Dissociation, aggregation and denaturation of sesame alpha-globulin in urea and guanidine hydrochloride solutions

The effect of urea and GuHCl on the major protein of sesame seed (Sesamum indicum L.), alpha-globulin, has been investigated by turbidity, sedimentation velocity, viscosity, difference spectra and fluorescence spectral measurements. The protein undergoes dissociation, aggregation and denaturation in the presence of the above denaturants. There is a critical concentration of the denaturant where aggregation is maximum. Both denaturation and aggregation are lower in buffers of high ionic strength. Dissociation and aggregation have been explained by considering two types of subunits present in the protein molecule, one leading to smaller sedimenting component and the other producing the aggregate. The amino acid analysis shows that the aggregated fraction is rich in aliphatic amino acid residues. The endothermic nature of the aggregation process has been considered to arise from hydrophobic interaction of aliphatic side chains of the relevant subunits. The protein exists in a more denatured state in GuHCl than in urea solution.