3-Methylaspartate ammonia-lyase (MAL) catalyzes the reversible amination of mesaconate to give both (2S,3S)-3-methylaspartic acid and (2S,3R)-3-methylaspartic acid as products. The deamination mechanism of MAL is likely to involve general base catalysis, in which a catalytic base abstracts the C3 proton of the respective stereoisomer to generate an enolate anion intermediate that is stabilized by coordination to the essential active-site Mg(II) ion. The crystal structure of MAL in complex with (2S,3S)-3-methylaspartic acid suggests that Lys331 is the only candidate in the vicinity that can function as a general base catalyst. The structure of the complex further suggests that two other residues, His194 and Gln329, are responsible for binding the C4 carboxylate group of (2S,3S)-3-methylaspartic acid, and hence are likely candidates to assist the Mg(II) ion in stabilizing the enolate anion intermediate. In this study, the importance of Lys331, His194, and Gln329 for the activity and stereoselectivity of MAL was investigated by site-directed mutagenesis. His194 and Gln329 were replaced with either an alanine or arginine, whereas Lys331 was mutated to a glycine, alanine, glutamine, arginine, or histidine. The properties of the mutant proteins were investigated by circular dichroism (CD) spectroscopy, kinetic analysis, and (1)H NMR spectroscopy. The CD spectra of all mutants were comparable to that of wild-type MAL, and this indicates that these mutations did not result in any major conformational changes. Kinetic studies demonstrated that the mutations have a profound effect on the values of k(cat) and k(cat)/K(M); this implicates Lys331, His194 and Gln329 as mechanistically important. The (1)H NMR spectra of the amination and deamination reactions catalyzed by the mutant enzymes K331A, H194A, and Q329A showed that these mutants have strongly enhanced diastereoselectivities. In the amination direction, they catalyze the conversion of mesaconate to yield only (2S,3S)-3-methylaspartic acid, with no detectable formation of (2S,3R)-3-methylaspartic acid. The results are discussed in terms of a mechanism in which Lys331, His194, and Gln329 are involved in positioning the substrate and in formation and stabilization of the enolate anion intermediate.