Structural and biochemical studies of alcohol dehydrogenase isozymes from Kluyveromyces lactis

The cytosolic and mitochondrial alcohol dehydrogenases from Kluyveromyces lactis (KlADHs) were purified and characterised. Both the N-terminally blocked cytosolic isozymes, KlADH I and KlADH II, were strictly NAD-dependent and exhibited catalytic properties similar to those previously reported for other yeast ADHs. Conversely, the mitochondrial isozymes, KlADH III and KlADH IV, displayed Ala and Asn, respectively, as N-termini and were able to oxidise at an increased rate primary alcohols with aliphatic chains longer than ethanol, such as propanol, butanol, pentanol and hexanol. Interestingly, the mitochondrial KlADHs, at variance with cytosolic isozymes and the majority of ADHs from other sources, were capable of accepting as a cofactor, and in some case almost equally well, either NAD or NADP. Since Asp-223 of horse liver ADH, thought to be responsible for the selection of NAD as coenzyme, is strictly conserved in all the KlADH isozymes, this amino-acid residue should not be considered critical for the coenzyme discrimination with respect to the other residues lining the coenzyme binding pocket of the mitochondrial isozymes. The relatively low specificity of the mitochondrial KlADHs both toward the alcohols and the cofactor could be explained on the basis of an enhanced flexibility of the corresponding catalytic pockets. An involvement of the mitochondrial KlADH isozymes in the physiological reoxidation of the cytosolic NADPH was also hypothesized. Moreover, both cytosolic and KlADH IV isozymes have an additional cysteine, not involved in zinc binding, that could be responsible for the increased activity in the presence of 2-mercaptoethanol.