Maltose binding protein (MBP) is a large, monomeric
two domain protein containing 370 amino acids. In the absence
of denaturant at neutral pH, the protein is in the native
state, while at pH 3.0 it forms a molten globule. The molten
globule lacks a tertiary circular dichroism signal but
has secondary structure similar to that of the native state.
The molten globule binds 8-anilino-1-naphthalene sulfonate
(ANS). The unfolding thermodynamics of MBP at both pHs
were measured by carrying out a series of isothermal urea
melts at temperatures ranging from 274–329 K. At
298 K, values of ΔG°, ΔCp,
and Cm were 3.1 ± 0.2 kcal mol−1,
5.9 ± 0.8 kcal mol−1 K−1
(15.9 cal (mol-residue)−1 K−1),
and 0.8 M, respectively, at pH 3.0 and 14.5 ± 0.4
kcal mol−1, 8.3 ± 0.7 kcal mol−1
K−1 (22.4 kcal (mol-residue)−1
K−1), and 3.3 M, respectively, at pH 7.1.
Guanidine hydrochloride denaturation at pH 7.1 gave values
of ΔG° and ΔCp
similar to those obtained with urea. The m values
for denaturation are strongly temperature dependent, in
contrast to what has been previously observed for small
globular proteins. The value of ΔCp
per mol-residue for the molten globule is comparable to
corresponding values of ΔCp for
the unfolding of typical globular proteins and suggests
that it is a highly ordered structure, unlike molten globules
of many small proteins. The value of ΔCp
per mol-residue for the unfolding of the native state is
among the highest currently known for any protein.