Thermophilic proteins: stability and function in aqueous and organic solvents.
The molecular stability of thermophilic and hyperthermophilic enzymes generally reflects the growth temperatures of the parent organisms. Extracellular enzymes from the hyperthermophilic Archaea typically show very high levels of thermal stability and a number of enzymes with Tm values of greater than 100 degrees C have been reported. The mechanisms responsible for high molecular stability are typically intrinsic characteristics of the protein, as shown by the comparative stabilities of many native and recombinant proteins. However, some extrinsic stabilisation mechanisms have been demonstrated. High levels of thermal stability are positively correlated with stability in the presence of other denaturing agents, including detergents and organic solvents. This correlation suggests a common denaturation pathway where molecular mobility/flexibility is the prime determinant of susceptibility to irreversible denaturation. In single phase organic-aqueous solvents, protein destabilisation occurs via solvent-induced alteration to the protein hydration shell. However, correlations between protein stability and solvent hydrophobicity are unreliable. In two-phase organic-aqueous systems, interfacial denaturation predominates and is a function of both interfacial tension and interfacial surface area. Intracellular enzymes are protected from interfacial denaturation but are potentially susceptible to direct organic solvent effects, possibly depending on the role of the cell wall and cell membrane in the partitioning of the organic solvent into the cell cytoplasm. Immobilisation of thermophilic enzymes provides a method for enhancing both the thermal and solvent stabilities of thermophilic and mesophilic enzymes. Multi-point covalent immobilisation to glyoxal-agarose enhances thermal stability and limits protein-protein inactivation mechanisms. Miscible organic solvents have a profound influence on the specificities of enzyme reactions. The presence of high concentrations of miscible organic solvents may induce gross changes in substrate specificity and/or more subtle alterations in chiral selectivity. Correlations between the variation in enantioselectivity and both solvent hydrophobicity and solvent dielectric constant have been demonstrated although some recent studies implicate the formation of specific solvent-enzyme complexes which directly affect reaction kinetics.