Conformational changes of alpha-lactalbumin and its fragment, Phe31-Ile59, induced by sodium dodecyl sulfate

Conformational changes of bovine alpha-lactalbumin in sodium dodecyl sulfate (SDS) solution were studied with the circular dichroism (CD) method using a dilute phosphate buffer of pH 7.0 and ionic strength 0.014. The proportions of alpha-helix and beta-structure in alpha-lactalbumin were 34% and 12%, respectively, in the absence of SDS. In the SDS solution, the helicity increased to 44%, while the beta-structure disappeared. In order to verify the structural change from beta-structure to alpha-helix, the moiety, assuming the beta-structure in the alpha-lactalbumin, was isolated by a chymotryptic digestion. The structure of this alpha-lactalbumin fragment, Phe31-Ile59, was almost disordered. However, the fragment adopted a considerable amount of alpha-helical structure in the SDS solution. On the other hand, the tertiary structure of alpha-lactalbumin, detected by changes of CD in the near-ultraviolet region, began to be disrupted before the secondary structural change in the surfactant solution. Dodecyl sulfate ions of 80 mol were cooperatively bound to alpha-lactalbumin. Although the removal of the bound dodecyl sulfate ions was tried by the dialysis against the phosphate buffer for 5 days, 4 mol dodecyl sulfates remained per mole of the protein. The remaining amount agreed with the number of stoichiometric binding site, determined by the Scatchard plot, indicating that the stoichiometric binding was so tight.