Abstract

The urea-induced unfolding reaction of the alpha subunit of tryptophan synthase was monitored by examining the chemical shifts and peak areas of the C epsilon protons of the four histidine residues with 1D NMR spectroscopy. In a native base-line region defined by tyrosine absorbance and far-UV circular dichroism spectroscopy, histidine-146 appears to undergo a rapid, local unfolding reaction at increasing denaturant concentrations. As the native form is converted to a previously detected stable intermediate between 2 and 3 M urea [Matthews, C. R., & Crisanti, M. M. (1981) Biochemistry 20, 784], histidines-92 and -146 in the amino folding unit (residues 1-188) and histidines-195 and -244 in the carboxy folding unit (residues 189-268) all experience a change in their environments which is slow on the NMR time scale. The subsequent conversion of this intermediate to a newly detected, stable, partially folded form populated at 5 M urea appears to have no effect on any of the histidines at 25 degrees C when an intermolecular association process involving His-244 is taken into account. Strikingly, a slow exchange process involving only His-92 is observed to begin at 5 M urea where the unfolding transitions monitored by absorbance or far-UV circular dichroism spectroscopy are essentially complete. This residual tertiary structure unfolds in a cooperative fashion as the urea concentration is increased to 8 M. Study holds ProTherm entries: 4639, 4640, 4641, 4642 Extra Details: additive : EDTA(0.2 mM),transtition is from native to intermediate chemical shifts; stable intermediate; intermolecular association ;,unfolding transitions

Submission Details

ID: pdjp3uBD

Submitter: Connie Wang

Submission Date: April 24, 2018, 8:26 p.m.

Version: 1

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