Mechanism of action of ethanolamine ammonia-lyase, an adenosylcobalamin-dependent enzyme. Interaction between the enzyme and a postulated organocobalamin intermediate.

Ethanolamine ammonia-lyase catalyzes the adenosylcobalamin (AdoCbl)-dependent conversion of ethanolamine to acetaldehyde and ammonia. It has been proposed that the mechanism of this and other AdoCbl-dependent rearrangements involves the formation and rearrangement of an organocobalamin in which a substrate fragment is coordinated to the corrin metal by a carbon-cobalt bond. In the case of ethanolamine ammonia-lyase, this mechanism states that one of the catalytic intermediates is a complex in which 5’-deoxyadenosine and /3-amino+-hydroxyethylcobalamin are bound to the active site of the enzyme. This complex, which arises when the substrate-containing organocobalamin initially formed at the active site undergoes rearrangement, then reacts further to release the products (acetaldehyde and NH,+) and regenerate the original cofactor, AdoCbl. To obtain evidence regarding this proposal, experiments were conducted to determine whether this complex, if constructed from its constituents, would react to form AdoCbl. For these experiments, formylmethylcobalamin plus ammonia (j3-amino+hydroxyethylcobalamin is the NH3 adduct of formylmethylcobalamin) were incubated with 5’-deoxyadenosine and enzyme, and AdoCbl formation was assayed by determining whether added [14Clethanolamine was converted to [Wlacetaldehyde. All three of the low molecular weight constituents of the complex, namely, formylmethylcobalamin, 5’-deoxyadenosine, and ammonia, have been shown in this and other studies to bind to the active site of ethanolamine ammonia-lyase. Nevertheless, the formation of AdoCbl was not detected by an assay sensitive enough to measure the conversion to cofactor of 0.0006% of the added formylmethylcobalamin. This finding supports other data which suggest that the alkylation of the corrin metal by the substrate does not occur during rearrangements catalyzed by AdoCbl-requiring enzymes.