Abstract
The effects of chemical modifications, esterification or reductive alkylation, on binding properties of several terpenes by beta-lactoglobulin were examined. As shown by fluorescence quenching, beta-lactoglobulin and its derivatives bind beta-ionone, but it does not bind alpha-ionone, neither geraniol nor R(+)- and S(-)-limonene. It is suggested that its binding site, presumably situated within the calyx-shaped protein fold (North, A. C. T. Int. J. Biol. Macromol. 1989, 11, 56-58), has a narrow specificity to the structure formed by the conjugated double bonds of the beta-ionone ring and isoprenoid chain, present in both terpenes--beta-ionone and retinol. The complexes of beta-ionone with the derivatives of beta-lactoglobulin (except N-ethyllysyl-BLG) exhibit lower Kd' values than that of the complex of beta-ionone with unmodified beta-lactoglobulin. N-Methyllysyl-BLG and EtBLG are binding beta-ionone stronger than the other even more extensively modified derivatives as, for example, N-ethyllysyl-BLG and MetBLG. The partial loss of BLG beta-barrel structure achieved during extensive esterification producing MetBLG, deduced from the analysis of circular dichroism spectra, could explain its less effective binding to beta-ionone.
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