Mineral Surfaces as Agents of Environmental Proteolysis: Mechanisms and Controls

Inhibition of Extracellular Enzyme Activity by Reactive Oxygen Species upon Oxygenation of Reduced Iron-Bearing Minerals

The dual roles of minerals in inhibiting and prolonging extracellular enzyme activity in soils and sediments are governed by enzyme adsorption to mineral surfaces. Oxygenation of mineral-bound Fe(II) generates reactive oxygen species (ROS), yet it is unknown whether and how this process alters the activity and functional lifespan of extracellular enzymes. Here, the effect of mineral-bound Fe(II) oxidation on the hydrolytic activity of a cellulose-degrading enzyme β-glucosidase (BG) was studied using two pre-reduced Fe-bearing clay minerals (nontronite and montmorillonite) and one pre-reduced iron oxide (magnetite) at pH 5 and 7. Under anoxic conditions, BG adsorption to mineral surfaces decreased its activity but prolonged its lifespan. Under oxic conditions, ROS was produced, with the amount of •OH, the most abundant ROS, being positively correlated with the extent of structural Fe(II) oxidation in reduced minerals. •OH decreased BG activity and shortened its lifespan via conformational change and structural decomposition of BG. These results suggest that under oxic conditions, the ROS-induced inhibitory role of Fe(II)-bearing minerals outweighed their adsorption-induced protective role in controlling enzyme activity. These results disclose a previously unknown mechanism of extracellular enzyme inactivation, which have pivotal implications for predicting the active enzyme pool in redox-oscillating environments.

Oxidation of Cr(III) to Cr(VI) and Production of Mn(II) by Synthetic Manganese(IV) Oxide

The heterogeneous oxidation of Cr(III) to Cr(VI), a toxic inorganic anion, by a synthetic birnessite (δ-MnO2) was investigated in batch reactions using a combination of analytical techniques including UV–Vis spectrophotometry, microwave plasma–atomic emission spectrometry, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR), to evaluate both the solution speciation of Cr(III)/Cr(VI) and the surface of the reacted δ-MnO2. The formation of dissolved Mn(II) was determined during the batch reactions to evaluate the extent and stoichiometry of the Cr(III) oxidation reaction. A stoichiometric 3:2 Mn(II):Cr(VI) molar relationship was observed in the reaction products. The reductive dissolution of the δ-MnO2 by Cr(III) resulted in a surface alteration from the conversion of Mn(IV) oxide to reduced Mn(II) and Mn(III) hydroxides. The results of this investigation show that naturally occurring Cr(III) will readily oxidize to Cr(VI) when it comes in contact with MnO2, forming a highly mobile and toxic groundwater contaminant.

Adsorption kinetics, conformational change, and enzymatic activity of β-glucosidase on hematite (α-Fe2O3) surfaces

Substantial fractions of extracellular enzymes are intimately associated with soil minerals, which may protect enzymes from denaturation, precipitation, proteolysis, or microbial consumption in soil environments. However, how mineral surface properties affect enzyme-mineral interactions and enzymatic activity of enzymes associated to mineral surface is still unclear. In the present study, adsorption behavior, conformational change, and enzymatic activity of β-glucosidase (BG) on hematite (001) face and (104) face, respectively, were investigated using in situ attenuated total reflectance FTIR spectroscopy and batch experiments. β-glucosidase undergoes greater conformational changes upon adsorption onto hematite (104) face than on hematite (001) face, probably due to the stronger protein-surface interactions on (104) face with the relatively higher surface hydroxyl density. On the other hand, the amount of BG sorbed on hematite (001) face was nearly two times higher than that on hematite (104) at the end of the 150-min adsorption experiments, due to the higher extent of conformational change in β-glucosidase on hematite (104) face. Correspondingly, the initial rate of cellobiose hydrolysis by per gram of β-glucosidase adsorbed on hematite (104) face was 1.7 times higher than that on hematite (001) face. However, when the density of hematite particles was same, the extent of cellobiose hydrolysis was 1.2 times higher on hematite (001) face than that on the (104) face, because of the higher adsorbed amount of β-glucosidase on the former. This study decoupled the effects of mineral surface properties on adsorption kinetics and conformational changes of soil enzymes bound to soil minerals and provided new insights into the correlation between mineral surface properties and catalytical activity of mineral-associated enzymes in soil environments.

Proteolysis of Iron Oxide-Associated Bovine Serum Albumin

Proteins are a substantial nitrogen source in soils provided that they can be hydrolyzed into bioavailable small peptides or amino acids. However, the strong associations between proteins and soil minerals restrict such proteolytic reactions. This study focused on how an extracellular fungal protease (Rhizopus sp.) hydrolyzed iron oxide-associated bovine serum albumin (BSA) and the factors that affected the proteolysis. We combined batch experiments with size-exclusion and reversed phase liquid chromatography and in situ infrared spectroscopic measurements to monitor the generation of proteolytic products in solution as well as the real-time changes of the adsorbed BSA during 24 h. Results showed that protease hydrolyzed the iron oxide-associated BSA directly at the surface without an initial desorption of BSA. Concurrently, the protease was adsorbed to vacant surface sites at the iron oxides, which significantly slowed down the rate of proteolysis. This inhibiting effect was counteracted by the presence of preadsorbed phosphate or by increasing the BSA coverage, which prevented protease adsorption. Fast initial rates of iron oxide-associated BSA proteolysis, comparable to proteolysis of BSA in solution, and very slow rates at prolonged proteolysis suggest a large variability in mineral-associated proteins as a nitrogen source in soils and that only a fraction of the protein is bioavailable.