Helix-rich transient and equilibrium intermediates of equine β-lactoglobulin in alkaline buffer

Abstract

Acidic buffer conditions are known to stabilize helix-rich states of even those proteins with a predominantly β-sheet native secondary structure. Here we investigated whether such states also exist under alkaline buffer conditions. The guanidine hydrochloride (GuHCl)-induced unfolding transition and kinetic refolding of equine β-lactoglobulin (ELG) by GuHCl-jump were investigated at pH 8.7 by far-ultraviolet circular dichroism. We found that an equilibrium intermediate appeared in 45% ethylene glycol (EGOH) buffer with 1.5 M GuHCl. The intermediate is rich in non-native α-helix, which is similar to the helix-rich state of ELG at pH 4.0. A kinetic study was done on the folding rate of ELG and compared with bovine β-lactoglobulin (BLG). Transient intermediates, which were observed as the burst phase of the refolding reaction, were also rich in α-helix. The activation enthalpy of ELG was calculated to be c.a. 80 kJ/mol, whereas that of BLG was c.a. 70 kJ/mol in the presence of 45% EGOH. The ellipticities of the transient intermediate of ELG show temperature dependence in the presence of 45% EGOH, whereas that of BLG did not show significant dependence. This study therefore extends the existence of helix-rich equilibrium and transient intermediates of predominantly β-sheet proteins to alkaline buffer conditions.