Molten globule state of equine β-Lactoglobulin

The acid-unfolded state of equine β-lactoglobulin was characterized by means of circular dichroism, nuclear magnetic resonance, analytical gel-filtration chromatography, and analytical centrifugation. The acid-unfolded state of equine β-lactoglobulin has a substantial secondary structure as shown by the far-ultraviolet circular dichroism spectrum but lacks persistent tertiary packing of the side chains as indicated by the near-ultraviolet circular dichroism and nuclear magnetic resonance spectra. It is nearly as compact as the native conformation as shown by the gel filtration and sedimentation experiments, and it has the exposed hydrophobic surface as indicated by its tendency to aggregate. All of these characteristics indicate that the acid-unfolded state of equine β-lactoglobulin is a molten globule state. The α helix content in the acid-unfolded state, which has been estimated from the circular dichroism spectrum, is larger than that in the native state, suggesting the presence of nonnative α helices in the molten globule state. This result suggests the generality of the intermediate with nonnative α helices during the folding of proteins having the β-clam fold. © 1997 Wiley-Liss Inc.