Human liver alcohol dehydrogenase

The primary structure of the γ 1 subunit of human liver alcohol dehydrogenase isoenzyme γ 1 γ 1 was deduced by characterization of 36 tryptic and 2 CNBr peptides. The polypeptide chain is composed of 373 amino acid residues. γ 1 differs from the β 1 subunit of human liver alcohol dehydrogenase at 21 positions, and from the E subunit of horse liver alcohol dehydrogenase at 43 positions including a gap at position 128 as in the β 1 subunit. All zinc‐liganding residues from the E subunit of the horse protein and the β 1 subunit of the human enzyme are conserved, but like β 1 , γ 1 also has an additional cysteine residue at position 286 (in the positional numbering system of the horse enzyme) due to a Tyr→Cys exchange. Most amino acid exchanges preserve the properties of the residues affected and are largely located on the surface of the molecules, away from the active site and the coenzyme binding region. However, eight positions with charge differences in relation to the E subunit of the horse enzyme are noticed. These result in a net positive charge increase of one in γ 1 versus E, explaining the electrophoretic mobilities on starch gels. Of functional significance is the conservation of Ser‐48 in γ 1 relative to E. The residue is close to the active site but different (Thr‐48) in the β 1 subunit of the human enzyme. Thus, the closer structural relationship between human γ 1 and horse E enzyme subunit than between β 1 and E is also reflected in functionally important residues, explaining a greater similarity between γ 1 γ 1 and EE than between β 1 β 1 and EE.