that a fraction of the folded state ensemble under native conditions has properties similar to those of the equilibrium intermediate populated at 3 M urea in full-length alphaTS. Refolding of alphaTS(1-188) is also complex, exhibiting two detectable kinetic phases and a burst phase that is complete within 5 ms. The two slowest isomerization phases observed in the refolding of the full-length protein are absent in the fragment, suggesting that these phases reflect contributions from the carboxy-terminal segment. The folding mechanism of alphaTS(1-188) appears to be a simplified version of the mechanism for the full-length protein [Bilsel, O., Zitzewitz, J. A., Bowers, K.E, and Matthews, C. R.(1999) Biochemistry 38, 1018-1029]. Four parallel channels in the full-length protein are reduced to a pair of channels that most likely reflect a cis/trans proline isomerization reaction in the amino-terminal fragment. The off- and on-pathway intermediates that exist for both full-length alphaTS and alphaTS(1-188) may reflect the preponderance of local interactions in the beta/alpha barrel motif.
authors.affiliation: Department of Chemistry, Life Sciences Consortium, Center for Biomolecular Structure and Function, The Pennsylvania State University, University Park 16802, USA.
authors.forename: J A
authors.initials: JA
authors.lastname: Zitzewitz
authors.forename: C R
authors.initials: CR
authors.lastname: Matthews
mesh_terms: D002942:Circular Dichroism; D004795:Enzyme Stability; D004926:Escherichia coli / Q000201:enzymology; D007700:Kinetics; D008958:Models, Molecular; D010446:Peptide Fragments / Q000737:chemistry* / Q000378:metabolism; D011489:Protein Denaturation; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D013329:Structure-Activity Relationship; D014367:Tryptophan Synthase / Q000737:chemistry* / Q000378:metabolism; D014461:Ultracentrifugation; D014508:Urea
chemical_list: D010446:Peptide Fragments; D014508:Urea; D014367:Tryptophan Synthase