Activité enzymatique extracellulaire dans Ie sol: effet du pH et de la force ionique sur l'interaction avec la montmorillonite de deux phosphates acides sécrdteés par le champignon ectomycorhizien Hebeloma cylindrosporum

Summary The interaction of extracellular enzymes with the solid phase of the soil affects their mobility and their catalytic properties. In particular, adsorption on clay minerals is known to shift the optimum pH of the catalytic activity towards alkaline values. Two conflicting interpretations of this phenomenon have been proposed: a surface pH effect and a pH–dependent modification of the conformation of the adsorbed enzyme. To assess the two mechanisms, we studied the adsorption on montmorillonite of two extracellular acid phosphatases of the ectomycorrhizal fungus Hebeloma cylindrosporum , and its consequences on catalytic activities. The results obtained are better interpreted by a pH–dependent modification of enzyme conformation due mainly to electrostatic interactions with the clay surface. At low pH, the two positively charged enzymes unfold on the negatively charged montmorillonite surface. At high pH, both the enzymes and the clay are negatively charged, and adsorption decreases. Adsorption and modification of conformation are largely irreversible, which should be taken into account when considering the fate of enzymes in soil. Finally, the comparison with the effect of clays on the catalytic activities of intracellular enzymes raises the hypothesis of a selection pressure of the soil solid phase leading to more stable extracellular enzymes.