Hydrophilic and hydrophobic fractions of extracellular fungal phosphatases interact differently with clay mineral surfaces

Abstract

The interaction of enzymes with soil organo-mineral surfaces determines their mobility, and hence zone of influence, and their catalytic activity. Two extracellular fungal phosphatases were purified and components separated using hydrophobic interaction chromatography. Affinity for montmorillonite and kaolinite surfaces and activity on these mineral surfaces of untreated samples and purified fractions were measured. The pH trends of catalytic activity in solution differed between fungi but were little affected by purification. The hydrophobic and hydrophilic fractions behaved differently in the presence of mineral surfaces. Adsorption resulted from both electrostatic and hydrophobic interactions with the mineral surfaces. The hydrophilic fraction of Hebeloma phosphatase had a much greater affinity for both clay surfaces than the hydrophobic fraction, which demonstrates the importance of electrostatic interactions. In contrast, the hydrophobic fraction of Suillus phosphatases had a greater affinity for both clay surfaces than the hydrophilic one. Competition with other proteins present reduced adsorption. The catalytic activity of Hebeloma phosphatases was well conserved on kaolinite over the pH range studied. However in contact with montmorillonite, hydrophilic phosphatase retained more activity than the hydrophobic fraction. For both hydrophilic and hydrophobic fractions more activity was lost at pH below the optimum pH when the enzyme would be positively charged. The specific catalytic activity of the hydrophobic fractions of Suillus phosphatase was greater after adsorption on both mineral surfaces than in initial solution. This was attributed to the presence of an inhibitor in solution, whose effect was masked in the presence of a competing protein, and not to surface enhancement of catalytic activity.