Pressure-induced unfolding of the molten globule of all-Ala α-lactalbumin

Pressure-induced unfolding of a molten globule (MG) was studied in a residue-specific manner with 1H-15N two-dimensional NMR spectroscopy using a variant of human α-lactalbumin (α-LA), in which all eight cysteines had been replaced with alanines (all-Ala α-LA). The NMR spectrum underwent a series of changes from 30 to 2000 bar at 20°C and from −18°C to 36°C at 2000 bar, showing a highly heterogeneous unfolding pattern according to the secondary structural elements of the native structure. Unfolding began in the loop part of the β-domain, and then extended to the remainder of the β-domain, after which the α-domain began to unfold. Within the α-domain, the pressure stability decreased in the order: D-helix ∼ 310-helix > C-helix ∼ B-helix > A-helix. The D-helix, C-terminal 310-helix and a large part of B- and C-helices did not unfold at 2000 bar, even at 36°C or at −18°C. The results verify that the MG state consists of a mixture of variously unfolded conformers from the mostly folded to the nearly totally unfolded that differ in stability and partial molar volume. Not only heat but also cold denaturation was observed, supporting the view that the MG state is stabilized by hydrophobic interactions.