Interpretation of the pH dependence of protein adsorption on clay mineral surfaces and its relevance to the understanding of extracellular enzyme activity in soil

Abstract

The pH dependence of the adsorption of bovine serum albumin (BSA) and Aspergillus niger β- d-glucosidase on montmorillonite and of its structural consequences was studied by several methods in order to understand the mechanisms of interaction between extracellular enzymes and clay minerals in soils. The relative influence of electrostatic and hydrophobic interactions on adsorption was deduced from experiments involving the coadsorption of BSA and methylated BSA. Also, the surface coverage of the clay by the BSA was studied by following the release of a paramagnetic charge-compensating cation on adsorption of the protein. This method is based on the line broadening effect of the released cation on the 31P NMR signal from phosphate in the solution, and the specific interfacial area of the BSA was deduced from the ratio of the surface covered by the quantity of protein adsorbed. Information on the secondary and overall structure of adsorbed BSA was obtained by Fourier transform IR spectroscopy from the frequency range and line width of the amide I/I′ signal and from the intensity of the COOH band. Finally, the catalytic activity of the β- d-glucosidase adsorbed on the clay was compared with its activity in solution, and the pH dependence of the adsorption was measured. The following general conclusions could be drawn from these experiments. (i) Below the isoelectric point (IEP), proteins unfold on the clay surfaces in response to electrostatic attractive interactions, a phenomenon which inhibits enzyme activity. (ii) Near the IEP proteins are adsorbed with little modification of conformation and thus enzymes preserve their catalytic activities. (iii) Above the IEP the proportion of adsorbed proteins decreases due to electrostatic repulsive interactions, permitting the diffusion of enzymes in the liquid-filled pore network of the soil.