Site‐Specific Substituted Cobalt(II) Horse Liver Alcohol Dehydrogenases

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Abstract
The specific substitution, using highly selective techniques, of catalytic and/or noncatalytic zinc ions by cobaltous ions in horse liver alcohol dehydrogenase (EC 1.I.1.1) has been studied with dissolved, crystalline and agaroseimmobilised enzyme, in order to examine the effect of protein structure on the specificity of the metal exchange. The different binding sites can be clearly distinguished by the absorption spectra of their cobalt derivatives.In solution an anaerobic column chromatographic method made it possible to exchange half of the zinc in the enzyme by cobalt ions in a much shorter time than previous procedures. By raising the temperature in the exchange step, even the slowly exchanging zinc ions were substituted by cobalt, yielding products similar to cobalt alcohol dehydrogenases described earlier.Treatment of crystal suspensions of the enzyme with chelating agents (preferentially dipicolinic acid) gave an inactive protein with two zinc ions remaining bound. The enzyme could be reactivated by treatment of the crystalline protein with 5 mM zinc or cobaltous ions or by dialysis of dissolved inactive protein against 20 μM zinc or 1 mM cobaltous ions. Higher metal concentrations led to denaturation but the inactive protein could be crystallized from solution and then reactivated completely at higher metal concentrations. The preparation and absorption spectrum show that cobalt is bound specifically at the catalytic sites. Since metal substitution at these sites critically depends on the maintenance of the correct tertiary and quaternary structure, these must be preserved in the crystal lattice and partially altered in solution when the catalytic zinc ions are removed (or when excess of metal ions is applied), thus demonstrating the structure‐stabilizing rôle of the catalytic metal ions.The enzyme immobilised on agarose, with unchanged content of active sites [Schneider‐Bernlöhr et al. (1978) Eur. J. Biochem. 41, 475–484], was treated like the crystal suspensions. Although half of the zinc was removed, some activity remained. After reactivation with cobaltous ions, a loss of about 30% active sites was measured. Thus the apparently homogeneous bound enzyme was rather heterogeneous in the properties of its catalytic metal binding sites. These results are taken as further proof for the dependence of the metal substitution on the proper tertiary and quaternary structure which is strained by multiple interactions in the covalently immobilised enzyme.