Enzymatic oxidation of ethanol in the gaseous phase

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Abstract
The enzymatic conversion of gaseous substrates represents a novel concept in bioprocessing. A critical parameter in such systems is the water activity, Aw The present article reports the effect of Aw on the catalytic performance of alcohol oxidase acting on ethanol vapors. Enzyme activity in the gas‐phase reaction increases several orders of magnitude, whereas the thermostability decreases drastically when Aw is increased from 0.11 to 0.97. The enzyme is active on gaseous substrates even at hydration levels below the monolayer coverage. Enhanced thermostability at lower hydrations results in an increase in the optimum temperature of the gas‐phase reaction catalyzed by alcohol oxidase. The apparent activation energy decreases as Aw increases, approaching the value obtained for the enzyme in aqueous solution. The formation of a pread‐sorbed ethanol phase on the surface of the support is not a prerequisite for the reaction, suggesting that the reaction occurs by direct interaction of the gaseous substrate with the enzyme. The gas‐phase reaction follows Michaelis–Menten kinetics, with a Km value almost 100 times lower than that in aqueous solution. Based on vapor–liquid equilibrium data and observed Km values, it is postulated that during the gas‐phase reaction the ethanol on the enzyme establishes an equilibrium with the ethanol vapor similar to that between ethanol in water and ethanol in the gas phase.