Analysis of protein–surfactant interactions—a titration calorimetric and fluorescence spectroscopic investigation of interactions between Humicola insolens cutinase and an anionic surfactant

Abstract

We have studied interactions of cutinase (HiC) from Humicula insolens and sodium dodecyl sulphate (SDS) by parallel calorimetric and fluorescence investigations of systems in which the concentration of both components was changed systematically. Results from the two methods exhibit a number of synchronous characteristics, when plotted against the total SDS concentration, [SDS] tot. The molecular origin of several of these anomalies was assigned, and five intervals of [SDS] tot in which different modes of interactions dominated were identified. Going from low to high [SDS] tot, these modes were: binding of (a few) SDS to native HiC, formation of oligomeric protein aggregates, denaturation of HiC and adsorption of SDS on denatured protein. For [SDS] tot > 3–6 mM (depending on the protein concentration), the adsorption saturated, and no further protein–detergent interaction could be detected. Two particularly conspicuous anomalies in the calorimetric data were ascribed to respectively denaturation and saturation. It was found that [SDS] tot at these points depended linearly on the (total) protein concentration, [HiC]. We suggest that this reflects the balance between bound and free SDS [SDS] tot = [SDS] aq + [HiC] N b where [SDS] aq and N b are, respectively, the aqueous (“free”) concentration of SDS and the average number of SDS bound per protein. Interpretation of the results along these lines showed that at 22 °C and pH 7.0, HiC denatures with ∼14 bound surfactant molecules at [SDS] aq = 1.0 mM. Saturation is characterized by N b ∼39 and [SDS] aq = 2.2 mM. The latter value is equal to CMC in the (protein free) buffer. These results are discussed with respect to the SDS-binding capacity of HiC and the origin and location of the saturation point.