title: Metal ion substitution at the catalytic site of horse-liver alcohol dehydrogenase: results from solvent magnetic relaxation studies. 2. Binding of manganese(II) and competition with zinc(II) and cadmium(II) ions.
abstract: The interaction of Mn2+ aquo ions with native horse-liver alcohol dehydrogenase demetalized specifically at the catalytic sites has been investigated by studying the magnetic field dependence and time dependence of the magnetic spin-lattice relaxation rate of solvent water protons. We find no detectable binding of Mn2+ ions to the catalytic sites in times on the order of hours; however, we do find that these ions bind to the enzyme at two previously unreported types of sites: one, characterized by a low dissociation constant (0.01 mM at pH 7.7, 5 degrees C), low relaxivity, and a stoichiometry of one per two catalytic sites, and a second, with a high dissociation constant (1.5 mM at pH 7.7, 5 degrees C) and high relaxivity. The stoichiometry of the second type of site could not be determined because of the relatively weak bindng of Mn2+ ions to these sites. Both Zn2+ and Cd2+ ions bind to the newly found tight-binding sites, displacing Mn2+ ions and thereby altering the relaxation rates of solvent protons. By monitoring the return to equilibrium of these altered rates, we find that Zn2+ ions enter the catalytic sites from the new tight-binding sites with an on-rate of approximately 0.1 M-1 s-1. It is not clear whether binding to these new sites is an obligatory intermediate for reintroduction of Zn2+ ions into the catalytic sites, but a small excess of Zn2+ ions beyond one per monomer causes the protein to precipitate. Cd2+ ions, by contrast, enter the catalytic sites at least 1 order of magnitude more rapidly than do Zn2+ ions, a rate too rapid to observe by our techniques. However, once the catalytic sites are filled, Cd2+ ions displace Mn2+ ions at the new sites as do Zn2+ ions.
mesh_terms: D000426:Alcohol Dehydrogenase; D000429:Alcohol Oxidoreductases / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D002104:Cadmium / Q000494:pharmacology*; D006736:Horses; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D009682:Magnetic Resonance Spectroscopy; D008345:Manganese / Q000494:pharmacology*; D011485:Protein Binding; D011487:Protein Conformation; D015032:Zinc / Q000494:pharmacology*
chemical_list: D002104:Cadmium; D008345:Manganese; D000429:Alcohol Oxidoreductases; D000426:Alcohol Dehydrogenase; D015032:Zinc