Calcium binding peptides from α-lactalbumin: Implications for protein folding and stability

The calcium binding protein α-lactalbumin folds via a molten globule intermediate. Calcium does not bind strongly to the unfolded protein or the molten globule, but does bind to the transition state between the molten globule and the native protein. Of interest are the structures formed in the transition state that promote calcium binding. To study the importance of local secondary structure on calcium binding, we have synthesized two peptides corresponding to the calcium binding site that include the flanking C-helix and 3₁₀-helix. The first peptide, elbow-A, consists of residues 72- 100 from bovine α-lactalbumin, but with Cys 73, Cys 77, and Cys 91 replaced by alanines. In the second peptide, denoted elbow, the cysteines at position 73 and 91 are included and the nativelike disulfide bond is formed. Both peptides are monomeric and unstructured in aqueous solution and bind calcium weakly with apparent K(d)'s on the order of 10^-2 M. In 50% trifluoroethanol (v/v), the peptides are 45% helical as judged by CD. NMR studies performed on elbow and elbow-A in TFE indicate that the helical structure is confined to the C-helix. In this solvent system elbow binds calcium one-to-one with a K(d) of 50 μM. Removing the disulfide bond reduces, but does not eliminate calcium binding (K(d) = 170 μM in 50% TFE). These results suggest that formation of the C-helix promotes calcium binding and may be a key determinant of calcium binding in the transition state.