O. Danielsson et al., FUNDAMENTAL MOLECULAR DIFFERENCES BETWEEN ALCOHOL-DEHYDROGENASE CLASSES, Proceedings of the National Academy of Sciences of the United Statesof America, 91(11), 1994, pp. 4980-4984
Two types of alcohol dehydrogenase in separate protein families are th
e ''medium-chain'' zinc enzymes (including the classical liver and yea
st forms) and the ''short-chain'' enzymes (including the insect form).
Although the medium-chain family has been characterized in prokaryote
s and many eukaryotes (fungi, plants, cephalopods, and vertebrates), i
nsects have seemed to possess only the short-chain enzyme. We have now
also characterized a medium-chain alcohol dehydrogenase in Drosophila
. The enzyme is identical to insect octanol dehydrogenase. It is a typ
ical class III alcohol dehydrogenase, similar to the corresponding hum
an form (70% residue identity), with mostly the same residues involved
in substrate and coenzyme interactions. Changes that do occur are con
servative, but Phe-51 is of functional interest in relation to decreas
ed coenzyme binding and increased overall activity. Extra residues ver
sus the human enzyme near position 250 affect the coenzyme-binding dom
ain. Enzymatic properties are similar-i.e., very low activity toward e
thanol (K-m beyond measurement) and high selectivity for formaldehyde/
glutathione (S-hydroxymethylglutathione; k(cat)/K-m = 160,000 min(-1).
mM(-1)). Between the present class m and the ethanol-active class I en
zymes, however, patterns of variability differ greatly, highlighting f
undamentally separate molecular properties of these two alcohol dehydr
ogenases, with class III resembling enzymes in general and class I sho
wing high variation. The gene coding for the Drosophila class III enzy
me produces an mRNA of about 1.36 kb that is present at all developmen
tal stages of the fly, compatible with the constitutive nature of the
vertebrate enzyme. Taken together, the results bridge a previously app
arent gap in the distribution of medium-chain alcohol dehydrogenases a
nd establish a strictly conserved class III enzyme, consistent with an
important role for this enzyme in cellular metabolism.