Denaturant mediated unfolding of both native and molten globule states of maltose binding protein are accompanied by large ΔCp's

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Abstract
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 plusmn; 0.2 kcal mol−1, 5.9 plusmn; 0.8 kcal mol−1 K−1 (15.9 cal (mol‐residue)−1 K−1), and 0.8 M, respectively, at pH3.0 and 14.5 plusmn; 0.4 kcal mol−1, 8.3 plusmn; 0.7 kcalmol−1 K−1 (22.4kcal (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.