Mammalian class IV alcohol dehydrogenase (stomachalcohol dehydrogenase): structure, origin, and correlation withenzymology.

The structure of a mammalian class IV alcoholdehydrogenase has been determined by peptide analysis of the protein isolatedfrom rat stomach. The structure indicates that the enzyme constitutes a separatealcohol dehydrogenase class, in agreement with the distinct enzymaticproperties; the class IV enzyme is somewhat closer to class I (the"classical" liver alcohol dehydrogenase; approximately 68% residueidentities) than to the other classes (II, III, and V; approximately 60% residueidentities), suggesting that class IV might have originated through duplicationof an early vertebrate class I gene. The activity of the class IV protein towardethanol is even higher than that of the classical liver enzyme. Both Km and kcatvalues are high, the latter being the highest of any class characterized so far.Structurally, these properties are correlated with replacements at the activesite, affecting both substrate and coenzyme binding. In particular, Ala-294(instead of valine) results in increased space in the middle section of thesubstrate cleft, Gly-47 (instead of a basic residue) results in decreased chargeinteractions with the coenzyme pyrophosphate, and Tyr-363 (instead of a basicresidue) may also affect coenzyme binding. In combination, these exchanges arecompatible with a promotion of the off dissociation and an increased turnoverrate. In contrast, residues at the inner part of the substrate cleft are bulky,accounting for low activity toward secondary alcohols and cyclohexanol.Exchanges at positions 259-261 involve minor shifts in glycine residues at areverse turn in the coenzyme-binding fold. Clearly, class IV is distinct instructure, ethanol turnover, stomach expression, and possible emergence fromclass I.