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 Ca(2+)-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 (Ca(2+)-free), while inducing a small, but significant, conformational change, does not cause the alpha-lactalbumin backbone conformation to change to that of the holo (Ca(2+)-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 Zn(2+)-binding site.(ABSTRACT TRUNCATED AT 250 WORDS)
Prestrelski
S J
SJ
Eastern Regional Research Center, U.S. Department of Agriculture, Philadelphia, Pennsylvania 19118.
Byler
D M
DM
Thompson
M P
MP
D000577:Amides / Q000737:chemistry; D000818:Animals; D002118:Calcium / Q000737:chemistry; D002417:Cattle; D005583:Fourier Analysis; D007768:Lactalbumin / Q000737:chemistry*; D008345:Manganese / Q000737:chemistry; D008670:Metals / Q000737:chemistry* / Q000494:pharmacology; D011485:Protein Binding / Q000187:drug effects; D011487:Protein Conformation / Q000187:drug effects; D012964:Sodium / Q000737:chemistry; D013055:Spectrophotometry, Infrared; D013329:Structure-Activity Relationship
D000577:Amides; D008670:Metals; D008345:Manganese; D007768:Lactalbumin; D012964:Sodium; D002118:Calcium