Factors influencing the operational stability of NADPH-dependent alcohol dehydrogenase and an NADH-dependent variant thereof in gas/solid reactors

The continuous enzymatic gas/solid bio-reactor serves both for the production of volatile fine chemicals and flavors on an industrial scale and for thermodynamically controlled investigation of substrate and water effects on enzyme preparations for research purposes. Here, we comparatively investigated the molecular effects on the operational stability of NADPH-dependent Lactobacillus brevis alcohol dehydrogenase and an NADH-dependent variant thereof, LbADH G37D, in the gas/solid bioreactor. The reference reaction is the reduction of acetophenone to (R)-1-phenylethanol with concomitant oxidation of 2-propanol to acetone for the purpose of regeneration of the redox cofactor. ld be clearly shown that not the thermostability of the cofactor, but the thermostability of the proteins in the solid dry state govern the order of magnitude of the operational stability of both purified enzymes in the gas/solid reactor at low thermodynamic activity of water and substrate. However, at higher thermodynamic activity the operational stability in the gas/solid reactor is overlaid by stabilizing and destabilizing effects of the substrates that require further investigation. We demonstrated first evidence that the substrate affinity of the two variants in the gas/solid reactor is similar to the affinity in aqueous medium. We could also show that partial unfolding of the proteins with subsequent aggregation are the factors governing protein thermo-in-stability both in the dissolved and in the dry state. Thus, stability investigations of enzymes in the dry state are suggested to predict their basal level of operational stability in gas/solid reactions.