Abstract
Abstract Mixed disulphide derivatives of bovine β-lactoglobulin B (BLG) were studied by circular dichroism (CD), gel-permeation HPLC and high-sensitivity differential scanning calorimetry (HSDSC). It was shown that modification of Cys121 of BLG results only in tertiary and quaternary structure changes. At neutral pH, the equilibrium dimer–monomer of modified BLG is shifted to monomeric form. Thermal denaturation of modified BLG is reversible both at neutral and acidic pH. The values of ΔG (37°C) for modified and unmodified BLG indicate substantial destabilisation of tertiary structure of the protein by modification of Cys121. The effect of destabilisation is of the same order of magnitude for disulphide derivative containing charged and neutral group. The nature of the stabilising role of Cys121 for tertiary structure of BLG is discussed. Activation of Cys121 by thermic treatment in milk is known to induce the disulphide bond formation of BLG with κ-casein. Its destabilisation by high pressure in β-lactoglobulin/α-lactalbumin (ALA) mixtures induce oligomerisation of ALA, triggering thiol ⇔ disulfide exchange reactions.
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