Active site-specifically reconstituted nickel(II) horse liver alcohol dehydrogenase: optical spectra of binary and ternary complexes with coenzymes, coenzyme analogues, substrates, and inhibitors.

Insertion of nickel ions into the empty catalytic site of horse liver alcohol dehydrogenase yields an active enzyme with 65% metal substitution and about 12% intrinsic activity. The electronic absorption spectrum is characterized by bands at 357 nm (2900 M-1 cm-1), 407 nm (3500 M-1 cm-1), 505 nm (300 M-1 cm-1), 570 nm (approximately equal to 130 M-1 cm-1), and 680 nm (approximately equal to 80 M-1 cm-1). The absorption and CD spectra are similar to those of nickel(II) azurin and nickel(II) aspartate transcarbamoylase and prove coordination of the nickel(II) ions to sulfur in a distorted tetrahedral coordination geometry. Changes of the spectra upon ligand binding at the metal or conformation changes of the protein induced by coenzyme, or both, indicate alterations of the metal geometry. The chromophoric substrate trans-4-(N, N-dimethylamino)-cinnamaldehyde forms a ternary complex with Ni(II) liver alcohol dehydrogenase and the coenzyme analogue 1,4,5,6-tetrahydronicotinamide-adenine-dinucleotide, stable between pH 6 and 10. The corresponding ternary complex with NADH is only stable at pH greater than 9.0. The spectral redshifts induced in the substrate are 11 nm larger than those found in the zinc enzyme. We suggest direct coordination of the substrate to the catalytic metal on which acts as a Lewis acid in both substrate coordination and catalysis.