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. Study holds ProTherm entries: 23835, 23836, 23837, 23838, 23839, 23840 Extra Details: Tryptophan synthase alpha-subunit (1-188), Absorbance at 287 nm, Added additives: 1) 0.2 mM K2EDTA, 2) 1mM beta-mercaptoethanol, State:Folding to unfolding folding mechanism, amino-terminal fragment, tryptophan synthase alpha-subunit

Submission Details

ID: AQ8pKzWm3

Submitter: Connie Wang

Submission Date: April 24, 2018, 8:54 p.m.

Version: 1

Publication Details
Zitzewitz JA;Matthews CR,Biochemistry (1999) 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. PMID:10433729
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