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Artefactual
intemediates and broad activation barriers in protein folding
Mikael Oliveberg, Lunds
University
Recent controversy about the role of populated intermediates in
protein folding emphasis the need to better characterise other
events
on the folding pathway. A complication is that these involve
high-energy states which are difficult to target experimentally
since
they do not accumulate kinetically. Here we explore the
energetics of
high-energy states, and map out the shape of the free-energy
profile
for folding of the two-state protein U1A. The analysis is based
on
non-linearities in the GdnHCl-dependence of the activation energy
for
unfolding, which we interpret in terms of structural changes of
the
protein-folding transition state. The result suggests that U1A
folds
by a high-energy pathway where most of the conformational search
takes
place isoenergetically at transition-state level. This is
manifested
in a very broad and flat activation barrier, the top of which
covers
more than 60 % of the reaction coordinate. The interpretation
favours
a folding mechanism where the pathway leading to the native
protein is
determined by the sequence's ability to stabilise productive
transition states.
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