Secondary structural changes in the intact and the disulfide bridges cleaved beta-lactoglobulin A and B in solutions of urea, guanidine hydrochloride, and sodium dodecyl sulfate

Abstract

The relative proportions of alpha-helix, beta-sheet, and unordered form in beta-lactoglobulin A and B were examined in solutions of urea, guanidine, and sodium dodecyl sulfate (SDS). In the curve-fitting method of circular dichroism (CD) spectra, the reference spectra of the corresponding structures determined by Chen et al. (1974) were modified essentially according to the secondary structure of beta-lactoglobulin B predicted by Creamer et al. (1983), i.e., that the protein has 17% alpha-helix and 41% beta-sheet. The two variants showed no appreciable difference in structural changes. The reduction of disulfide bridges in the proteins increased beta-sheet up to 48% but did not affect the alpha-helical proportion. The alpha-helical proportions of nonreduced beta-lactoglobulin A and B were not affected below 2 M guanidine or below 3 M urea, but those of the reduced proteins began to decrease in much lower concentrations of these denaturants. By contrast, the alpha-helical proportions of the nonreduced and reduced proteins increased to 40-44% in SDS. The beta-sheet proportions of both nonreduced and reduced proteins which remained unaffected even in 6 M guanidine and 9 M urea, decreased to 24-25% in SDS.