A Synthetic Approach to Analysis of the Structural Zinc Site of Alcohol Dehydrogenase

The mammalian alcohol dehydrogenases are medium-sized dimeric zinc-containing enzymes with a subunit molecular mass of about 40 kDa. Each subunit contains two zinc atoms, one of which is part of the catalytic site, whereas the other appears to fulfil a structural role (Akeson, 1964; Drum et al., 1967; Drum and Vallee, 1970) and is bound to a separate loop of the protein that contributes to subunit interactions (Eklund et al., 1976). However, the precise role of this non-catalytic zinc is not known. It is tetrahedrally coordinated by four cysteine residues, which are fairly adjacent in a short segment of the polypeptide backbone (Cys97, CyslOO, Cysl03 and Cys111; cf. Eklund et al., 1976). This structure follows the general rules for ligands to non-catalytic zinc atoms, as deduced from similarities with the relationships in other proteins (Vallee and Auld, 1990; Vallee and Auld, 1991). The cysteines are also conserved in medium-chain (Persson et al, 1991) alcohol dehydrogenases proper (Jornvall et al., 1987a), but other residues in this region are highly variable and species-specific (Jornvall, 1985; Jornvall et al., 1987b). Consequently, the cysteine spacing, i.e. Cys-Xaa2-Cys-Xaa2-Cys-Xaa7-Cys, appears to be of major importance for the protein/zinc complex to form. This structural arrangement differs distinctly from that of other sulfur-containing, zinc-binding motifs, e.g. zinc fingers, zinc twists, and zinc clusters.