Alcohol-induced changes of beta-lactoglobulin-retinol-binding stoichiometry

It has been demonstrated using CD that ethanol induces important secondary structure changes of beta-lactoglobulin. CD spectra indicate that beta-lactoglobulin secondary structure, which is mainly composed of beta-strands, becomes mostly alpha-helical under the influence of the solvent polarity changes. The midpoint of beta-strand/alpha-helix transition in beta-lactoglobulin is observed at dielectric constant approximately 60 (35% ethanol; v/v). According to CD measurements, the ethanol-dependent secondary structure changes are reversible. The alkylation of lysines epsilon-NH2 in beta-lactoglobulin weakens the central beta-barrel structure, since the beta-strand/alpha-helix transition midpoint of alkylated beta-lactoglobulin is shifted to lower ethanol concentration (25% ethanol; v/v). beta-Lactoglobulin structural changes are triggering the dissociation of the beta-lactoglobulin-retinol complex as judged from complete quenching of its fluorescence in ethanol concentration greater than 30% (v/v). However, in 20% ethanol (v/v), beta-lactoglobulin still retains most of its native secondary structure as shown by CD and, in this condition, one beta-lactoglobulin molecule binds an additional second retinol molecule. This suggests that the highly populated species observed around 20% ethanol (v/v) might represent an intermediate state able to bind two molecules of retinol.