LIVER CLASS-I ALCOHOL-DEHYDROGENASE ISOZYME RELATIONSHIPS AND CONSTANT PATTERNS IN A VARIABLE BASIC STRUCTURE - DISTINCTIONS FROM CHARACTERIZATION OF AN ETHANOL DEHYDROGENASE IN COBRA, NAJA-NAJA

The major ethanol dehydrogenase of cobra liver was characterized in or der to clarify isozyme relationships and functional motifs of the vert ebrate enzyme. The cobra protein is a class-I form, most related to on e of the isozyme subunits (the a form) in Uromastix (lizard) liver. Th is positions the isozyme duplication and defines the main-line alterna tive. The new structure also allows extensive correlations with struct ure/function relationships for alcohol dehydrogenases in general, of w hich 38 animal variants (still disregarding strain and allelic differe nces) now have been characterized. Architectural features are discerne d, distinguishing the enzyme at large, the classes, and the functional interactions at the sites of substrate binding and coenzyme binding. Variability is greater at the substrate-binding site, with only one of 13 residues strictly conserved (His67, one of the active-site zinc li gands) but all other residues differing among and frequently within cl asses. However, many substrate-interacting residues are class preferen tial and may be used in predictive assignments. Class-I/III difference s concern position 48 (typically Ser in class I, Thr in class III), po sition 93 (Phe versus Tyr), position 141 (branch-chained aliphatic res idue versus methionine), position 57 (hydrophobic residue versus Asp), position 115 (Asp versus Arg), position 116 (Leu or Ile versus Val), position 306 (Met or Leu/Ile versus Phe), position 309 (Phe or Leu/Ile versus Val) and position 318 (Val or Ile versus Ala). In contrast, co enzyme binding is more conserved. A characteristic coenzyme-binding mo tif, covering only a 50-residue stretch, is defined as tVDiK (residues 178, 203, 223, 224, 228; capital letters for residues strictly conser ved and small-cases letters for residues nearly so). This motif is cla ss independent and unique to animal alcohol dehydrogenases. Therefore, the novel enzyme structure establishes class-I isozyme relationships, shows characteristic 'constant' residues also in the 'variable' class -I line, and defines residue-specific patterns which may have a predic tive value in functional assignments of an increasing number of undefi ned further forms expected to result from gene projects.