(S)-1-Phenylethanol dehydrogenase of Azoarcus sp. strain EbN1, an enzyme of anaerobic ethylbenzene catabolism

Abstract.

The initial steps in the anaerobic oxidation of the aromatic hydrocarbon ethylbenzene by denitrifying bacteria are two sequential dehydrogenation reactions of ethylbenzene to (S)-1-phenylethanol and further to acetophenone. The enzyme catalysing the second oxidation step, (S)-1-phenylethanol dehydrogenase, was analysed in the denitrifying bacterium Azoarcus sp. strain EbN1. An NAD⁺-dependent 1-phenylethanol dehydrogenase for each of the enantiomers of 1-phenylethanol was identified in this bacterium; the two enzymes were induced under different growth conditions. (S)-1-phenylethanol dehydrogenase from ethylbenzene-grown cells was purified and biochemically characterised. The enzyme is a typical secondary alcohol dehydrogenase and consists of two subunits of 25.5 kDa. The enantioselective enzyme catalyses the oxidation of (S)-1-phenylethanol or the reduction of acetophenone and is inhibited by high concentrations of (R)-1-phenylethanol. The enzyme exhibits low apparent K _m values for (S)-1-phenylethanol and acetophenone and is rather substrate-specific, using only a few chemically similar secondary alcohols, such as 1-phenylpropanol and isopropanol.