1H, 13C and 15N resonance assignments for human all-Ala α-lactalbumin in its molten globule and urea-denatured states

Molten globules are compact, partially-folded proteins postulated to be general intermediates in protein folding. Human α-lactalbumin (α-LA) is a Ca²⁺-binding, four-disulphide protein whose native structure is divided into two lobes, one is largely helical, the α-domain, and the other has a significant β-sheet content, the β-domain. α-LA forms a "classical" molten globule at low pH which has been studied widely as a model system of a partially-folded protein. The α-LA molten globule is compact and has a native-like helical secondary structure content. All-Ala α-LA, which has all eight native cysteines mutated to alanine, also adopts a partially-folded molten globule conformation and gives a high-quality ¹H-¹⁵N HSQC spectrum at pH 2 and 40 °C. The lack of cysteine residues makes all-Ala α-LA a suitable template for spin-labelling studies. In this report we present ¹H, ¹³C and ¹⁵N assignments for human all-Ala α-LA in its molten globule and 8 M urea-denatured states. Analysis of the chemical shift data for the molten globule state shows they are consistent with high populations of conformations in the α region of φ,ψ space for residues in the α domain of the protein. In contrast, the data for the urea-denatured state are closely similar to those expected for a random coil.