Recently many studies on enzyme reaction in non-aqueous phase system are being carried out. Among them gas phase enzyme reaction is very interesting and promising. This gas phase enzyme reaction can be applied to practically such as biosensors and the existing chemical process replacements.
This study shows the effects of reaction temperature and the level of hydration (water activity) on the gas phase reactions of alcohol oxidase and alcohol dehydrogenase which were immobilized on DEAE-cellulose and CPG. In gas phase reactions, reaction temperature and water activity were important parameters. Especially water activity was a critical parameter. Optimum reaction temperature zone was similar to that of aqueous phase reaction and enzyme activity increased dramatically whereas the thermostability decreased when water activity was increased from 0.3 to 0.9. The apparent activation energy of gas phase reaction was a function of water activity and higher than that of aqueous phase reaction. That is, the apparent activation energy decreased as water activity increased and approached the values obtained in the aqueous phase system.
In case of alcohol oxidase, enzyme reaction follow M-M kinetics, with a Km value of 0.24mM, which was 2 order of magnitude smaller than that in aqueous phase system. These results were explained with vapor-liquid equilibrium data, and to supplement and verify this explanation alcohol dehydrogenase was used.
In case of alcohol dehydrogenase, enzyme reaction followed ordered bi-bi mechanism, with $K_a$, $K_b$, and $K_{ia}$ values were $7.41\times 10^{-4}$, 0.21, $3.03\times 10^{-3}$mM. These values in the gas phase are also lower than that in aqueous phase system by 2 order of magnitude.
