New information regarding the binding of coenzyme B12 analogs to propanediol dehydratase was obtained from a study of apoenzyme-coenzyme Bi2 analog complexes which may be used as a model for the holoenzyme. Requirements for the binding of some coenzyme analogs, such as cyano-and methylcobalamin, to the apoenzyme were the same as those for coenzyme B22 binding. A monovalent cation such as potassium ion was absolutely required; the substrate also facilitated the binding, but to a lesser degree. Cyanocobalamin was bound to the apoenzyme to approximately the same extent as coenzyme B12. Treatment of the apoenzyme with a sulfhydryl inhibitor, p-chloromercuribenzoate, caused a remarkable inhibition of the binding of these cobalamins. The apoenzyme-cyanocobalamin and apoenzymemethylcobalamin complexes, like the holoenzyme, resisted inactivation by p-chloromercuribenzoate. These results suggest that the binding of coenzyme B22 to the apoenzyme is similar to the binding of these coenzyme analogs, and that c X^Voenzyme B,»* 1 dependent propanediol dehydratase (dl-1,2-propanediol hydro-lyase, EC 4.2.1.28) from Aerobacter aerogenes catalyzes the conversion of l-or d-1,2-propanediol to propionaldehyde and of 1,2-ethanediol to acetaldehyde (Lee and Abeles, 1963). Some analogs of coenzyme Bí2, such as cyano-, hydroxo-, and methylcobalamin, inhibit coenzyme B12 requiring enzymes, such as propanediol dehydratase (Lee and Abeles, 1963), glycerol dehydratase (Pawelkiewicz and Schneider, 1967), and ethanolamine deaminase (Kaplan and Stadtman, 1968b;Babior, 1969), very strongly and almost irreversibly. It has been generally accepted that the powerful inhibition by these cobalamins is due to their tight binding to an apoenzyme, forming inactive, undissociable apoenzymecoenzyme analog complexes. These complexes themselves are catalytically inactive, and no method for resolving the complexes into the native apoenzyme and cobalamin under mild conditions has hitherto been found. Therefore, the properties of apoenzyme-coenzyme Bi2 analog complexes remain almost unknown in any coenzyme Bi2 dependent enzymes, except for some properties of the glycerol dehydratase apoenzyme-hydroxocobalamin complex (Schneider et al., 1970).
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