An ethanol‐inducible MDR ethanol dehydrogenase/acetaldehyde reductase in Escherichia coli

Abstract

An ethanol-active medium-chain dehydrogenase/reductase (MDR) alcohol dehydrogenase was isolated and characterized from Escherichia coli. It is distinct from the fermentative alcohol dehydrogenase and the class III MDR alcohol dehydrogenase, both already known in E. coli. Instead, it is reminiscent of the MDR liver enzyme forms found in vertebrates and has a K_m for ethanol of 0.7 mm, similar to that of the class I enzyme in humans, however, it has a very high k_cat, 4050 min⁻¹. It is also inhibited by pyrazole (K_i = 0.2 µm) and 4-methylpyrazole (K_i= 44 µm), but in a ratio that is the inverse of the inhibition of the human enzyme. The enzyme is even more efficient in the reverse direction of acetaldehyde reduction (K_m = 30 µm and k_cat = 9800 min⁻¹), suggesting a physiological function like that seen for the fermentative non-MDR alcohol dehydrogenase. Growth parameters in complex media with and without ethanol show no difference. The structure corresponds to one of 12 new alcohol dehydrogenase homologs present as ORFs in the E. coli genome. Together with the previously known E. coli MDR forms (class III alcohol dehydrogenase, threonine dehydrogenase, ζ-crystallin, galactitol-1-phosphate dehydrogenase, sensor protein rspB) there is now known to be a minimum of 17 MDR enzymes coded for by the E. coli genome. The presence of this bacterial MDR ethanol dehydrogenase, with a structure compatible with an origin separate from that of yeast, plant and animal ethanol-active MDR forms, supports the view of repeated duplicatory origins of alcohol dehydrogenases and of functional convergence to ethanol/acetaldehyde activity. Furthermore, this enzyme is ethanol inducible in at least one E. coli strain, K12 TG1, with apparently maximal induction at an enthanol concentration of ≈17 mm. Although present in several strains under different conditions, inducibility may constitute an explanation for the fairly late characterization of this E. coli gene product.