Amino acids important in enzyme activity and dimer stability for Drosophila alcohol dehydrogenase

Abstract

We have determined the nucleotide sequences of eight ethyl methanesulphonate-induced mutants in Droosphila alcohol dehydrogenase (ADH), of which six were previously characterized by Hollocher and Place. Four of these ADH mutants contain a single amino acid change: glycine-17 to arginine, glycine-93 to glutamic acid, alanine-159 to threonine, and glycine-184 to aspartic acid. Although these mutants are inactive, three mutants (Gly17Arg, Gly93Glu and Gly184Asp) form stable homodimers, as well as heterodimers with wild-type ADH, in which the wild-type ADH subunit retains full enzyme activity. Interestingly, the Ala159Thr mutant does not form either stable homodimers or heterodimers with wild-type ADH, suggesting that alanine-159 is important in stabilizing ADH dimers. The mutations were analysed in terms of a three-dimensional model of ADH using bacterial 20β-hydroxysteroid dehdrogenase and rat dihydropteridine reductase as templates. The model indicates that mutations in glycine-17 and glycine-93 affect the binding of NAD+. It also shows that alanine-159 is part of a hydrophobic anchor on the dimer interface of ADH. Replacement of alanine-159 with threonine, which has a larger side chain and can hydrogen bond with water, is likely to reduce the strength of the hydrophobic interaction. The three-dimensional model shows that glycine-184 is close to the substrate binding site. Replacement of glycine-184 with aspartic acid is likely to alter the position of threonine-186, which we propose hydrogen bonds to the carboxamide moiety of NAD+. Also, the negative charge on the aspartic acid side chain may interact with the substrate and/or residues in the substrate binding site. These mutations provide information about ADH catalysis and the stability of dimers, which may also be useful in understanding homologous dehydrogenases, which include the human l7β-hydroxysteroid, 11β-hydroxysteroid and 15-hydroxyprostaglandin dehydrogenases.