Active site substituted Cd(II) horse liver alcohol dehydrogenase has been studied by Perturbed Angular Correlation of Gamma rays Spectroscopy during turnover conditions for benzaldehyde and 4-trans-(N,Ndimethylamino)cinnamaldehyde. The ternary complex between alcohol dehydrogenase NAD + and CI-, and the binary complex between alcohol dehydrogenase and orthophenanthroline have also been studied. The Nuclear Quadrupole Interaction parameters have been interpreted in terms of different coordination geometries for Cd(II) in the catalytic zinc site of the enzyme. Calculation of the nuclear quadrupole interaction for cadmium in the catalytic site of the enzyme with and without coenzyme, based upon the four coordinated geometries determined from X-ray diffraction, agrees with the experimentally determined values. The ternary complexes between enzyme, NAD + and either C1-or trifluoroethanol and the binary complex between enzyme and orthophenanthroline have almost identical spectral parameters which are not consistent with a four coordinated geometry, but are consistent with a five coordinated geometry. The nonprotein ligands for the ternary complex with trifluoroethanol are suggested to be an alkoxide group and a water molecule. The Nuclear Quadrupole Interaction parameters for the productive ternary complex between enzyme, NADH and an aldehyde is consistent with the * The experimental work was carried out at the Niels Bohr Institute Riso, 4000 Roskilde and Blegdamsvej 19, 2•00 Copenhagen, Denmark Abbreviations: LADH, Horse liver alcohol dehydrogenase; H4ZnzLADH , derivative of LADH free of zinc in the catalytic site; IllCdZn2LADH, derivative of LADH with 111Cd (carrier free) in the catalytic site; Cd2ZnzLADH , derivative of LADH with 2 mole of Cd(II) per mole LADH in the catalytic site; PAC, perturbed angular correlation of gamma rays; NQI, Nuclear quadrupole interaction; AOM, Angular overlap model; trifluoroethanol, 2,2,2-trifluoroethanol;
DACA, trans-4-(N,N-dimethylamino)cinnamaldehyde; NAD + and NADH, oxidized and reduced nicotinamide adenine dinucleotide; NADHz, reduced 1,4,5,6tetrahydronicotinamide adenine dinucleotide Offprint requests to: R. Bauer four coordinated geometry predicted from X-ray diffraction data having the carbonyl group of the aldehyde substituting the water molecule as ligand to the metal.