Catalyzes the stereoinversion of LL-2,6-diaminoheptanedioate (L,L-DAP) to meso-diaminoheptanedioate (meso-DAP), a precursor of L-lysine and an essential component of the bacterial peptidoglycan (PubMed:10194362, PubMed:16723397). Only accepts DAP isomers with the L configuration (PubMed:16723397). (2S,6S)-2,6-diaminoheptanedioate = meso-2,6-diaminoheptanedioate Inhibited by LL-aziridino (LL-AziDAP), DL-aziridino (DL-AziDAP) (PubMed:16723397). Also inhibited by (2S,3R,6S)-2,6-diamino-3-fluoropimelate (L,L-3-fluoro-DAP) and (2R,3S,6S)-2,6-diamino-3-fluoropimelate (D,L-3-fluoro-DAP) (Ref.3). kcat is 128 and 82 sec(-1) for L,L-DAP and D,L-DAP, respectively. Amino-acid biosynthesis; L-lysine biosynthesis via DAP pathway; DL-2,6-diaminopimelate from LL-2,6-diaminopimelate: step 1/1. Homodimer (Potential). Previously DapF has been proposed to be a monomer, however it seems that it adopts a dimeric structure (PubMed:9843410, PubMed:17889830). DapF utilizes a two-base mechanism involving a pair of cysteine residues (Cys-73 and Cys-217). Belongs to the diaminopimelate epimerase family.