Studies on the Mechanism of Action of Cobamide Coenzymes: CHEMICAL PROPERTIES OF THE ENZYME-COENZYME COMPLEX

Abstract

The complex consisting of dioldehydrase and dimethylbenzimidazolycobamide coenzyme (DBC) can, in addition to its catalytic function, react with oxygen, glycolaldehyde, and glyoxal. The enzyme-coenzyme complex does not serve a catalytic function in these reactions, but participates as a reactant. However, the properties of the noncatalytic reaction suggest that similar mechanisms may be involved in the catalytic and noncatalytic reaction. The reaction with oxygen leads to the inactivation of the enzyme-DBC complex. Spectral changes suggest the formation of hydroxocobalamin. Upon denaturation of the inactive complex, hydroxocobalamin and several nucleosides derived from the adenosyl moiety of DBC are detected. When glycolaldehyde is added to the enzyme-coenzyme complex, the complex becomes inactivated and glycolaldehyde is converted to glyoxal and to an as yet unidentified minor compound. In this reaction, 1 mole of enzyme-coenzyme complex is inactivated per mole of glycolaldehyde consumed. The reaction with glycolaldehyde also results in changes in the spectrum of the coenzyme. When the inactivated complex is denatured, DBC is no longer present; instead, hydroxocobalamin is isolated. A single nucleoside is formed from the adenosyl moiety, which has been tentatively identified as 5'-deoxyadenosine. The noncatalytic reactions show that the enzyme-coenzyme complex can function as electron donor and acceptor and that DBC participates in both of these processes. The ability of the coenzyme to function as oxidizing and reducing agent suggests that its chemical properties are modified as a result of the interaction with the apoenzyme. It is proposed that the modified coenzyme functions in both the catalytic and the noncatalytic reaction and that the ability of the enzyme-coenzyme complex to act as electron donor and acceptor is utilized in the catalytic reaction.