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

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] Nb where [SDS]aq and Nb 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 Nb ∼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.