Crystal-structure determination of reduced nicotinamide adenine dinucleotide complex with horse liver alcohol dehydrogenase maintained in its apo conformation by zinc-bound imidazole

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Abstract
A crystallographic study to 2.4-.ANG. resolution of the ternary complex between horse liver alcohol dehydrogenase (LADH), NADH and the effector molecule imidazole (Im) (LADH-NADH-Im) is presented. The ligand binding and the changes in the protein structure due to ligand interactions were interpreted from difference electron density maps calculated with phase angles derived from the refined native enzyme model. The complex crystallizes in the orthorhombic space group C2221, and the enzyme structure remains in the apo conformation in which the active-site cleft is not entirely shielded from the solvent. NADH binds in an extended conformation, and the protein-coenzyme interactions are weaker compared to other complexes. The B-stereospecific side of the nicotinamide ring faces the catalytic center (LADH is known to be an A-side-specific enzyme). However, the reactive carbon atom C4 of the ring has a similar position in relation to active-center groups in this structure compared to LADH complexes where the A side of the ring faces the substrate site. The carboxamide group is situated within H-bonding distance to the sulfur of Cys-46, which is one of the 3 protein ligands to the active-site Zn atom. The imidazole molecule is directly ligated to the metal ion, which has a roughly tetrahedral geometry in the complex.