Catalytic properties of lipoamide dehydrogenase from Mycobacterium smegmatis

The metabolic acclimation of Arabidopsis thaliana to arsenate is sensitized by the loss of mitochondrial LIPOAMIDE DEHYDROGENASE2, a key enzyme in oxidative metabolism

Mitochondrial lipoamide dehydrogenase is essential for the activity of four mitochondrial enzyme complexes central to oxidative metabolism. The reduction in protein amount and enzyme activity caused by disruption of mitochondrial LIPOAMIDE DEHYDROGENASE2 enhanced the arsenic sensitivity of Arabidopsis thaliana. Both arsenate and arsenite inhibited root elongation, decreased seedling size and increased anthocyanin production more profoundly in knockout mutants than in wild-type seedlings. Arsenate also stimulated lateral root formation in the mutants. The activity of lipoamide dehydrogenase in isolated mitochondria was sensitive to arsenite, but not arsenate, indicating that arsenite could be the mediator of the observed phenotypes. Steady-state metabolite abundances were only mildly affected by mutation of mitochondrial LIPOAMIDE DEHYDROGENASE2. In contrast, arsenate induced the remodelling of metabolite pools associated with oxidative metabolism in wild-type seedlings, an effect that was enhanced in the mutant, especially around the enzyme complexes containing mitochondrial lipoamide dehydrogenase. These results indicate that mitochondrial lipoamide dehydrogenase is an important protein for determining the sensitivity of oxidative metabolism to arsenate in Arabidopsis.

Biological and structural characterization of the Mycobacterium smegmatis nitroreductase NfnB, and its role in benzothiazinone resistance

Tuberculosis is still a leading cause of death in developing countries, for which there is an urgent need for new pharmacological agents. The synthesis of the novel antimycobacterial drug class of benzothiazinones (BTZs) and the identification of their cellular target as DprE1 (Rv3790), a component of the decaprenylphosphoryl-β-d-ribose 2'-epimerase complex, have been reported recently. Here, we describe the identification and characterization of a novel resistance mechanism to BTZ in Mycobacterium smegmatis. The overexpression of the nitroreductase NfnB leads to the inactivation of the drug by reduction of a critical nitro-group to an amino-group. The direct involvement of NfnB in the inactivation of the lead compound BTZ043 was demonstrated by enzymology, microbiological assays and gene knockout experiments. We also report the crystal structure of NfnB in complex with the essential cofactor flavin mononucleotide, and show that a common amino acid stretch between NfnB and DprE1 is likely to be essential for the interaction with BTZ. We performed docking analysis of NfnB-BTZ in order to understand their interaction and the mechanism of nitroreduction. Although Mycobacterium tuberculosis seems to lack nitroreductases able to inactivate these drugs, our findings are valuable for the design of new BTZ molecules, which may be more effective in vivo.

Disruption of ptLPD1 or ptLPD2, genes that encode isoforms of the plastidial lipoamide dehydrogenase, confers arsenate hypersensitivity in Arabidopsis

Arsenic is a ubiquitous environmental poison that inhibits root elongation and seed germination to a variable extent depending on the plant species. To understand the molecular mechanisms of arsenic resistance, a genetic screen was developed to isolate arsenate overly sensitive (aos) mutants from an activation-tagged Arabidopsis (Arabidopsis thaliana) population. Three aos mutants were isolated, and the phenotype of each was demonstrated to be due to an identical disruption of plastidial LIPOAMIDE DEHYDROGENASE1 (ptLPD1), a gene that encodes one of the two E3 isoforms found in the plastidial pyruvate dehydrogenase complex. In the presence of arsenate, ptlpd1-1 plants exhibited reduced root and shoot growth and enhanced anthocyanin accumulation compared with wild-type plants. The ptlpd1-1 plants accumulated the same amount of arsenic as wild-type plants, indicating that the aos phenotype was not due to increased arsenate in the tissues but to an increase in the innate sensitivity to the poison. Interestingly, a ptlpd1-4 knockdown allele produced a partial aos phenotype. Two loss-of-function alleles of ptLPD2 in Arabidopsis also caused elevated arsenate sensitivity, but the sensitivity was less pronounced than for the ptlpd1 mutants. Moreover, both the ptlpd1 and ptlpd2 mutants were more sensitive to arsenite than wild-type plants, and the LPD activity in isolated chloroplasts from wild-type plants was sensitive to arsenite but not arsenate. These findings show that the ptLPD isoforms are critical in vivo determinants of arsenite-mediated arsenic sensitivity in Arabidopsis and possible strategic targets for increasing arsenic tolerance.

Enzymatic characterization of dihydrolipoamide dehydrogenase from Streptococcus pneumoniae harboring its own substrate

This study describes the enzymatic characterization of dihydrolipoamide dehydrogenase (DLDH) from Streptococcus pneumoniae and is the first characterization of a DLDH that carries its own substrate (a lipoic acid covalently attached to a lipoyl protein domain) within its own sequence. Full-length recombinant DLDH (rDLDH) was expressed and compared with enzyme expressed in the absence of lipoic acid (rDLDH(-LA)) or with enzyme lacking the first 112 amino acids constituting the lipoyl protein domain (rDLDH(-LIPOYL)). All three proteins contained 1 mol of FAD/mol of protein, had a higher activity for the conversion of NAD(+) to NADH than for the reaction in the reverse direction, and were unable to use NADP(+) and NADPH as substrates. The enzymes had similar substrate specificities, with the K(m) for NAD(+) being approximately 20 times higher than that for dihydrolipoamide. The kinetic pattern suggested a Ping Pong Bi Bi mechanism, which was verified by product inhibition studies. The protein expressed without lipoic acid was indistinguishable from the wild-type protein in all analyses. On the other hand, the protein without a lipoyl protein domain had a 2-3-fold higher turnover number, a lower K(I) for NADH, and a higher K(I) for lipoamide compared with the other two enzymes. The results suggest that the lipoyl protein domain (but not lipoic acid alone) plays a regulatory role in the enzymatic characteristics of pneumococcal DLDH.

Lipoamide dehydrogenase mediates retention of coronin-1 on BCG vacuoles, leading to arrest in phagosome maturation

Mycobacterium tuberculosis evades the innate antimicrobial defenses of macrophages by inhibiting the maturation of its phagosome to a bactericidal phagolysosome. Despite intense studies of the mycobacterial phagosome, the mechanism of mycobacterial persistence dependent on prolonged phagosomal retention of the coat protein coronin-1 is still unclear. The present study demonstrated that several mycobacterial proteins traffic intracellularly in M. bovis BCG-infected cells and that one of them, with an apparent subunit size of M(r) 50,000, actively retains coronin-1 on the phagosomal membrane. This protein was initially termed coronin-interacting protein (CIP)50 and was shown to be also expressed by M. tuberculosis but not by the non-pathogenic species M. smegmatis. Cell-free system experiments using a GST-coronin-1 construct showed that binding of CIP50 to coronin-1 required cholesterol. Thereafter, mass spectrometry sequencing identified mycobacterial lipoamide dehydrogenase C (LpdC) as a coronin-1 binding protein. M. smegmatis over-expressing Mtb LpdC protein acquired the capacity to maintain coronin-1 on the phagosomal membrane and this prolonged its survival within the macrophage. Importantly, IFNgamma-induced phagolysosome fusion in cells infected with BCG resulted in the dissociation of the LpdC-coronin-1 complex by a mechanism dependent, at least in part, on IFNgamma-induced LRG-47 expression. These findings provide further support for the relevance of the LpdC-coronin-1 interaction in phagosome maturation arrest.