A comparative study of the α-subdomains of bovine and human α-lactalbumin reveals key differences that correlate with molten globule stability

The α-lactalbumins form stable molten globule states under a range of conditions, with the low pH form being the best characterized. The stability of the molten globule varies among different members of this family, but the origin of the stability difference is not clear. We compare the folding and stability of α-subdomain constructs of human and bovine α-lactalbumin. Previous studies have demonstrated that the isolated α-subdomain of human α-lactalbumin folds and forms a molten globule state. The minimum core construct has been defined to include the A, B, and D α-helices and the C-terminal 3₁₀ helix. A construct corresponding to the same region of bovine α-lactalbumin is much less structured and less stable than the human α-lactalbumin construct. Addition of the C-helix to generate a 75-residue bovine construct does not lead to a significant increase in structure or stability. This construct (AB-CD/3₁₀) contains the entire α-subdomain of bovine α-lactalbumin. Thus molten globule formation in the human protein, but not in the bovine protein, can be rationalized on the basis of a stable α-subdomain. Interactions involving more of the protein chain are required to generate a well structured molten globule in the bovine protein. Comparison of AB-CD/3₁₀ to the molten globule formed by the intact protein and to the protein with the 6-120 disulfide reduced indicates that both the β-subdomain and the 6-120 disulfide play a role in stabilizing the bovine α-lactalbumin molten globule.