Kinetics and mechanism of syn-elimination of ammonia from (2S,3R)-3-methylaspartic acid by methylaspartase

Methylaspartase catalyses the slow syn-elimination of ammonia from the (2S,3R)-[L-erythro]-diastereoisomer of the natural substrate (2S,3S)-3-methylaspartic acid, to give mesaconic acid; the reaction does not involve C-3 epimerisation followed by normal anti-elimination, ruling-out the possibility of a carbanion intermediate, but, displays large primary deuterium isotope effects consistent with concerted C–H and C–N bond cleavage.