meso-Diaminopimelate dehydrogenase (meso-DAPDH) is an NADP؉ -dependent enzyme which catalyzes the reversible oxidative deamination on the D-configuration of meso-2,6-diaminopimelate to produce L-2-amino-6-oxopimelate. In this study, the gene encoding a meso-diaminopimelate dehydrogenase from Symbiobacterium thermophilum was cloned and expressed in Escherichia coli. In addition to the native substrate meso-2,6-diaminopimelate, the purified enzyme also showed activity toward Dalanine, D-valine, and D-lysine. This enzyme catalyzed the reductive amination of 2-keto acids such as pyruvic acid to generate D-amino acids in up to 99% conversion and 99% enantiomeric excess. Since meso-diaminopimelate dehydrogenases are known to be specific to meso-2,6-diaminopimelate, this is a unique wild-type meso-diaminopimelate dehydrogenase with a more relaxed substrate specificity and potential for D-amino acid synthesis. The enzyme is the most stable meso-diaminopimelate dehydrogenase reported to now. Two amino acid residues (F146 and M152) in the substrate binding sites of S. thermophilum meso-DAPDH different from the sequences of other known meso-DAPDHs were replaced with the conserved amino acids in other meso-DAPDHs, and assay of wild-type and mutant enzyme activities revealed that F146 and M152 are not critical in determining the enzyme's substrate specificity. The high thermostability and relaxed substrate profile of S. thermophilum meso-DAPDH warrant it as an excellent starting enzyme for creating effective D-amino acid dehydrogenases by protein engineering. N ot only do D-amino acids serve as specialized components of many types of machineries in living organisms, such as neural signaling (20), and bacterial cell walls (19), but they also are important components or building blocks in the production of pharmaceuticals and other fine chemicals (4, 5, 12). As such, many methods for the synthesis of D-amino acids and their derivatives have been developed. Just as L-amino acid dehydrogenases are useful for preparation of L-amino acids from the corresponding 2-keto acids (6,7,13,21), the use of D-amino acid dehydrogenase (D-AADH) should offer a straightforward approach, in which the enzyme catalyzes the reductive amination of 2-keto acid to give D-amino acid. However, NAD(P)H-dependent D-amino acid dehydrogenase is much less abundant in nature than its L-amino acid counterpart and largely unexplored. The most-known D-AADH is meso-diaminopimelate dehydrogenase (meso-DAPDH; EC 1.4.1.16), which is a key enzyme in the lysine biosynthetic pathway and has been found in bacteria, such as Bacillus sphaericus (17) and Corynebacterium glutamicum (14). In addition, meso-DAPDH has also been isolated from plants, for example, soybeans (Glycine max) (24). meso-DAPDH is NADP ϩ dependent and catalyzes the reversible oxidative deamination on the D-configuration center of meso-2,6-diaminopimelate (meso-DAP) to yield L-2-amino-6-oxopimelate (17). However, previously reported meso-DAPDHs are generally specific toward meso-DAP and showed only very ...