Class III human liver alcohol dehydrogenase: a novel structural type equidistantly related to the class I and class II enzymes

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Abstract
The primary structure of class III alcohol dehydrogenase (dimeric with .chi. subunits) from human liver has been determined by peptide analyses. The protein chain is a clearly distinct type of subunit distantly related to those of both human class I and class II alcohol dehydrogenases (with .alpha., .beta., .gamma., and .pi. subunits, respectively). Disregarding a few gaps, residue differences in the .chi. protein chain with respect to .beta.1 and .pi. occur at 139 and 140 positions, respectively. Compared to class I, the 373-residue .chi. structure has an extra residue, Cys after position 60, and two missing ones, the first two residues relative to class I, although the N-terminus is acetylated like that for those enzymes. The .chi. subunit contains two more tryptophan residues than the class I subunits, accounting for the increased absorbance at 280 nm. There are also four additional acidic and two fewer basic side chains than in the class I .beta. structure, compatible with the markedly different electrophoretic mobility of the class III enzyme. Residue differences between class III and the other classes occur with nearly equal frequency in the coenzyme-binding and catalytic domains. The similarity in the number of exchanges relative to that of the enzymes of the other two classes supports conclusions that the three classes of alcohol dehydrogenase reflect stages in the development of separate enzymes with distinct functional roles. In spite of the many exchanges, the residues critical to basic functional properties are either completely unchanged-all zinc ligands and space-restricted Gly residues-or partly unchanged-residues at the coenzyme-binding pocket. This suggests that the overall conformations are related and that some functional replacements account for the different enzymatic properties. Thus, several of the residues at the substrate-binding pocket are markedly changed in .chi., including some substitutions for smaller residues, and have an increased polarity, with one charge. This is compatible with an altered substrate specificity for the class III enzyme. The corresponding isozyme from horse liver was also characterized in part, revealing that between the two species the structure of this enzyme class is more conserved than that of the class I enzymes. This further distinguishes the classes and suggests a strict functional importance for the class III alcohol dehydrogenase.
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