Human liver alcohol dehydrogenase: amino acid substitution in the beta 2 beta 2 Oriental isozyme explains functional properties, establishes an active site structure, and parallels mutational exchanges in the yeast enzyme.

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Abstract
The homodimeric Oriental .beta.2.beta.2 isozyme of human liver alcohol dehydrogenase, corresponding to an allelic variant at the ADH2 gene locus, was studied in order to define the amino acid exchange in relation to the .beta.1.beta.1 isozyme, the predominant allelic form among Caucasians. Sequence analysis reveals that the amino acid substitution occurs at position 7 of the largest CNBr fragment, corresponding to position 47 of the whole protein chain. The .beta.2 form has a His residue, while, in common with other characterized mammalian liver alcohol dehydrogenases, the .beta.1 form has an Arg residue. This exchange does not affect the adjacent Cys-46 residue, which is a protein ligand to the active-site Zn atom, thus clarifying previously inconsistent results. The His/Arg-47 mutational replacement corresponds to a position that binds the pyrophosphate group of the coenzyme NAD(H); this explains the functional differences between the .beta.1.beta.1 and .beta.2.beta.2 isozymes, including both a lower pH optimum and higher turnover number of .beta.2.beta.2, which is likely to be the mutant form. The exchange demonstrates the existence of parallel but separate mutations in the evolution of alcohol dehydrogenases because these mammalian enzymes differ at exactly the same position by the same type of substitution as is found between a mutant and the wild-type constitutive forms of the corresponding yeast enzyme.