Effect of metal ion binding on the secondary structure of bovine alpha-lactalbumin as examined by infrared spectroscopy

We have examined the influence of monovalent and divalent cations on the secondary structure of bovine alpha-lactalbumin at neutral pH using Fourier-transform infrared spectroscopy. Our present studies are based on previously reported amide I' component band assignments for this protein [Prestrelski, S. J., Byler, D. M., & Thompson, M. P. (1991) Int. J. Pept. Protein Res. 37, 508-512]. The results indicate that upon dissolution, alpha-lactalbumin undergoes a small, but significant, time-dependent conformational change, regardless of the ions present. Additionally, these studies provide the first quantitative measure of the well-known secondary structural change which accompanies calcium binding. Results indicate that removal of Ca2+ from holo alpha-lactalbumin results in local unfolding of the Ca2+-binding loop; the spectra indicate that approximately 16% of the backbone chain changes from a rigid coordination complex to an unordered loop. We have also examined the effects of binding of several other metal ions. Our studies have revealed that binding of Mn2+ to apo alpha-lactalbumin (Ca2+-free), while inducing a small, but significant, conformational change, does not cause the alpha-lactalbumin backbone conformation to change to that of the holo (Ca2+-bound) form as characterized by infrared spectroscopy. Similar changes to those induced by Mn2+ are observed upon binding of Na+ to apo alpha-lactalbumin, and furthermore, even at very high concentrations (0.2 M), Na+ does not stabilize a structure similar to the holo form. Binding of Zn2+ to the apo form of alpha-lactalbumin does not result in significant backbone conformational changes, suggesting a rigid Zn2+-binding site. Further, as characterized by infrared spectroscopy, binding of Zn2+ to holo alpha-lactalbumin does not induce a reversion to an apolike conformer. These results suggest that current models of the metal-binding properties of alpha-lactalbumin need to be carefully reexamined in light of these findings.