In order to enlarge the substrate binding pocket of the meso-diaminopimelate dehydrogenase from Symbiobacterium thermophilum to accommodate larger 2-keto acids, four amino acid residues (Phe146, Thr171, Arg181, and His227) were targeted for site saturation mutagenesis.Among all mutants, the single mutant H227V had a specific activity of 2.39 ± 0.06 U· mg-1, which was 35.1-fold enhancement over the wild-type enzyme.