High helicity of peptide fragments corresponding to β-strand regions of β-lactoglobulin observed by 2D-NMR spectroscopy

Abstract

Whereas protein fragments, when they are structured, adopt conformations similar to that found in the native state, the high helical propensity of beta-lactoglobulin, a predominantly beta-sheet protein, suggested that the fragments of beta-lactoglobulin can assume the non-native helical conformation. In order to assess this possibility, we synthesized four 17-18-residue peptides corresponding to three beta-strand regions and one helical region (as a control) of beta-lactoglobulin and examined their conformation. We observed residual helicities of up to 17% in water, by far-UV CD, for all four peptide fragments. The helices could be significantly stabilized by the addition of TFE, and the NMR analyses in a mixture of 50% water/TFE indicated that helical structures are formed in the central region whereas both termini are frayed. Thus, the very same residues that form strands in the native beta-lactoglobulin showed high helical preferences. These results stand out from the current general view that peptide fragments isolated from proteins either are unfolded or adopt native-like secondary structures. The implications of the results in the mechanism of protein folding and in designing proteins and peptides are significant.