Characterization of a thermostable dihydrodipicolinate synthase from Thermoanaerobacter tengcongensis.

Dihydrodipicolinate synthase (DHDPS) catalyses the first reaction of the (S)-lysine biosynthesis pathway in bacteria and plants. The hypothetical gene for dihydrodipicolinate synthase (dapA) of Thermoanaerobacter tengcongensis was found in a cluster containing several genes of the diaminopimelate lysine-synthesis pathway. The dapA gene was cloned in Escherichia coli, DHDPS was subsequently produced and purified to homogeneity. The T. tengcongensis DHDPS was found to be thermostable (T0.5=3 h at 90 degrees C). The specific condensation of pyruvate and (S)-aspartate-beta -semialdehyde was catalyzed optimally at 80 degrees C at pH 8.0. Enzyme kinetics were determined at 60 degrees C, as close as possible to in vivo conditions. The established kinetic parameters were in the same range as for example E. coli dihydrodipicolinate synthase. The specific activity of the T. tengcongensis DHDPS was relatively high even at 30 degrees C. Like most dihydrodipicolinate synthases known at present, the DHDPS of T. tengcongensis seems to be a tetramer. A structural model reveals that the active site is well conserved. The binding site of the allosteric inhibitor lysine appears not to be conserved, which agrees with the fact that the DHDPS of T. tengcongensis is not inhibited by lysine under physiological conditions.