Protein denaturation due to the action of surfactants: a study by SAXS and ITC

Full text: Proteins are the major constituent of biological systems along with carbohydrates, lipids and nucleic acids (DNA and RNA). According to their structure and composition, proteins perform several functions in the organism, starting from the macroscopic level, with participation on the olfaction of animals, down to the cellular level, allocated in the membrane and making the connection between extra and intracellular environment. The function of a protein (which may be enzymatic, hormonal, structural, energetic, transport etc) is related to several factors including its structure (primary, secondary, tertiary or quaternary). Denaturation occurs when the secondary structure and/or tertiary is lost, which is almost always followed by the loss of the associated biological function. Temperature, pH and the action of surfactants influence the process of the denaturation. The influence of surfactants to the protein structure and function is the aim of this work. Therefore we are using an isolated protein, α-lactalbumin, that is found in the milk and whose function is related to the synthesis of galactose. The purpose is to characterize, in a thermodynamic-structural point of view, the denaturation of alpha-lactalbumin in the presence of surfactants anionic (sodium dodecyl sulfate - SDS), cationic (tetradecyltrimethylammonium bromide - TTAB), zwitterionic (2-diheptanoyl-sn-glycero-3- phosphocholine - DHPC) and nonionic (decyl-β-D-Maltopyranoside - DM). The isothermal titration calorimetry (ITC) technique, which provides information of structural changes from changes in energy, represents the starting point for the study, while the technique of small angle X-ray scattering (SAXS) provides information about the structural characteristics of surfactant-protein complexes formed at each step of the denaturation process. The data analysis is in the initial stage, but it was possible to obtain general parameters related to the complex formed from the correlation of both calorimetric and scattering data. As will be shown, each class of surfactant denatures protein in a particular way, as shown in the literature [1], providing complexes with different characteristics. The studies will provide the basis for a characterization of other protein-surfactant systems, in addition to future correlation of data obtained with those from other techniques such as circular dichroism (CD), nuclear magnetic resonance (NMR) and fluorescence spectroscopy, which might provide additional information for these systems. [1] D. E. Otzen et al. α-Lactalbumin is unfolded by all classes of surfactants but by different mechanisms. Journal of Colloidand Interface Science 329 (2009) 273 - 283. (author)