Extracellular enzyme activity in soil: effect of pH and ionic strength on the interaction with montmorillonite of two acid phosphatases secreted by the ectomycorrhizal fungus Hebeloma cylindrosporum

Abstract

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.ActivitC enzymatique extracellulaire dans Ie sol: effet du pH et de la force ionique sur l'interaction avec la montmorillonite de deux phosphates acides sCcrdteds par le champignon ectomycorhizien Hebeloma cylindrosporum RCsumC L'interaction des enzymes extracellulaires avec la phase solide du sol modifie leur mobilitt et leur proprittts catalytiques. En particulier, I'adsorption sur les minCraux argileux provoque une augmentation du pH optimal de I'activitt catalytique. Deux interprktations opposkes de ce phtnombne ont Ctt propostes: un effet de pH de surface et une modification de conformation de I'enzyme adsorbte qui dtpend du pH. De faGon ? I tvaluer ces deux mtcanismes, I'adsorption sur la montmorillonite de deux phosphatases acides extracellulaires du champignon ectomycorhizien Hebeloma cylindrosporum a Ct C CtudiCe, ainsi que les constquences de I'adsorption sur leurs activitts catalytiques. Les rtsultats obtenus s'interprbtent mieux par une modification de conformation des enzymes dtpendant du pH et principalement due aux interactions klectrostatiques avec la surface d'argile. A bas pH, les deux enzymes positivement chargtes se dtnaturent sur la surface de montmorillonite chargke ntgativement. A pH tlevt, les enzymes et I'argile sont ntgativement chargtes, et ]'adsorption diminue. L'adsorption et la modification de conformation sont largement irrkversibles, ceci devrait Ctre prise en compte dans I'ttude de I'activitk enzymatique du sol. Enfin, la comparaison avec I'effet des argiles sur I'activitt catalytique d'enzymes intracellulaires soulBve I'hypothbse d'une pression de stlection de la phase solide du sol conduisant A des enzymes extracellulaires plus stables.

Monitoring Bacterial Transport by Stable Isotope Enrichment of Cells

“…3). These data are consistent with prior findings which indicate that soils and sediments bind nucleic acids (5,23), proteins (12), viruses (4), and presumably other cell-free biological macromolecules quite strongly.…”

“…The objective of this experiment was to demonstrate that the isotope values of samples obtained from bacterial injection experiments reflect transport of intact bacterial cells and not cell fragments or soluble cell components possibly released by the demise of added bacteria. To accomplish this, an intact-core experiment using SGW was performed to compare the breakthrough be- Since there is no a priori reason to expect differences in the behavior of cellular macromolecules synthesized using carbon in the form of 12 C, 13 C, or 14 C, provided that it is incorporated into bacterial cells in the same chemical form (in this case acetate) and under the same conditions, the 14 C isotope should function as a faithful analog of 13 C enrichment. For this experiment, a single culture of DA001 was grown on [ 14 C]acetate and then split into two equal aliquots, of which one was thoroughly lysed by sonication.…”

Variation in pH Optima of Hydrolytic Enzyme Activities in Tropical Rain Forest Soils

“…Although sorption on surfaces usually reduces enzyme activity, it also stabilizes the enzyme to the extent that much of the activity measured in soil enzyme assays may be due to stabilized enzymes (46). Sorption of an enzyme can also change its optimum pH by one or two pH units compared to that of the free enzyme in solution (31,40), with an increase in the optimum pH expected for enzymes sorbed on negatively charged surfaces, such as clays (26). For example, sorption on kaolinite increased the optimum pH of wheat acid phosphomonoesterase from 5.0 to 5.7 (40) and that of chitinase from 4.7 to 5.7 (48).…”

“…For example, the sorption of an enzyme on a clay surface can increase its optimum pH by one or two pH units relative to that of the same enzyme in solution (31,40). This is due to "unfolding" of enzymes on solid surfaces, which is most likely to occur at soil pH values below the isoelectric point of the enzyme (26,38).…”

Functional Relationships with N Deposition Differ According to Stand Maturity in Calluna-Dominated Heathland

“…Other studies have reported positive relationships between N deposition and PME activity (e.g., Phoenix et al 2003), although this was not found in our survey. PME activity is highly sensitive to differences in substrate pH above and below an optimum pH of 4.5 (Leprince and Quiquampoix 1996) and, in the present study, reduced N deposition significantly lowered soil pH (soil pH range in the survey 3.5-5.4), a phenomenon that has been observed in studies elsewhere (Stevens et al 2004;Power et al 2006). The influence of soil pH may, therefore, have confounded potential relationships between PME activity and N deposition.…”