Differential capacity of kaolinite and birnessite to protect surface associated proteins against thermal degradation

Abstract

It is widely accepted that soil organic carbon cycling depends on the presence and catalytic functionality of extracellular enzymes. Recent reports suggest that combusted and autoclaved soils may have the capacity to degrade organic test substrates to a larger extent than the living, enzyme-bearing soils. In search of the underlying mechanisms, we adsorbed Beta-Glucosidase (BG) and Bovine Serum Albumin (BSA) on the phyllosilicate kaolinite and the manganese oxide birnessite at pH 5 and pH 7. The protein-mineral samples were then subjected to gradual energy inputs of a magnitude equivalent to naturally occurring wildfire events. The abundance and molecular masses of desorbed organic compounds were recorded after ionization with tunable synchrotron vacuum ultraviolet radiation (VUV). The mechanisms controlling the fate of proteins varied with mineralogy. Kaolinite adsorbed protein largely through hydrophobic interactions and, even at large energy inputs, produced negligible amounts of desorption fragments compared to birnessite. Acid birnessite adsorbed protein through coulombic forces at low energy levels, became a hydrolyzing catalyst at low energies and low pH, and eventually turned into a reactant involving disintegration of both mineral and protein at higher energy inputs. Fragmentation of proteins was energy dependent and did not occur below an energy threshold of 0.20 MW cm−2. Neither signal abundance nor signal intensity were a function of protein size. Above the energy threshold value, BG that had been adsorbed to birnessite at pH 7 showed an increase in signal abundance with increasing energy applications. Signal intensities differed with adsorption pH for BSA but only at the highest energy level applied. Our results indicate that proteins adsorbed to kaolinite may remain intact after exposure to such energy inputs as can be expected to occur in natural ecosystems. Protein fragmentation and concomitant loss of functionality must be expected in surface soils replete with pedogenic manganese oxides. We conclude that minerals can do both: protect enzymes at high energy intensities in the case of kaolinite and, in the case of birnessite, substitute for and even exceed the oxidative functionality that may have been lost when unprotected oxidative enzymes were denatured at high energy inputs