Structural and Functional Studies on the Interaction of Sodium Dodecyl Sulfate with β-Galactosidase

The effect of sodium dodecyl sulfate (SDS) on enzyme activity, electrophoretic behavior, and conformation of Escherichia coli β-galactosidase is presented. Fourier-transform infrared spectroscopy (FT-IR), previously used to study the structure of native β-galactosidase has been applied to examine the detergent effects on the enzyme. At 20°C, the presence of 1% SDS does not cause appreciable changes in the secondary structure, and enzyme activity is preserved; however, 10% SDS produces complete enzyme inactivation and FT-IR spectroscopy indicates a concomitant change in conformation. Thermal denaturation of β-galactosidase starts at ≈53°C in the absence and at ≈46°C in the presence of 1% SDS, indicating tertiary structure changes; also, a good correlation between structural (FT-IR) and functional (Arrhenius plots) data is observed. The secondary structure of thermally denatured β-galactosidase contains mainly extended structures, and intermolecular interactions produce protein aggregation. In the presence of 10% SDS, however, the hydrophobic segments of the protein are stabilized by SDS into helical structures without protein aggregation. At 30°C, in the presence of 1% SDS, two protein bands are resolved by gel electrophoresis, only one of them being active. A model for SDS-galactosidase interaction is proposed, according to which, at low surfactant concentrations, SDS molecules bind the outer surface of the protein, without affecting the protein core. Higher detergent concentrations produce a larger conformational change involving enzyme inactivation and increased accessibility of the solvent to the protein core. Increasing temperature in the presence of 10% SDS leads to a facilitated access of surfactant molecules to the inner protein regions and to an increase of the β-galactosidase α-helical content.