The mechanism of action of ethanolamine ammonia-lyase, an adenosylcobalamin-dependent enzyme. Studies with isopropanolamine, a true substrate.

Ethanolamine ammonia-lyase catalyzes the adenosylcobalamin (AdoCbl)-dependent deamination of 1-amino-3-propanol (isopropanolamine) to acetone and NH3. During the course of the reaction a hydrogen is transferred from the carbinol carbon of the substrate to one of the methyl carbons of the product, the cofactor serving as intermediate hydrogen carrier. This result, together with previous results obtained with this and other AdoCbl-requiring enzymes, suggests that an initial step in the catalytic mechanism is the abstraction of a hydrogen atom from the substrate to produce the 1-amino-2-hydroxyprop-2-yl radical. This is a bulky species which for steric reasons is unlikely to alkylate cob(II)alamin. Thus, an organocobalamin formed by a reaction between the substrate radical and cob(II)alanin is probably not involved in the mechanism of deamination of isopropanolamine by ethanolamine ammonia-lyase.