Crystallization and preliminary X-ray study of the 8-amino-7-oxopelargonate synthase fromBacillus sphaericus

The Biotin Biosynthetic Pathway in Mycobacterium tuberculosis is a Validated Target for the Development of Antibacterial Agents

Mycobacterium tuberculosis, responsible for Tuberculosis (TB), remains the leading cause of mortality among infectious diseases worldwide from a single infectious agent, with an estimated 1.7 million deaths in 2016. Biotin is an essential cofactor in M. tuberculosis that is required for lipid biosynthesis and gluconeogenesis. M. tuberculosis relies on de novo biotin biosynthesis to obtain this vital cofactor since it cannot scavenge sufficient biotin from a mammalian host. The biotin biosynthetic pathway in M. tuberculosis has been well studied and rigorously genetically validated providing a solid foundation for medicinal chemistry efforts. This review examines the mechanism and structure of the enzymes involved in biotin biosynthesis and ligation, summarizes the reported genetic validation studies of the pathway, and then analyzes the most promising inhibitors and natural products obtained from structure-based drug design and phenotypic screening.

Pyridoxal-5'-phosphate-dependent enzymes involved in biotin biosynthesis: structure, reaction mechanism and inhibition

The four last steps of biotin biosynthesis, starting from pimeloyl-CoA, are conserved among all the biotin-producing microorganisms. Two enzymes of this pathway, the 8-amino-7-oxononanoate synthase (AONS) and the 7,8-diaminopelargonic acid aminotransferase (DAPA AT) are dependent on pyridoxal-5'-phosphate (PLP). This review summarizes our current understanding of the structure, reaction mechanism and inhibition on these two interesting enzymes. Mechanistic studies as well as the determination of the crystal structure of AONS have revealed a complex mechanism involving an acylation with inversion of configuration and a decarboxylation with retention of configuration. This reaction mechanism is shared by the homologous 5-aminolevulinate synthase and serine palmitoyltransferase. While the reaction catalyzed by DAPA AT is a classical PLP-dependent transamination, the inactivation of this enzyme by amiclenomycin, a natural antibiotic that is active against Mycobacterium tuberculosis, involves the irreversible formation of an adduct between PLP and amiclenomycin. Mechanistic and structural studies allowed the complete description of this unique inactivation mechanism. Several potent inhibitors of these two PLP-dependent enzymes have been prepared and might be useful as starting points for the design of herbicides or antibiotics. This article is part of a Special Issue entitled: Pyridoxal Phospate Enzymology.

Biosynthesis of biotin and lipoic acid

The genetics and mechanistic enzymology of biotin biosynthesis have been the subject of much investigation in the last decade, owing to the interest for biotin production by fermentation, on the one hand, and for the design of inhibitors with potential herbicidal properties, on the other hand. Four enzymes are involved in the synthesis of biotin from its two precursors, alanine and pimeloyl-CoA. They are now well-characterized and the X-ray structures of the first three have been published. 8-Amino-7-oxopelargonic acid synthase is a pyridoxal 5'-phosphate (PLP) enzyme, very similar to other acyl-CoA alpha-oxoamine synthases, and its detailed mechanism has been determined. The origin of its specific substrate, pimeloyl-CoA, however, is not completely established. It could be produced by a modified fatty acid pathway involving a malonyl thioester as the starter. 7,8-Diaminopelargonic acid (DAPA) aminotransferase, although sharing sequence and folding homologies with other transaminases, is unique as it uses S-adenosylmethionine (AdoMet) as the NH2 donor. The mechanism of dethiobiotin synthethase is also now well understood. It catalyzes the formation of the ureido ring via a DAPA carbamate activated with ATP. On the other hand, the mechanism of the last enzyme, biotin synthase, which has long raised a very puzzling problem, is only starting to be unraveled and appears indeed to be very complex. Biotin synthase belongs to the family of AdoMet-dependent enzymes that reductively cleave AdoMet into a deoxyadenosyl radical, and it is responsible for the homolytic cleavage of C-H bonds. A first radical formed on dethiobiotin is trapped by the sulfur donor, which was found to be the iron-sulfur (Fe-S) center contained in the enzyme, and cyclization follows in a second step. Two important features come from these results: (1) a new role for an Fe-S center has been revealed, and (2) biotin synthase is not only a catalyst but also a substrate for the reaction. Lipoate synthase, which catalyzes the formation of two C-S bonds from octanoic acid, has a very high sequence similarity with biotin synthase. Although no in vitro enzymology has been carried out with lipoate synthase, the sequence homology as well as the results of in vivo studies support the conclusion that both enzymes are strongly mechanistically related.

Slow-binding and competitive inhibition of 8-amino-7-oxopelargonate synthase, a pyridoxal-5'-phosphate-dependent enzyme involved in biotin biosynthesis, by substrate and intermediate analogs. Kinetic and binding studies

8-Amino-7-oxopelargonate synthase catalyzes the first committed step of biotin biosynthesis in micro-organisms and plants. Because inhibitors of this pathway might lead to antibacterials or herbicides, we have undertaken an inhibition study on 8-amino-7-oxopelargonate synthase using six different compounds. d-Alanine, the enantiomer of the substrate of this pyridoxal-5'-phosphate-dependent enzyme was found to be a competitive inhibitor with respect to l-alanine with a Ki of 0.59 mm. The fact that this inhibition constant was four times lower than the Km for l-alanine was interpreted as the consequence of the inversion-retention stereochemistry of the catalyzed reaction. Schiff base formation between l or d-alanine and pyridoxal-5'-phosphate, in the active site of the enzyme, was studied using ultraviolet/visible spectroscopy. It was found that l and d-alanine form an external aldimine with equilibrium constants K = 4.1 mm and K = 37.8 mm, respectively. However, the equilibrium constant for d-alanine aldimine formation dramatically decreased to 1.3 mm in the presence of saturating concentration of pimeloyl-CoA, the second substrate. This result strongly suggests that the binding of pimeloyl-CoA induces a conformational change in the active site, and we propose that this new topology is complementary to d-alanine and to the putative reaction intermediate since they both have the same configuration. (+/-)-8-Amino-7-oxo-8-phosphonononaoic acid (1), the phosphonate derivative of the intermediate formed during the reaction, was our most potent inhibitor with a Ki of 7 microm. This compound behaved as a reversible slow-binding inhibitor, competitive with respect to l-alanine. Kinetic investigation showed that this slow process was best described by a one-step mechanism (mechanism A) with the following rate constants: k1 = 0.27 x 103 m-1.s-1, k2 = 1.8 s-1 and half-life for dissociation t1/2 = 6.3 min. The binding of compound 1 to the enzyme was also studied using ultraviolet/visible spectroscopy, and the data were consistent with the kinetic data (K = 4.2 microm). Among the other compounds tested, two potential transition state analogs, 4-carboxybutyl(1-amino-1-carboxyethyl)phosphonate (4) and 2-amino-3-hydroxy-2-methylnonadioic acid (5) were found to be competitive inhibitors with respect to l-alanine with Ki of 68 microm and 80 microm, respectively.

The crystal structure of 8-amino-7-oxononanoate synthase: a bacterial PLP-dependent, acyl-CoA-condensing enzyme

8-Amino-7-oxononanoate synthase (or 8-amino-7-ketopelargonate synthase; EC 2.3.1.47; AONS) catalyses the decarboxylative condensation of l-alanine and pimeloyl-CoA in the first committed step of biotin biosynthesis. We have cloned, over-expressed and purified AONS from Escherichia coli and determined the crystal structures of the apo and PLP-bound forms of the enzyme. The protein is a symmetrical homodimer with a tertiary structure and active site organisation similar to, but distinct from, those of other PLP-dependent enzymes whose three-dimensional structures are known. The critical PLP-binding lysine of AONS is located at the end of a deep cleft that allows access of the pantothenate arm of pimeloyl-CoA. A cluster of positively charged residues at the entrance to this cleft forms a putative diphosphate binding site for CoA. The structure of E. coli AONS enables identification of the key residues of the PLP-binding site and thus provides a framework with which to understand the biochemical mechanism, which is similar to that catalysed by 5-aminolevulinate synthase and two other alpha-oxoamine synthases. Although AONS has a low overall sequence similarity with the catalytic domains of other alpha-oxoamine synthases, the structure reveals the regions of significant identity to be functionally important. This suggests that the organisation of the conserved catalytic residues in the active site is similar for all enzymes of this sub-class of PLP-dependent enzymes and they share a common mechanism. Knowledge of the three-dimensional structure of AONS will enable characterisation of the structural features of this enzyme sub-family that are responsible for this important type of reaction.