Structural stability of beta-globulin, the low molecular weight protein fraction from sesame seed (Sesamum indicum...

Beta-globulin, a single polypeptide chain of molecular weight 15,000 +/- 1,000, undergoes denaturation in alkaline pH (7.0-13.0), thereby affecting the hydrodynamic properties of the protein, viz. a decrease in sedimentation coefficient from a value of 2.0s to 1.4s at pH 11.3, an increase in reduced viscosity from 0.042 dl/g to 0.158 dl/g at pH 12.6 and a decrease in partial specific volume resulting in a volume change of 6.3 +/- 1.0 ml/mole residue at pH 11.7. The perturbation of tryptophanyl residues and ionization of tyrosyl residues are preceded by alteration in conformational status of the protein. The fluorescence emission measurements indicate initial unfolding of the protein molecule which exposes the tryptophan and tyrosyl residues to the solvent. The tyrosyl phenolic group ionization is anomalous having a pKint value of 11.2. The reduced viscosity value reaches a plateau region at pH 12.5.