The mechanism of action of ethanolamine ammonia-lyase, an adenosylcobalamin-dependent enzyme. Evidence for a carboxyl group at the active site

Ethanolamine ammonia-lyase is an adenosylcobalamin-dependent enzyme that catalyzes the rearrangement of ethanolamine and other vicinal amino alcohols to oxo-compounds and ammonia. Treatment of this enzyme with the sulfhydryl group-blocking reagent methyl methanethiosulfonate produces a species with diminished catalytic activity. When methyl methanethiosulfonate -treated ethanolamine ammonia-lyase was incubated with a carboxyl-blocking reagent consisting of glycine ethyl ester plus a water-soluble carbodiimide, the enzyme lost more than 80% of its residual activity, while at the same time glycine ethyl ester was incorporated into it at a stoichiometry of 6 mol/mol of enzyme. Both the loss of activity and the incorporation of glycine ethyl ester were prevented if ethanolamine was included in the glycine ethyl ester-containing incubation mixture. These results suggest that an active site carboxyl group plays a role in the mechanism of catalysis by ethanolamine ammonia-lyase, and that this carboxyl group is amidated when the enzyme is incubated with glycine ethyl ester plus carbodiimide.