Mechanism of action of ethanolamine ammonia-lyase, an adenosylcobalamin-dependent enzyme. Proton nuclear magnetic resonance studies of the binding of adenine nucleosides and substrate to ethanolamine ammonia-lyase.

Abstract

Proton NMR spectroscopy was used to study the binding of adenosine, 5'-deoxyadenosine, adenine, and ethanolamine to the adenosylcobalamin-dependent enzyme ethanolamine ammonia-lyase. Broadening of ligand resonances in the presence of ethanolamine ammonia-lyase indicated that adenosine, 5'-deoxyadenosine, and ethanolamine all formed complexes with the enzyme (KD(mM) = 3.5, 3.0, and 2.5 respectively). The methyl group of enzyme-bound 5'-deoxyadenosine rotated at a rate exceeding 10(7) revolutions/s. Adenine did not appear to bind to the enzyme. Rates of dissociation of nucleosides from the enzyme were fast on the NMR time scale, precluding measurements of rate constants for the binding reaction. The departure of ethanolamine was slow, however, permitting their determination. The values for these rate constants were: k1 = 4.4 times 10(5) M-1 S-1; k-1 = 1.1 times 10(3) S-1. Addition of 1 mol of cyanocobalamin/mol of active sites led to narrowing of the enzyme-broadened ligand resonances. With 5'-deoxyadenosine, linewidths still exceeded those of the free ligand, indicating that binding to enzyme was weakened but not abolished. The KD for this nucleoside in the presence of CNCbl was 8.0 mM. With ethanolamine and adenosine, however, linewidths reverted to values characteristic of the unbound ligand, indicating either that CNCbl greatly lowered the rate of dissociation of the ligand or displaced the ligand from the enzyme. A decision between these two possibilities could not be made from the data at hand, although analogy with the situation obtaining with 5'-deoxyadenosine suggests that adenosine is displaced from the enzyme by CNCbl. 5'-Deoxyadenosine inhibited catalytic activity of the enzyme, competing with adenosylcobalamin (Ki = 2.7 mM). Adenosine had no effect, despite NMR evidence indicating that it formed a complex with free enzyme. These experiments showed that ethanolamine ammonia-lyase possesses binding sites for adenine nucleosides, a class of compounds chemically related to the Cobeta-ligand of the cofactor, as well as for ethanolamine. Binding to the enzyme has now been demonstrated for all three categories of low molecular weight compounds thought to be involved in the reaction; namely, substrate (ethanolamine), corrin, and adenine nucleoside.