Abstract
The noncatalytic zinc in horse liver alcohol dehydrogenase was selectively replaced by nickel(II). This novel species, Zn(c) 2Ni(n) 2 § horse liver alcohol dehydrogenase (where c denotes the catalytic and n denotes the noncatalytic site) was compared to Zn(c) 2Co(n) 2 horse liver alcohol dehydrogenase with respect to its absorption, circular dichroism and magnetic circular dichroism spectra, as well as its magnetic moment. For Zn(c) 2Co(n) 2 horse liver alcohol dehydrogenase (prepared according to refs. 1 and 2) the extinction coefficients were redetermined in the UV, visible and near-infrared region and the molar ellipticities in the range 300-800 nm. The average magnetic moment was determined by the NMR method as 4.5-5.0 B.M. The results confirm a tetrahedral structure in the zinc-cobalt enzyme. In contrast, the spectroscopic data and the zero magnetic moment support a planar geometry for the nickel(Il) bound in the noncatalytic site. Zn(c) 2Ni(n) 2 horse liver alcohol dehydrogenase is very temperature-sensitive and precipitates after short exposure to room temperature. Stored in the cold it has the same activity as the native enzyme. The results indicate that the protein is flexible in the loop region binding the noncatalytic metal ion and that it may retain catalytic activity even in a partially distorted conformation.
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