Dihydrodipicolinate synthase
4-Hydroxy-tetrahydrodipicolinate synthase (, "dihydrodipicolinate synthase", "dihydropicolinate synthetase", "dihydrodipicolinic acid synthase", "L-aspartate-4-semialdehyde hydro-lyase (adding pyruvate and cyclizing)", "dapA (gene)") is an enzyme with the systematic name "L-aspartate-4-semialdehyde hydro-lyase (adding pyruvate and cyclizing; (4S)-4-hydroxy-2,3,4,5-tetrahydro-(2S)-dipicolinate-forming)". This enzyme catalyses the following chemical reaction The reaction proceeds in three consecutive steps. Function. This enzyme belongs to the family of lyases, specifically the amine-lyases, which cleave carbon-nitrogen bonds. 4-hydroxy-tetrahydrodipicolinate synthase is the key enzyme in lysine biosynthesis via the diaminopimelate pathway of prokaryotes, some phycomycetes, and higher plants. The enzyme  catalyses the condensation of L-aspartate-beta-semialdehyde and pyruvate to 4-hydroxy-tetrahydropicolinic acid via a ping-pong mechanism in which pyruvate binds to the enzyme by forming a Schiff base with a lysine residue. Related enzymes. Three other proteins are structurally related to this enzyme and probably also act via a similar catalytic mechanism. These are "Escherichia coli" N-acetylneuraminate lyase () (protein NanA), which catalyses the condensation of "N"-acetyl-D-mannosamine and pyruvate to form "N"-acetylneuraminate; "Rhizobium meliloti" ("Sinorhizobium meliloti") protein MosA, which is involved in the biosynthesis of the rhizopine 3-"O"-methyl-scyllo-inosamine; and "E. coli" hypothetical  protein YjhH. Structure. The sequences of 4-hydroxy-tetrahydrodipicolinate synthase from different sources are well-conserved. The structure takes the form of a homotetramer, in which 2 monomers are related by an approximate 2-fold symmetry. Each monomer comprises 2 domains: an 8-fold alpha-/beta-barrel, and a C-terminal alpha-helical domain. The fold resembles that of "N"-acetylneuraminate lyase. The active site lysine is located in the barrel domain, and has access via 2 channels on the C-terminal side of the barrel.