The Mechanism of Action of Ethanolamine Ammonia Lyase, a B12-dependent Enzyme

Abstract

Abstract Ethanolamine ammonia lyase catalyzes the coenzyme B12-dependent conversion of ethanolamine to acetaldehyde and NH3. When the enzyme-coenzyme complex is frozen in liquid nitrogen during the act of catalysis, an electron spin resonance signal equivalent to about 0.04 spin per active site can be detected. By saturation experiments, the signal consists of two components, a broad derivative signal composed of two peaks (g = 2.34 and g = 2.08) assigned to the cobalt of B12r, and a narrow signal (g = 2.007) assigned to a free radical. With [1,1-D2]ethanolamine as substrate, the shape and position of the signal was unchanged, but its size was increased. In the absence of substrate, a signal was still observed, but the concentration of unpaired electrons was reduced. The broad metal signal was unchanged except for a decrease in size. The radical signal was narrower (10 gauss, compared with 18 gauss in the presence of substrate) and was shifted (g = 2.003) with respect to the substrate-dependent signal, suggesting that the radical in the absence of substrate may have been different from the substrate-dependent radical. These observations support the proposal that the mechanism of action of this coenzyme B12-dependent enzyme involves homolysis of the carbon-cobalt bond of the coenzyme.