bi9805083_si_001.pdf (244.63 kB)
Proline Isomerization-Independent Accumulation of an Early Intermediate and Heterogeneity of the Folding Pathways of a Mixed α/β Protein, Escherichia coli Thioredoxin†
journal contribution
posted on 1998-06-20, 00:00 authored by Roxana E. Georgescu, Jian-Hua Li, Michel E. Goldberg, Maria Luisa Tasayco, Alain F. ChaffotteOxidized Escherichia coli thioredoxin (Trx) is a small protein of 108 residues with one disulfide
bond (C32−C35 essentially involved in the activity) and no prosthetic moieties, which folds into a structural
motif containing a central twisted β-sheet flanked by helices that is found in many larger proteins. The
kinetics of refolding of Trx in vitro have been investigated using a newly developed active site titration
assay and continuous or stopped-flow (SF) methods in conjunction with circular dichroism (CD) and
fluorescence (Fl) spectroscopy. These studies revealed the presence of early folding intermediates with
“molten globule or pre-molten globule” characteristics. Measurements of the ellipticity at 222 nm indicated
that about 68% of the total change associated with refolding occurred during the dead time (4 ms) of the
stopped-flow instrument, suggesting the formation of substantial secondary structure. The reconstruction
of the far-UV CD spectrum of the burst intermediate using combined continuous and stopped-flow methods
showed the formation of a defined secondary structure that contains more β-structure than the native
state. Kinetic measurements using SF far-UV CD and Fl over a wide range (0.087−6 M) of GuHCl
concentrations at two temperatures (6 and 20 °C) demonstrated that the population formed during the 4
ms dead time contained multiple species that are stabilized mainly by hydrophobic interactions and undergo
further folding along alternative pathways. One of these species leads directly and rapidly to the native
state as demonstrated by active site titration, while the two others fold into a fourth intermediate that is
slowly converted to the native protein. Double-jump experiments suggest that the heterogeneity in folding
behavior results from proline isomerizations occurring in the unfolded state. Conversely, the accumulation
of the burst intermediate does not depend on proline isomerizations.