An ethanol-inducible MDR ethanol dehydrogenase/acetaldehyde reductase in Escherichia coli - Structural and enzymatic relationships to the eukaryotic protein forms

An ethanol-active medium-chain dehydrogenase/reductase (MDR) alcohol dehydr ogenase was isolated and characterized from Escherichia coli. It is distinc t from the fermentative alcohol dehydrogenase and the class III MDR alcohol dehydrogenase, both already known in E. coli. Instead, it is reminiscent o f 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 ha s a very high k(cat), 4050 min(-1). It is also inhibited by pyrazole (K-i = 0.2 mu M) and 4-methylpyrazole (K-i = 44 mu M), but in a ratio that is the inverse of the inhibition of the human enzyme. The enzyme is even more eff icient in the reverse direction of acetaldehyde reduction (K-m = 30 mu M an d k(cat) = 9800 min(-1)), suggesting a physiological function like that see n for the fermentative non-MDR alcohol dehydrogenase. Growth parameters in complex media with and without ethanol show no difference. The structure co rresponds to one of 12 new alcohol dehydrogenase homologs present as ORFs i n the E. coli genome. Together with the previously known E. coil MDR forms (class III alcohol dehydrogenase, threonine dehydrogenase, zeta-crystallin, galactitol-1-phosphate dehydrogenase, sensor protein rspB) there is now kn own 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 comp atible with an origin separate from that of yeast, plant and animal ethanol -active MDR forms, supports the view of repeated duplicatory origins of alc ohol 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 conc entration of approximate to 17 mM. Although present in several strains unde r different conditions, inducibility may constitute an explanation for the fairly late characterization of this E. coli gene product.