Abstract

Many food proteins have multiple subunits. Beta-lactoglobulin (beta-1g) dimer was assessed for its resistance to subunit dissociation and unfolding in 0-8 M urea. Equilibrium denaturation profiles were monitored by ultraviolet difference spectrophotometry. The results were analysed by a new dissociation coupled unfolding (DCU) model. The Gibbs free energy change for denaturing beta-1g (Delta G(DCU)(0)) was 60 (+/- 2.3) kJ mol(-1) at pH 7 and 72 (+/- 1.6) kJ mol(-1) at pH 2.6. By comparison, the dissociation free energy for beta-1g dimer (Delta G(D)) is 26.0 kJ mol(-1) at pH 7 and 22.6 kJ mol(-1) at pH 2.6. Hence, dimerization accounts for 43% (pH 7) and 32% (pH 2.6) of the stability of native beta-1g dimer. Such results indicate a 3-state denaturation process in urea with native beta-1g monomer acting as a stable intermediate during DCU. The issues raised by a study of beta-1g may be relevant to other multisubunit proteins in food systems.