Enzyme activity and cation exchange as tools for the study of the conformation of proteins adsorbed on mineral surfaces

Abstract

The modification of the conformation of proteins adsorbed on different mineral surfaces was followed indirectly using two methods. Firstly, the effect of the pH on enzyme activity of two β-D-glucosidases on different mineral surfaces was studied; a diminished activity would indicate a departure from the active conformation. The mineral surfaces were chosen for their electrical charge and hydrophobicity/hydrophilicity (montmorillonite, talc, goethite). Secondly, the exchange of paramagnetic charge compensating cations on adsorption of bovine serum albumin was studied by an NMR method and the specific interfacial area occupied by the protein on the clay surface was deduced.

The results with montmorillonite showed that below the i.e.p. the proteins were adsorbed up to the saturation level of the surface and adsorbed proteins unfolded; above the i.e.p. the surface coverage decreased and there was no major change in the conformation. In addition to these electrostatic interactions, the comparison between the results obtained with goethite and talc showed that hydrophobic interactions can also induce conformational changes.