Scavenging of Cytosolic Octanoic Acid by Mutant LplA Lipoate Ligases Allows Growth of Escherichia coli Strains Lacking the LipB Octanoyltransferase of Lipoic Acid Synthesis

The LipB octanoyltransferase catalyzes the first step of lipoic acid synthesis in Escherichia coli, transfer of the octanoyl moiety from octanoyl-acyl carrier protein to the lipoyl domains of the E2 subunits of the 2-oxoacid dehydrogenases of aerobic metabolism. Strains containing null mutations in lipB are auxotrophic for either lipoic acid or octanoic acid. We report the isolation of two spontaneously arising mutant strains that allow growth of lipB strains on glucose minimal medium; we determined that suppression was caused by single missense mutations within the coding sequence of the gene (lplA) that encodes lipoate-protein ligase. The LplA proteins encoded by the mutant genes have reduced K_m values for free octanoic acid and thus are able to scavenge cytosolic octanoic acid for octanoylation of lipoyl domains.Escherichia coli has three lipoic acid-dependent enzyme systems: pyruvate dehydrogenase (PDH), 2-oxoglutarate dehydrogenase (OGDH), and the glycine cleavage system (GCV) (8). PDH catalyzes the oxidative decarboxylation of pyruvate to acetyl-coenzyme A (CoA), the tricarboxylic acid (TCA) cycle substrate and fatty acid building block. OGDH functions in the TCA cycle, where it catalyzes the decarboxylation of 2-oxoglutarate to succinyl-CoA, the precursor of several amino acids. GCV is involved in the breakdown of glycine into ammonia and C₁ units. Whereas GCV is expressed only in the presence of glycine, PDH and OGDH are required for aerobic growth. (During anaerobic growth, acetyl-CoA is synthesized by other enzymes and an OGDH-independent branched form of the TCA cycle forms succinyl-CoA from succinate.) The three enzyme systems contain subunits (the E2 subunits of PDH and OGDH and the H protein of GCV) which contain at least one lipoyl domain, a conserved structure of ca. 80 residues (8). Lipoic acid is attached in an amide bond to a specific lysine residue of these domains, where it functions as a classical “swinging arm,” carrying reaction intermediates between the active sites of the lipoate-dependent systems (27).Lipoic acid [R-5-(1,2-dithiolan-3-yl)pentanoic acid, also called 6,8-dithiooctanoic acid and thioctic acid] is composed of an eight-carbon fatty acid backbone to which two sulfur atoms are attached at carbons 6 and 8 (Fig. ​(Fig.1).1). In the oxidized state, the sulfur atoms are in a disulfide linkage forming a five-membered ring with three backbone carbons. The disulfide bond is reduced upon binding of the intermediates (an acetyl moiety in the case of PDH, a succinyl moiety in the case of OGDH, and an aminomethyl moiety in the case of GCV). Following release of the intermediates to form the products of the enzyme complexes, the reduced lipoyl moiety must be reoxidized before entering another catalytic cycle. Oxidation is catalyzed by lipoamide dehydrogenase, a subunit component of the three lipoic acid-dependent enzyme systems (8). E. coli strains defective in lipoic acid biosynthesis are unable to grow on aerobic glucose minimal media unless the media are supplemented with acetate and succinate to bypass the need for the two lipoic acid-dependent dehydrogenases (15, 32).Open in a separate windowFIG. 1.Lipoic acid metabolism in E. coli. (A) LplA lipoate ligase reaction, in which lipoate reacts with ATP to form the activated intermediate, lipoyl-adenylate (lipoyl-AMP), which remains firmly bound within the active site. The lipoyl-adenylate mixed anhydride bond is then attacked by the ɛ-amino group of the target lysine residue of the acceptor lipoyl domain to form lipoylated protein. LplA also utilizes octanoic acid. (B) Lipoic acid synthesis in E. coli. LipB transfers an octanoyl moiety from the fatty acid biosynthetic intermediate, octanoyl-ACP, to the lipoyl domain of a lipoate-accepting protein (in this case the E2 subunit of a 2-oxoacid dehydrogenase). The octanoylated domain is the substrate of LipA, an S-adenosylmethionine radical enzyme that replaces one hydrogen atom on each of octanoate carbons 6 and 8 with sulfur atoms. For a review, see reference 8.Studies in our laboratory and others have elucidated the lipoic acid synthesis pathway of E. coli (Fig. ​(Fig.1).1). The LipB octanoyl-[acyl carrier protein {ACP}]:protein N-octanoyltransferase (20, 33, 34) transfers the octanoyl moiety from octanoyl-ACP, a fatty acid biosynthetic intermediate, to lipoyl domains. This reaction proceeds through an acyl enzyme intermediate in which the octanoyl moiety is in thioester linkage to a conserved cysteine residue in the enzyme active site (22, 33). The thioester bond is then attacked by the ɛ-amino group of the target lipoyl domain lysine residue to give the amide-linked lipoate moiety. The product of this catalysis, an octanoyl domain, is the substrate of the LipA lipoate synthase, an S-adenosylmethionine radical enzyme which inserts sulfur atoms at carbons 6 and 8. In addition to the LipB-LipA pathway of lipoic acid synthesis, E. coli also contains an enzyme that scavenges lipoic acid from the growth medium, the LplA lipoate-protein ligase. LplA uses ATP to activate lipoic acid to lipoyl-adenylate, the mixed anhydride of which is attacked by the lipoyl domain lysine reside to give the lipoylated domain (Fig. ​(Fig.1).1). LplA is also active with octanoic acid and efficiently attaches exogenous octanoate to lipoyl domains both in vivo and in vitro (11, 25, 26, 34). lplA null mutants have no phenotype in strains having an intact lipoic acid synthesis pathway (26).The subject of this report is the behavior of lipB null mutants, which (as expected from the above discussion) are lipoic acid auxotrophs (26, 32). Growth of lipB strains can also be supported by supplementation of the medium with octanoate (34). Upon plating of lipB null mutants on plates of minimal glucose medium, colonies arise that no longer require lipoic acid (26). These are suppressor mutations because the block in lipoic acid synthesis remains. Suppression in the strains studied in this work maps to the lplA gene. The LplA proteins encoded by these suppressor mutants contain point mutations that greatly decrease the Michaelis constant for free octanoic acid and allow efficient scavenging of cytosolic octanoate.     

Solubilization of organic and inorganic phosphates by three highly efficient soil bacterial isolates

Screening soil samples collected from a diverse range of slightly alkaline soil types, we have isolated 22 competent phosphate solubilizing bacteria (PSB). Three isolates identified as Pantoea agglomerans strain P5, Microbacterium laevaniformans strain P7 and Pseudomonas putida strain P13 hydrolyzed inorganic and organic phosphate compounds effectively. Bacterial growth rates and phosphate solubilization activities were measured quantitatively under various environmental conditions. In general, a close association was evident between phosphate solubilizing ability and growth rate which is an indicator of active metabolism. All three PSB were able to withstand temperature as high as 42°C, high concentration of NaCl upto 5% and a wide range of initial pH from 5 to 11 while hydrolyzing phosphate compounds actively. Such criteria make these isolates superior candidates for biofertilizers that are capable of utilizing both organic and mineral phosphate substrates to release absorbable phosphate ion for plants.     

Targeted double domain nanoplex based on galactosylated polyethylenimine enhanced the delivery of IL-12 plasmid

The objective of the present investigation was to design a targeted polyethylenimine (PEI)-based polyplex by conjugating lactose bearing galactose groups on low molecular weight PEI (LMW PEI) grafted to a high molecular weight PEI (HMW PEI) via a succinic acid linker in order to restore the amine content of the whole conjugate used for ligand conjugation. The PEI conjugate was synthesized and characterized in terms of buffering capacity, particle size, zeta potential, plasmid condensation ability, and protection of DNA against degrading enzymes. Also, the transfection efficiency and cytotoxicity were evaluated in the cell line over-expressing asialoglycoprotein receptors (ASGPRs) and compared with the cells lacking the receptors. The results demonstrated the ability of PEI conjugate in condensation of plasmid DNA and protection against enzyme degradation. The PEI conjugate formed nanoparticles of around 75 nm with higher buffering capacity compared with unmodified PEI. The polyplexes prepared by the modified PEI could increase the level of transgene up to four folds in the cells over-expressing the receptor. The results demonstrated the separation of targeting and delivery domains could be considered as a strategy to restore the amine content of the PEI molecule utilized for targeting ligand conjugation.     

Assessing the viability of three Lactobacillus bacterial species protected in the cryoprotectants containing whey and maltodextrin during freeze-drying process

Freeze-drying of bacteria associates with different stresses such as osmotic pressure, temperature and oxidation, and decreases bacterial viability, which seem to reduce by applying cryoprotectants. The present study evaluated the effect of four cryoprotectants on decreasing the stress caused by freeze-drying process among three Lactobacillus species. Additionally, it highlighted the use of whey and maltodextrin as a substitute for peptone and sucrose in cryoprotectants respectively. The viability of lactobacilli was measured after freeze-drying, 1 month of storage at 25 and 4°C. Based on the results, the viability rate of bacteria in protectants during freeze-drying stage was dependent on their strains. The best viability of Lacticaseibacillus rhamnosus GG and Ligilactobacillus salivarius 20687 was, respectively, observed in the protectants containing sucrose and whey, while Lactiplantibacillus plantarum NRRL B-14768 viability was equal in all protectants. The number of live bacteria reduced significantly by storing bacteria for 1 month at 25°C compared to the 4°C storage. During the storage period, the viability of L. salivarius improved by adding sucrose in protectant. Due to the positive effect of whey and sucrose in the drying and storage stage, on bacterial viability, the protectant consisting of whey and sucrose is suggested for all of the species under study.     

Facile Preparation of Internally Self-assembled Lipid Particles Stabilized by Carbon Nanotubes

We present a facile method to prepare nanostructured lipid particles stabilized by carbon nanotubes (CNTs). Single-walled (pristine) and multi-walled (functionalized) CNTs are used as stabilizers to produce Pickering type oil-in-water (O/W) emulsions. Lipids namely, Dimodan U and Phytantriol are used as emulsifiers, which in excess water self-assemble into the bicontinuous cubic Pn3m phase. This highly viscous phase is fragmented into smaller particles using a probe ultrasonicator in presence of conventional surfactant stabilizers or CNTs as done here. Initially, the CNTs (powder form) are dispersed in water followed by further ultrasonication with the molten lipid to form the final emulsion. During this process the CNTs get coated with lipid molecules, which in turn are presumed to surround the lipid droplets to form a particulate emulsion that is stable for months. The average size of CNT-stabilized nanostructured lipid particles is in the submicron range, which compares well with the particles stabilized using conventional surfactants. Small angle X-ray scattering data confirms the retention of the original Pn3m cubic phase in the CNT-stabilized lipid dispersions as compared to the pure lipid phase (bulk state). Blue shift and lowering of the intensities in characteristic G and G'' bands of CNTs observed in Raman spectroscopy characterize the interaction between CNT surface and lipid molecules. These results suggest that the interactions between the CNTs and lipids are responsible for their mutual stabilization in aqueous solutions. As the concentrations of CNTs employed for stabilization are very low and lipid molecules are able to functionalize the CNTs, the toxicity of CNTs is expected to be insignificant while their biocompatibility is greatly enhanced. Hence the present approach finds a great potential in various biomedical applications, for instance, for developing hybrid nanocarrier systems for the delivery of multiple functional molecules as in combination therapy or polytherapy.     

Differentiation of multipotent stem cells to insulin-producing cells for treatment of diabetes mellitus: bone marrow- and adipose tissue-derived cells comparison

Molecular Biology Reports - Mesenchymal stem cells (MSCs) from human adipose tissue and bone marrow have a great potential for use in cell therapy due to their ease of isolation, expansion, and...     

Effect of Zinc Supplementation on Physical and Psychological Symptoms,Biomarkers of Inflammation,Oxidative Stress,and Brain-Derived Neurotrophic Factor in Young Women with Premenstrual Syndrome: a Randomized,Double-Blind,Placebo-Controlled Trial

Biological Trace Element Research - To examine sex-specific associations of neonatal and childhood exposure to eight trace elements with cognitive abilities of school-age children. The association...     

Cyclooxygenase-2 inhibition by novel Bisaryl imidazolyl imidazole derivatives increases Bax/Bcl-2 ratio and upregulates Caspase-3 gene expression in Caco-2 colorectal cancer cell line

Cyclooxygenase-2 (COX-2) inhibitors including celecoxib inhibit cell growth and induce apoptosis in cancer cells. In this study, the relation of Bax (an apoptosis promoter) to Bcl-2 (an apoptosis inhibitor) ratio with the apoptosis co-ordination enzyme, caspase-3 was investigated in correlation with the treatment of 4,5-bisaryl imidazolyl imidazoles as novel selective COX-2 inhibitors in Caco-2 colorectal cancer cells. Recently, the organic reactions under microwave irradiation attracted attention of scientists due to their high reaction rate, mild reaction conditions and the formation of clean products. Therefore, a microwave-assisted method was used to synthesize our compounds. The effects of these COX-2 inhibitors on the proliferation of Caco-2 cells were evaluated by MTT assay. cDNA microarray and clustering analysis were used to evaluate effects of our synthetic compounds on gene expression pattern of 112 genes involved in apoptosis pathways. Bax, Bcl-2 and caspase-3 mRNA expression and their relationship were analyzed by quantitative real-time PCR. Results indicated that proliferation of Caco-2 cells after treatment with 4,5-bisaryl imidazolyl imidazoles on Caco-2 cells were time and dose dependent. We conclude that increase in Bax/Bcl-2 ratio leads to an up-regulation in caspase-3 mRNA expression.