Methylasparate ammonium lyase (MAL) is involved in the methylaspartate cycle and catalyses the formation of the alpha,beta-unsaturated bond by the reversible anti elimination of ammonia from L-threo-beta-methylaspartate (L-threo-(2S,3S)-3-methylaspartate) to give mesaconate. It can also use L-erythro-beta-methylaspartate (L-erythro-(2S,3R)-3-methylaspartate), L-aspartate, fumarate and ethylfumarate as substrates. It will also catalyse the stereo- and regioselective addition of ammonia to several derivatives of mesaconic acid to give a limited number of homochiral substituted aspartic acids. The structure of MAL is similar to that of members of the enolase superfamily, which all catalyse removal of a proton alpha to a carboxylic acid as the first step in their mechanism.