THE REFINED CRYSTAL-STRUCTURE OF DROSOPHILA-LEBANONENSIS ALCOHOL-DEHYDROGENASE AT 1.9 ANGSTROM RESOLUTION

Drosophila alcohol dehydrogenase (DADH; EC 1.1.1.1) is a NAD(H)depende nt oxidoreductase belonging to the short-chain dehydrogenases/ reducta ses (SDR) family. This homodimeric enzyme catalyzes the dehydrogenatio n of alcohols to their respective ketones or aldehydes in the fruit-fl y Drosophila, both for metabolic assimilation and detoxification purpo ses. The crystal structure of the apo form of DADH, one of the first b iochemically characterized member of the SDR family, was solved at 1.9 Angstrom resolution by Patterson methods. The initial model was impro ved by crystallographic refinement accompanied by electron density ave raging, R-factor = 20.5%, X-free = 23.8%. DADH subunits show an alpha/ beta single domain structure with a characteristic NAD(H) binding moti f (Rossmann fold). The peptide chain of a subunit is folded into a cen tral eight-stranded beta-sheet flanked on each side by three alpha-hel ices. The dimers have local 2-fold symmetry. Dimer association is domi nated by a four-helix bundle motif as well as two C-terminal loops fro m each subunit, which represent a unique structural feature in SDR enz ymes with known structure. Three structural features are characteristi c for the active site architecture. (1) A deep cavity which is covered by a flexible loop (33 residues) and the C-terminal tail (11 residues ) from the neighboring subunit. The hydrophobic surface of the cavity is likely to increase the specificity of this enzyme towards secondary aliphatic alcohols. (2) The residues of the catalytic triad (Ser138, Tyrl51, Lys155) are known to be involved in enzymatic catalysis in the first Line. The Tyr151 OH group is involved in an ionic bond with the Lys155 side-chain. Preliminary electrostatic calculations have provid ed evidence that the active form of Tyrl51 is a tyrosinate ion at phys iological pH. (3) Three well-ordered water molecules in hydrogen bond distance to side-chains of the catalytic triad may be significant for the proton release steps in DADH catalysis. A ternary structure-based sequence alignment with ten members of the SDR family with known three -dimensional structure has suggested to define a model consisting of f our groups of residues, which relates the observed low degree of seque nce identity to quite similar folding patterns and nearly identical di stributions of residues involved in catalysis. (C) 1998 Academic Press .