The Reduction of Allyl Alcohols byClostridiumspecies is Catalyzed by the Combined Action of Alcohol Dehydrogenase and Enoate Reductase

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Abstract
Cells and crude extracts of C. kluyveri or Clostridium sp. La 1, catalyze the hydrogenation of (E)- or (Z)-2-butenol to n-butanol. No single enzyme was detected which directly accomplishes this reaction. The reduction occurs as follows: 2-butenol .fwdarw. 2-butenal .fwdarw. n-butanal .fwdarw. n-butanol. The 1st step is catalyzed by the NAD-dependent alcohol dehydrogenase in C. kluyveri, the 2nd by the recently detected enoate reductase which reduces not only nonactivated .alpha.,.beta.-unsaturated acylates but also .alpha.,.beta.-unsaturated aldehydes in a NADH-dependent reaction and the 3rd step is again catalyzed by alcohol dehydrogenase. In Clostridium sp. La 1, the alcohol dehydrogenase is NADP-dependent. The rate of the reduction of 2-butenol to n-butanol depends on the enzymes and on the NAD(P)/NAD(P)H ratio. In the presence of the methylviologen cation radical which is formed by the reduction of methylviologen by the system H2/hydrogenase, the NAD(P)/NAD(P)H ratio is too small for the dehydrogenation of 2-butenal to 2-butenol. This explains the antagonistic effect of methylviologen in the hydrogenation of allyl alcohols and 2-enoates by both Clostridium spp. The mechanism explains the finding that from a preparative point of view ethanol is a better electron donor than hydrogen for the stereospecific reduction of allyl alcohols.