Mechanistic Aspects of Enzymatic Catalysis in Anhydrous Organic Solvents

Abstract

Enzymatic catalysis in non-aqueous media has progressed from using enzymes in aqueous solutions containing relatively low fractions of water-miscible organic cosolvents to that in biphasic aqueous-organic mixtures, to that in microemulsions and reversed micelles, to that in monophasic organic media containing small amounts of water, to that in anhydrous organic solvents. It is easy to understand why enzymes retain catalytic activity in the first three types of reaction media, for in all of them the enzyme molecules are located in aqueous environments (and therefore the inherent enzymatic properties in such systems are usually not significantly different from those in water). Conversely, the phenomenon of enzymes vigorously functioning in organic solvents with no water goes against the conventional wisdom and universally accepted dogmas. Nevertheless, it has now been firmly established (see Ref. [1] for a review) that this phenomenon exists, that it is quite general and highly beneficial for bioprocessing [2], and that enzymes in organic solvents exhibit remarkable novel properties, e.g., greatly enhanced thermal stability and dramatically altered substrate specificity and stereoselectivity. In this presentation, fundamental questions concerning enzymatic catalysis in organic solvents will be addressed: How does enzymatic activity depend on the nature of the solvent and why? What physicochemical rules govern substrate, inhibitor, enantiomeric, and positional specificities of enzymes in organic solvents? What is the enzyme’s structure and mechanisms of action in anhydrous media? and What additional new properties do enzymes acquire when placed in non-aqueous solvents?