Quinoprotein alcohol dehydrogenase from a non-methylotroph, Acinetobacter calcoaceticus

Acinetobacter calcoaceticus grown on ethanol contains an NAD(P)+-independent alcohol dehydrogenase which resembles methanol dehydrogenase from methylotrophic bacteria in many respects. Likewise, the prosthetic group of this enzyme appears to be identical to that of methanol dehydrogenase, namely, pyrrolo quinoline quinone. The organism is unable to grow on methanol, which means that quinoprotein alcohol dehydrogenases are not restricted to methylotrophs. Arguments are presented for the idea that quinoprotein alcohol dehydrogenases exist in other alkane- or alcohol-grown bacteria. Although the enzyme from A. calcoaceticus can be best compared with that from Rhodopseudomonas acidophila in that both have very low affinities for methanol and are activated by aliphatic amines, the two enzymes are immunologically and electrophoretically unrelated. Furthermore, the A. calcoaceticus enzyme shows the broadest substrate specificity hitherto known for this type of enzyme in that it also oxidizes higher aldehydes. The extent of hydration of aldehydes cannot account for the aldehyde substrate specificity of these enzymes but the concept of a dual substrate specificity for alcohols and aldehydes can explain this very well. The different properties of the two enzymes compared with those of methanol dehydrogenases cannot be ascribed to the presence of iron as both enzymes contained a negligible amount of this metal.