Reduced catalytic activity of an exogenous extracellular β-D-glucosidase due to adsorption on a model humic-clay complex and different soils under wetting and drying cycles

Abstract

To ascertain the role of extracellular enzymes in soil biochemical reactions, we followed the changes in catalytic activity of an exogenous β-glucosidase (GLU) enzyme after its adsorption on a synthetic model humic-clay complex, composed by a lignite humic acid coupled by Al bridges to a Ca-montmorillonite (HM), and on three sterilised soils (DS, ISC and IST) with different properties. Either HM or the selected soils enabled a stable GLU adsorption that induced a significant decrease of GLU activity. In the case of soils, both the largest GLU adsorption and reduction of catalytic activity was observed for the clayey and organic matter-rich ISC soil. When the GLU-soil adducts were subjected to wetting and drying (W/D) cycles for 3 and 6 weeks, the enzyme activity was further largely reduced after the first 3 weeks of W/D, while the decrease progressed more slowly during the following 3 weeks. This was attributed to the increasing modification of the enzyme conformational structure due to formation of dispersive and hydrogen bonds with the inorganic and organic components of HM and soils. Our results showed that an exogenous extracellular enzyme, such as GLU, is quantitatively immobilised on model and real soil aggregates, and that the catalytic activity is significantly and progressively reduced by soil physical-chemical changes, thereby implying that soil biochemical transformations are to be accounted more to intracellular than extracellular enzymes.