A residue-specific NMR view of the non-cooperative unfolding of a molten globule.

Molten globules are partially folded forms of proteins that are thought to be general intermediates in protein folding. Nonetheless, there is limited structural information about such species because they possess conformational heterogeneity and complex dynamical properties that lead to extreme line broadening in NMR spectra. Here we use a 2-D NMR approach that overcomes this difficulty by detecting the unfolding of individual residues in a molten globule in increasing concentrations of denaturant. The results show that the structure in the low pH form of alpha-lactalbumin (alpha-LA) is not formed cooperatively. Moreover, a core region remains collapsed under extremely denaturing conditions, even when the majority of the polypeptide chain is completely unfolded. Our results support a model for protein folding in which the core provides a template for correct assembly of the remainder of the structure.