SYNTHESIS AND TESTING OF HETEROCYCLIC-ANALOGS OF DIAMINOPIMELIC ACID (DAP) AS INHIBITORS OF DAP DEHYDROGENASE AND DAP EPIMERASE

Substrate analogues were synthesized and examined as inhibitors of diaminopimelic acid (DAP) dehydrogenase from Bacillus sphaericus and of DAP epimerase from Escherichia coli. These enzymes produce meso-DAP (3) (a precursor for L-lysine and for peptidoglycan) from L-tetrahydrodipicolinic acid (1) and LL-DAP (2), respectively. The epimerase was purified by an improved procedure and confirmed to require both carboxyl and both amino groups for substrate recognition using deuterium-exchange experiments with DAP isomers, L-lysine, D-lysine, L-alpha-aminopimelate, and racemic alpha-aminopimelate. An imidazole analogue of DAP, (2S)-2-amino-3-(4-carboxyimidazol-1-yl)propanoic acid (4), was synthesized by condensation of benzyl imidazole-4-carboxylate (8) with N-benzyloxycarbonyl(Cbz)-L-serine beta-lactone (9) (product structure confirmed by X-ray analysis) followed by hydrogenolytic deprotection. Two other analogues, (2S,5'R)-2-amino-3-(3-carboxy-2-isoxazolin-5-yl)propanoic acid (5) and its 5'S diastereomer 6, were prepared by condensation of methyl N-Cbz-L-allylglycinate (13) with methyl chlorooximidoacetate (14) followed by separation of isomers and deprotection with NaOH and Me(3)SiCl/NaI. Similar condensation of ethyl chlorooximidoacetate with ethylene and of 14 with ethyl acrylate generated isoxazolines, which were saponified to 2-isoxazoline-3-carboxylic acid (25) and 2-isoxazoline-3,5-dicarboxylic acid (26), respectively. None of the compounds show significant inhibition of DAP epimerase or DAP dehydrogenase with the exception of 6, which is a potent and specific inhibitor of DAP dehydrogenase. At pH 7.5 or 7.8, compound 6 shows competitive inhibition (K-i = 4.2 mu M) with tetrahydrodipicolinic acid (1) for the forward reaction and noncompetitive inhibition (K-i = 23 mu M) with meso-DAP (3) for the reverse process. Preliminary tests for antimicrobial activity demonstrate that 6 inhibits the growth of B. sphaericus, which relies exclusively on DAP dehydrogenase to produce 3.