Description of the Kinetic Mechanism and the Enantioselectivity of Quinohaemoprotein Ethanol Dehydrogenase from Comamonas testosteroni in the Oxidation of Alcohols and Aldehydes

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Abstract
Initial rate studies were performed on the oxidation of (racemic) alcohols as well as aldehydes by quinohaemoprotein ethanol dehydrogenase, type 1, from Comamonas testosteroni with potassium ferricyanide as electron acceptor. The data could be fitted with an equation derived for a mechanism (hexa‐uni ping‐pong) in which alcohols are oxidized to the corresponding carboxylic acids and the intermediate aldehyde becomes released from the enzyme. However, for some substrates it was necessary to assume that they exert uncompetitive inhibition. The same model was used to fit the data of conversion processes. Reversible inactivation of the enzyme takes place during the conversion, the extent being inversely proportional to the concentration of ferricyanide present at the start. From the values of the kinetic parameters obtained for (R)‐ and (S)‐solketal [2,2‐dimethyl‐4‐(hydroxymethyl)‐1,3‐dioxolane] and their corresponding aldehydes, it appeared that the second step in (S)‐solketal conversion is much faster than the first one and that opposite enantiomeric preferences exist for the alcohol and the aldehyde substrates. Since the initial rate measurements as well as the progress curve analysis gave similar kinetic parameter values and product analysis revealed intermediates in the amounts predicted, it is concluded that the kinetic and enantioselective behaviour of the enzyme is adequately described by the model presented here. Finally, the results indicate that both kinetic approaches should be used in conversions with consecutive reactions since they provide complementary information.

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