Divergent effects of anionic surfactants on laundry enzymes: Structural stability, activity modulation, and self-cleavage mechanisms

Research on interactions between surfactants and proteins typically studies protein denaturation. Resistance to surfactant-induced denaturation is exploited in the use of detergent enzymes which are commonly used with petrochemically derived anionic surfactants. The basis for this resistance remains unclear but has great technological relevance. To address this, we use different biophysical techniques: circular dichroism, enzymatic activity assays, isothermal titration calorimetry, and small-angle X-ray scattering, to provide a detailed and global description of how surfactants impact protein structure and function. We study three laundry-relevant enzymes, an α-amylase, a cellulase, and a protease, which are combined with the model anionic surfactant sodium dodecyl sulphate as well as three anionic biosurfactants (rhamnolipid, sophorolipid, and surfactin). Our studies uncover a wide range of interactions and effects. As expected, lack of measurable interactions between all surfactants and the amylase translates to lack of effect on enzymatic activity. However, enzymatic activity is enhanced by surfactant in cases involving an insoluble substrate (cellulose) or conformational changes (protease). This last effect occurs in parallel with protease self-cleavage as observed by the appearance of small fragments and significant changes in scattering profiles. These processes rationalize previous reports of detergent-mediated inactivation of alkaline proteases, a main component in detergent formulations.