Structure-function relationship of beta-lactoglobulin in the presence of dodecyltrimethyl ammonium bromide.

Bovine beta-lactoglobulin (beta-LG) present in milks has been found "in vivo" in complexes with lipids such as butyric and oleic acids. To elucidate the still unknown structure-function relationship in this protein, the structural changes of beta-lactoglobulin variant A (beta-LG A) in the presence of cationic surfactant such as dodecyltrimethyl ammonium bromide (DTAB) have been investigated using various experimental techniques such as UV-vis spectrophotometry, fluorimetry, isothermal titration calorimetry (ITC) and circular dichroism (CD). Subsequently, the retinol binding by beta-LG has been investigated in the presence of various amounts of this surfactant as its extrinsic functional binding fluorophore. Comparison of the results allowed to determine the binding of retinol by beta-LG in the presence of DTAB. The results of UV-vis and fluorescence studies showed a red shift in wavelength and an increase in absorbance and enhancement in the intensity of the quantum yield of protein during its interaction with DTAB. The results of UV-vis also showed two distinct conformational changes corresponding first to precipitation and second to solubilization of the precipitated beta-LG at pH 6.7 and 8.0. The results indicate the cooperative character of binding at pH 2.0. The results of fluorescence studies showed that the binding strength of beta-LG/DTAB complex increases with the increase of the pH. CD results showed the shifts in positions of the major minima and change in magnitude of ellipticity and subsequently signified two significant changes in structure of beta-LG between 10-30 and 50-100 molar ratio of [DTAB]/[beta-LG]. ITC measurements indicated the endothermic nature of beta-LG/DTAB interactions at pH 6.7 and the exothermic nature of beta-LG/DTAB interactions at pH 8.0. The analysis of the binding data demonstrates the absence of significant changes in retinol-binding properties of beta-LG in the presence of various amounts of this surfactant. This implies that surfactant binding does not change the conformation of beta-LG in the regions defining retinol-binding site nor interferes with retinol binding by a competition for the same binding site(s).