Aspartate ammonia lyases (also referred to as aspartases) catalyze the revers-ible deamination of l-aspartate to yield fumarate and ammonia. In the pro-posed mechanism for these enzymes, an active site base abstracts a proton from C3 ofl-aspartate to form an enzyme-stabilized enediolate intermediate. Ketonization of this intermediate eliminates ammonia and yields the prod-uct, fumarate.