The antibiotic amiclenomycin blocks the biosynthesis of biotin by inhibiting the pyridoxal-phosphate-dependent enzyme diaminopelargonic acid synthase. Inactivation of the enzyme is stereoselective, i.e. the cis isomer of amiclenomycin is a potent inhibitor, whereas the trans isomer is much less reactive. The crystal structure of the complex of the holoenzyme and amiclenomycin at 1.8 Å resolution reveals that the internal aldimine linkage between the cofactor and the side chain of the catalytic residue Lys-274 is broken. Instead, a covalent bond is formed between the 4-amino nitrogen of amiclenomycin and the C4 carbon atom of pyridoxal-phosphate. The electron density for the bound inhibitor suggests that aromatization of the cyclohexadiene ring has occurred upon formation of the covalent adduct. This process could be initiated by proton abstraction at the C4 carbon atom of the cyclohexadiene ring, possibly by the proximal side chain of Lys-274, leading to the tautomer Schiff base followed by the removal of the second allylic hydrogen. The carboxyl tail of the amiclenomycin moiety forms a salt link to the conserved residue Arg-391 in the substrate-binding site. Modeling suggests steric hindrance at the active site as the determinant of the weak inhibiting potency of the trans isomer.The biosynthesis of the vitamin biotin, a cofactor in biological carboxylation reactions, occurs in micro-organisms and plants and involves at least four different steps (Scheme 1) (1-12). The second step in this pathway, the conversion of 7-keto-8-aminopelargonic acid (KAPA) 1 into 7,8-diaminopelargonic acid (DAPA), is catalyzed by DAPA synthase, an aminotransferase that requires PLP as a cofactor (6, 7). DAPA synthase from Escherichia coli is a homodimer with a molecular mass of 94 kDa (6, 13) and contains 429 residues per monomer (14). The enzyme is unique among aminotransferases in that it uses S-adenosyl-L-methionine (SAM) as an amino group donor. The reaction catalyzed by DAPA synthase is typical of that of an aminotransferase and follows the general mechanism seen in other PLP-dependent enzymes (15).The crystal structure of DAPA synthase (16) shows that DAPA synthase belongs to the fold type I family of PLPdependent enzymes (17, 18). The monomer of DAPA synthase consists of two domains, a small domain comprising the N-and C-terminal part of the polypeptide chain (residues 1-49 and 330 -429) and a large domain formed by the intervening residues containing the cofactor-binding site. In the crystal, the two subunits of the homodimer of DAPA synthase are related by a 2-fold non-crystallographic axis (16). The PLP-binding site is located between the two domains of the monomer at the interface of the two subunits. In the enzyme-PLP complex, the cofactor is covalently linked to the ⑀-amino group of Lys-274, a residue functionally invariant in the whole family of fold-type I PLP-dependent enzymes.Because biotin synthesis is unique to plants and microorganisms, enzymes of this pathway are potential targets for the development of antimicr...