Dimerization, stability and electrostatic properties of porcine beta-lactoglobulin

The study of homologous proteins belonging to the same family can provide a rationale for important molecular properties such as oligomer formation, f olding mechanism and mode of binding. We report here a physico-chemical cha racterization of porcine beta -lactoglobulin, purified from pooled milk: si ze-exclusion chromatography, CD and NMR measurements were used to study the aggregation and stability of this protein. In spite of the high sequence i dentity and homology of porcine beta -lactoglobulin with the widely studied bovine species, the two proteins exhibit very different behaviours. The po rcine protein shows a monomer-dimer equilibrium with a pH dependence opposi te to that observed for the bovine species. Unfolding experiments revealed the presence of an intermediate that probably has excess alpha helices, as reported for equine species. Modelling studies were performed on bovine, po rcine and equine proteins, and, interestingly, electrostatic surface potent ial calculations led to results consistent with the different dimer interfa ce found for porcine beta -lactoglobulin in the crystal structure. Interact ion studies revealed that porcine beta -lactoglobulin is unable to bind fat ty acids at any pH, thus questioning the main functional role proposed for lactoglobulins as fatty acid transporters or solubilizers.