Regulation of malonyl-CoA metabolism by acyl-acyl carrier protein and beta-ketoacyl-acyl carrier protein synthases in Escherichia coli

Functional disparities of malonyl-ACP decarboxylase between Xanthomonas campestris and Xanthomonas oryzae

Xanthomonas campestris pv. campestris (Xcc) and X. oryzae pv. oryzae (Xoo) are crucial plant pathogenic bacteria, causing crucifer black rot and rice leaf blight, respectively. Both bacterial species encode a protein containing the YiiD_C domain, designated MadB, which exhibits an 87.5% sequence identity between their MadBs. The madB genes from either Xoo or Xcc successfully restored the growth defect in Ralstonia solanacearum and Escherichia coli fabH mutants in vivo. In vitro assays demonstrated that MadB proteins possess malonyl-ACP decarboxylase activity, although Xcc MadB exhibited lower activity compared with Xoo MadB. Mutation of madB in both Xoo and Xcc strains led to decreased pathogenicity in their respective host plants. Interestingly, the Xoo madB mutant exhibited a significant increase in branched-chain fatty acid production, whereas the Xcc madB mutant showed only minor changes in fatty acid composition. Despite the reduction in exopolysaccharide (EPS) synthesis due to madB mutation in both Xoo and Xcc, EPS production in the Xoo madB mutant could be restored by exogenous sodium acetate supplementation. In contrast, sodium acetate failed to restore EPS synthesis in the Xcc madB mutant. Biochemical and genetic analyses indicated that these divergent physiological roles arise from the distinct biochemical functions of MadB in the two bacteria. In Xoo, the fatty acid synthesis (FAS) pathway mediated by MadB operates independently of the FAS pathway mediated by FabH. Conversely, in Xcc, the FAS pathway mediated by FabH is the primary route, with MadB's pathway serving a supplementary and regulatory role. Further analysis of gene organization and expression regulation of madB in both bacteria corroborates these distinctions.

IMPORTANCE

Despite the high conservation of the mad gene within the Proteobacteria, the physiological roles of the Mad protein remain largely unclear. Xoo and Xcc are bacteria with very close phylogenetic relationships, both encoding malonyl-ACP decarboxylase (MadB). However, MadB demonstrates substantial physiological function variations between these two species. This study demonstrates that even in closely related bacteria, homologous genes have adopted different evolutionary pathways to adapt to diverse living environments, forming unique gene expression regulation mechanisms. This has led to the biochemical functional divergence of homologous proteins within their respective species, ultimately resulting in distinct physiological functions.

Erucic acid utilization by Lactobacillus johnsonii N6.2

A multivariate nutritional analysis indicated that the consumption of erucic acid-rich food, a fatty acid (FA) found primarily in rapeseed and mustard oil, was positively correlated with higher counts of lactic acid bacteria (LAB). Furthermore, we showed Lactobacillus johnsonii N6.2, as well as other species of LAB tested from the former Lactobacillus genus, were able to efficiently use erucic acid (EA) as the source of FA. In this work, we identified significant changes induced in the FA profiles of L. johnsonii cultured with EA as the source of FA. We performed global transcriptomics to identify genes and pathways involved in EA utilization. It was found that L. johnsonii incorporates external fatty acids via a FakA/FakB and the plsX/plsY/plsC pathway for phosphatidic acid synthesis. It was found that cells grown in MRS with EA (MRS-E) significantly upregulated fakB2 and fakB4 when compared to cells grown in standard MRS with tween 80 as the source of FA. Additionally, in MRS-E, L. johnsonii N6.2 induced the expression of plsY2, plsC2 and plsC4 while the expression of pslX was constitutive during short term EA exposure. LC-MS analyses revealed that L. johnsonii N6.2 rapidly incorporates EA and synthesizes a variety of long chain fatty acids, including the health-relevant omega-9 monounsaturated fatty acids such as nervonic and gondoic acids.

Proteomic Profiling in Pediococcus pentosaceus SF11 Exposed to Condensed Tannins from Sainfoin

The antibacterial mechanism of condensed tannins (CTs) obtained from tea has been elucidated, but the mechanism of legume-derived CTs remains unclear. The mechanisms of legume- and tea-derived CTs probably differ due to the diverse compositions of CTs. Previous research found that sainfoin CTs directly inhibited the growth of Pediococcus. The present study investigated the inhibition mechanism of CTs against Pediococcus pentosaceus SF11 (SF11) through proteomic analysis. The results showed that the minimum inhibitory concentration (MIC) of CTs against SF11 was 1500 mg/L and that CTs increased cell membrane permeability in a dose-dependent manner. In total, 418 differentially expressed proteins (DEPs) were identified between the CT treatment and the control, among which 341 were down-regulated and 77 were up-regulated in the CT treatment. The protein interaction network showed that the expression of only two DEPs was highly different between CT treated and control (|log2FC|> 2); the atpD protein was up-regulated in the CT-treated group, which was involved in ATP synthesis; down-regulated DEPs were most involved in lipoteichoic acid synthesis, peptidoglycan synthesis, and glycine metabolism. Twenty-seven proteins were not detected after CT treatment, which were involved in functions including fatty acid synthesis, RNA synthesis and translation, drug resistance, and cell membrane permeability in SF11. Therefore, the findings suggest that the inhibition mechanism of CTs may be related to cell membrane damage and inhibition of cell reproduction.

Self-controlled in silico gene knockdown strategies to enhance the sustainable production of heterologous terpenoid by Saccharomyces cerevisiae

Microbial bioengineering is a growing field for producing plant natural products (PNPs) in recent decades, using heterologous metabolic pathways in host cells. Once heterologous metabolic pathways have been introduced into host cells, traditional metabolic engineering techniques are employed to enhance the productivity and yield of PNP biosynthetic routes, as well as to manage competing pathways. The advent of computational biology has marked the beginning of a novel epoch in strain design through in silico methods. These methods utilize genome-scale metabolic models (GEMs) and flux optimization algorithms to facilitate rational design across the entire cellular metabolic network. However, the implementation of in silico strategies can often result in an uneven distribution of metabolic fluxes due to the rigid knocking out of endogenous genes, which can impede cell growth and ultimately impact the accumulation of target products. In this study, we creatively utilized synthetic biology to refine in silico strain design for efficient PNPs production. OptKnock simulation was performed on the GEM of Saccharomyces cerevisiae OA07, an engineered strain for oleanolic acid (OA) bioproduction that has been reported previously. The simulation predicted that the single deletion of fol1, fol2, fol3, abz1, and abz2, or a combined knockout of hfd1, ald2 and ald3 could improve its OA production. Consequently, strains EK1∼EK7 were constructed and cultivated. EK3 (OA07△fol3), EK5 (OA07△abz1), and EK6 (OA07△abz2) had significantly higher OA titers in a batch cultivation compared to the original strain OA07. However, these increases were less pronounced in the fed-batch mode, indicating that gene deletion did not support sustainable OA production. To address this, we designed a negative feedback circuit regulated by malonyl-CoA, a growth-associated intermediate whose synthesis served as a bypass to OA synthesis, at fol3, abz1, abz2, and at acetyl-CoA carboxylase-encoding gene acc1, to dynamically and autonomously regulate the expression of these genes in OA07. The constructed strains R_3A, R_5A and R_6A had significantly higher OA titers than the initial strain and the responding gene-knockout mutants in either batch or fed-batch culture modes. Among them, strain R_3A stand out with the highest OA titer reported to date. Its OA titer doubled that of the initial strain in the flask-level fed-batch cultivation, and achieved at 1.23 ± 0.04 g L-1 in 96 h in the fermenter-level fed-batch mode. This indicated that the integration of optimization algorithm and synthetic biology approaches was efficiently rational for PNP-producing strain design.

Loss of β-Ketoacyl Acyl Carrier Protein Synthase III Activity Restores Multidrug-Resistant Escherichia coli Sensitivity to Previously Ineffective Antibiotics

Gram-negative pathogens are a major concern for global public health due to increasing rates of antibiotic resistance and the lack of new drugs. A major contributing factor toward antibiotic resistance in Gram-negative bacteria is their formidable outer membrane, which acts as a permeability barrier preventing many biologically active antimicrobials from reaching the intracellular targets and thus limiting their efficacy.

Synthetic protein quality control to enhance full-length translation in bacteria

Coupled transcription and translation processes in bacteria cause indiscriminate translation of intact and truncated messenger RNAs, inevitably generating nonfunctional polypeptides. Here, we devised a synthetic protein quality control (ProQC) system that enables translation only when both ends of mRNAs are present and followed by circularization based on sequence-specific RNA-RNA hybridization. We demonstrate that the ProQC system dramatically improved the fraction of full-length proteins among all synthesized polypeptides by selectively translating intact mRNA and reducing abortive translation. As a result, full-length protein synthesis increased up to 2.5-fold without changing the transcription or translation efficiency. Furthermore, we applied the ProQC system for 3-hydroxypropionic acid, violacein and lycopene production by ensuring full-length expression of enzymes in biosynthetic pathways, resulting in 1.6- to 2.3-fold greater biochemical production. We believe that our ProQC system can be universally applied to improve not only the quality of recombinant protein production but also efficiencies of metabolic pathways.

Dynamic control over feedback regulatory mechanisms improves NADPH flux and xylitol biosynthesis in engineered E. coli

We report improved NADPH flux and xylitol biosynthesis in engineered E. coli. Xylitol is produced from xylose via an NADPH dependent reductase. We utilize 2-stage dynamic metabolic control to compare two approaches to optimize xylitol biosynthesis, a stoichiometric approach, wherein competitive fluxes are decreased, and a regulatory approach wherein the levels of key regulatory metabolites are reduced. The stoichiometric and regulatory approaches lead to a 20-fold and 90-fold improvement in xylitol production, respectively. Strains with reduced levels of enoyl-ACP reductase and glucose-6-phosphate dehydrogenase, led to altered metabolite pools resulting in the activation of the membrane bound transhydrogenase and an NADPH generation pathway, consisting of pyruvate ferredoxin oxidoreductase coupled with NADPH dependent ferredoxin reductase, leading to increased NADPH fluxes, despite a reduction in NADPH pools. These strains produced titers of 200 g/L of xylitol from xylose at 86% of theoretical yield in instrumented bioreactors. We expect dynamic control over the regulation of the membrane bound transhydrogenase as well as NADPH production through pyruvate ferredoxin oxidoreductase to broadly enable improved NADPH dependent bioconversions or production via NADPH dependent metabolic pathways.

Enhancing glutaric acid production in Escherichia coli by uptake of malonic acid

Glutaric acid is an important organic acid applied widely in different fields. Most previous researches have focused on the production of glutaric acid in various strains using the 5-aminovaleric acid (AMV) or pentenoic acid synthesis pathways. We previously utilized a five-step reversed adipic acid degradation pathway (RADP) in Escherichia coli BL21 (DE3) to construct strain Bgl146. Herein, we found that malonyl-CoA was strictly limited in this strain, and increasing its abundance could improve glutaric acid production. We, therefore, constructed a malonic acid uptake pathway in E. coli using matB (malonic acid synthetase) and matC (malonic acid carrier protein) from Clover rhizobia. The titer of glutaric acid was improved by 2.1-fold and 1.45-fold, respectively, reaching 0.56 g/L and 4.35 g/L in shake flask and batch fermentation following addition of malonic acid. Finally, the highest titer of glutaric acid was 6.3 g/L in fed-batch fermentation at optimized fermentation conditions.

Fatty acid starvation activates RelA by depleting lysine precursor pyruvate

Bacteria undergoing nutrient starvation induce the ubiquitous stringent response, resulting in gross physiological changes that reprograms cell metabolism from fast to slow growth. The stringent response is mediated by the secondary messengers pppGpp and ppGpp collectively referred to as (p)ppGpp or 'alarmone'. In Escherichia coli, two paralogs, RelA and SpoT, synthesize (p)ppGpp. RelA is activated by amino acid starvation, whereas SpoT, which can also degrade (p)ppGpp, responds to fatty acid (FA), carbon and phosphate starvation. Here, we discover that FA starvation leads to rapid activation of RelA and reveal the underlying mechanism. We show that FA starvation leads to depletion of lysine that, in turn, leads to the accumulation of uncharged tRNA^Lys and activation of RelA. SpoT was also activated by FA starvation but to a lower level and with a delayed kinetics. Next, we discovered that pyruvate, a precursor of lysine, is depleted by FA starvation. We also propose a mechanism that explains how FA starvation leads to pyruvate depletion. Together our results raise the possibility that RelA may be a major player under many starvation conditions previously thought to depend principally on SpoT. Interestingly, FA starvation provoked a ~100-fold increase in relA dependent ampicillin tolerance.

Structural insights into repression of the Pneumococcal fatty acid synthesis pathway by repressor FabT and co-repressor acyl-ACP

The Streptococcus pneumoniae fatty acid synthesis (FAS) pathway is globally controlled at the transcriptional level by the repressor FabT and its co-repressor acyl carrier protein (acyl-ACP), the intermediate of phospholipid synthesis. Here, we report the crystal structure of FabT complexed with a 23-bp dsDNA, which indicates that FabT is a weak repressor with low DNA-binding affinity in the absence of acyl-ACP. Modification of ACP with a long-chain fatty acid is necessary for the formation of a stable complex with FabT, mimicked in vitro by cross-linking, which significantly elevates the DNA-binding affinity of FabT. Altogether, we propose a putative working model of gene repression under the double control of FabT and acyl-ACP, elucidating a distinct repression network for Pneumococcus to precisely coordinate FAS.

Engineering Escherichia coli for Glutarate Production as the C5 Platform Backbone

Glutarate is a linear-chain dicarboxylic acid with wide applications in the production of polyesters and polyamides such as nylon-4,5 and nylon-5,5. Previous studies focused on the biological production of glutarate from lysine with low yields and titers. Here, we report on glutarate production by Escherichia coli using a five-step reverse adipate degradation pathway (RADP) identified in Thermobifida fusca By expressing the enzymes of RADP, the glutarate was detected by strain Bgl146 in shaken flasks. After fermentation optimization, the titer of glutarate by Bgl146 was increased to 4.7 ± 0.2 mM in shaken flasks. We further eliminated pathways for the major metabolites competing for carbon flux by CRISPR/Cas9 (ΔarcA, ΔldhA, ΔatoB, and ΔpflB). Moreover, the final strain Bgl4146 produced 36.5 ± 0.3 mM glutarate by fed-batch fermentation. These results constitute the highest glutarate titer reported in E. coliIMPORTANCE Glutarate is an important C₅ linear-chain dicarboxylic acid, which is widely used in polyesters and polyamides such as nylon-4,5 and nylon-5,5 in the chemical industry. Glutarate is currently produced from the feedstocks derived from petroleum, specifically by oxidation of a mixture of cyclohexanone and cyclohexanol catalyzed by nitric acid. However, the chemical synthesis results in high pollution and dramatic greenhouse gas emission. Thus, the biological production of glutarate directly from the substrate is of great importance. Although there have been reports using Corynebacterium glutamicum to produce glutarate, it has serious limitations due to the limited lysine supply and long fermentation time. To solve this problem, a novel synthetic pathway was constructed in this study, and the highest glutarate titer was reported in Escherichia coli using a short fermentation time without lysine addition, making bio-based glutarate production much more feasible.

FapR: From Control of Membrane Lipid Homeostasis to a Biotechnological Tool

Phospholipids and fatty acids are not only one of the major components of cell membranes but also important metabolic intermediates in bacteria. Since the fatty acid biosynthetic pathway is essential and energetically expensive, organisms have developed a diversity of homeostatic mechanisms to fine-tune the concentration of lipids at particular levels. FapR is the first global regulator of lipid synthesis discovered in bacteria and is largely conserved in Gram-positive organisms including important human pathogens, such as Staphylococcus aureus, Bacillus anthracis, and Listeria monocytogenes. FapR is a transcription factor that negatively controls the expression of several genes of the fatty acid and phospholipid biosynthesis and was first identified in Bacillus subtilis. This review focuses on the genetic, biochemical and structural advances that led to a detailed understanding of lipid homeostasis control by FapR providing unique opportunities to learn how Gram-positive bacteria monitor the status of fatty acid biosynthesis and adjust the lipid synthesis accordingly. Furthermore, we also cover the potential of the FapR system as a target for new drugs against Gram-positive bacteria as well as its recent biotechnological applications in diverse organisms.

Improvement of free fatty acid production using a mutant acyl-CoA thioesterase I with high specific activity in Escherichia coli

BACKGROUND

Microbial production of oleochemicals has been actively studied in the last decade. Free fatty acids (FFAs) could be converted into a variety of molecules such as industrial products, consumer products, and fuels. FFAs have been produced in metabolically engineered Escherichia coli cells expressing a signal sequence-deficient acyl-CoA thioesterase I ('TesA). Nonetheless, increasing the expression level of 'TesA seems not to be an appropriate approach to scale up FFA production because a certain ratio of each component including fatty acid synthase and 'TesA is required for optimal production of FFAs. Thus, the catalytic activity of 'TesA should be rationally engineered instead of merely increasing the enzyme expression level to enhance the production of FFAs.

RESULTS

In this study, we constructed a sensing system with a fusion protein of tetracycline resistance protein and red fluorescent protein (RFP) under the control of a FadR-responsive promoter to select the desired mutants. Fatty acid-dependent growth and RFP expression allowed for selection of FFA-overproducing cells. A 'TesA mutant that produces a twofold greater amount of FFAs was isolated from an error-prone PCR mutant library of E. coli 'TesA. Its kinetic analysis revealed that substitution of Arg⁶⁴ with Cys⁶⁴ in the enzyme causes an approximately twofold increase in catalytic activity.

CONCLUSIONS

Because the expression of 'TesA in E. coli for the production of oleochemicals is almost an indispensable process, the proposed engineering approach has a potential to enhance the production of oleochemicals. The use of the catalytically active mutant 'TesA^R64C should accelerate the manufacture of FFA-derived chemicals and fuels.

Mass-Spectrometry-Based Quantification of Protein-Bound Fatty Acid Synthesis Intermediates from Escherichia coli

The production of fatty acids from simple nutrients occurs via a complex biosynthetic pathway with dozens of intermediate compounds and multiple branch points. Despite its importance for microbial physiology and biotechnology, critical aspects of fatty acid biosynthesis, especially dynamics of in vivo regulation, remain poorly characterized. We have developed a liquid chromatography/mass spectroscopy (LC-MS) method for relative quantification of fatty acid synthesis intermediates in Escherichia coli, a model organism for studies of fatty acid metabolism. The acyl carrier protein, a vehicle for the substrates and intermediates of fatty acid synthesis, is extracted from E. coli, proteolytically digested, resolved using reverse-phase LC, and detected using electrospray ionization coupled with a tandem MS. Our method reliably resolves 21 intermediates of fatty acid synthesis, with an average relative standard deviation in ratios of individual acyl-ACP species to total ACP concentrations of 20%. We demonstrate that fast sampling and quenching of cells is essential to accurately characterize intracellular concentrations of ACP species. We apply our method to examine the rapid response of fatty acid metabolism to the antibiotic cerulenin. We anticipate that our method will enable the characterization of in vivo regulation and kinetics of microbial fatty acid synthesis at unprecedented detail and will improve integration of fatty acid synthesis into models of microbial metabolism.

Genetically encoded sensors enable real-time observation of metabolite production

Engineering cells to produce valuable metabolic products is hindered by the slow and laborious methods available for evaluating product concentration. Consequently, many designs go unevaluated, and the dynamics of product formation over time go unobserved. In this work, we develop a framework for observing product formation in real time without the need for sample preparation or laborious analytical methods. We use genetically encoded biosensors derived from small-molecule responsive transcription factors to provide a fluorescent readout that is proportional to the intracellular concentration of a target metabolite. Combining an appropriate biosensor with cells designed to produce a metabolic product allows us to track product formation by observing fluorescence. With individual cells exhibiting fluorescent intensities proportional to the amount of metabolite they produce, high-throughput methods can be used to rank the quality of genetic variants or production conditions. We observe production of several renewable plastic precursors with fluorescent readouts and demonstrate that higher fluorescence is indeed an indicator of higher product titer. Using fluorescence as a guide, we identify process parameters that produce 3-hydroxypropionate at 4.2 g/L, 23-fold higher than previously reported. We also report, to our knowledge, the first engineered route from glucose to acrylate, a plastic precursor with global sales of $14 billion. Finally, we monitor the production of glucarate, a replacement for environmentally damaging detergents, and muconate, a renewable precursor to polyethylene terephthalate and nylon with combined markets of $51 billion, in real time, demonstrating that our method is applicable to a wide range of molecules.

ucFabV Requires Functional Reductase Activity to Confer Reduced Triclosan Susceptibility in Escherichia coli

BACKGROUND/AIMS

We previously identified the Triclo1 fosmid in a functional metagenomic selection for clones that increased triclosan tolerance in Escherichia coli. The active enzyme encoded by Triclo1 is ucFabV. Although ucFabV is homologous to FabV from other organisms, ucFabV contains substitutions at key positions that would predict differences in substrate binding. Therefore, a detailed characterization of ucFabV was conducted to link its biochemical activity to its ability to confer reduced triclosan sensitivity.

METHODS

ucFabV and a catalytic mutant were purified and used to reduce crotonoyl-CoA in vitro. The mutant and wild-type enzymes were introduced into E. coli, and their ability to confer triclosan tolerance as well as suppress a temperature-sensitive mutant of FabI were measured.

RESULTS

Purified ucFabV, but not the mutant, reduced crotonoyl-CoA in vitro. The wild-type enzyme confers increased triclosan tolerance when introduced into E. coli, whereas the mutant remained susceptible to triclosan. Additionally, wild-type ucFabV, but not the mutant, functionally replaced FabI within living cells.

CONCLUSION

ucFabV confers increased tolerance through its function as an enoyl-ACP reductase. Furthermore, ucFabV is capable of restoring viability in the presence of compromised FabI, suggesting ucFabV is likely facilitating an alternate step within fatty acid synthesis, bypassing FabI inhibition.

Biosynthesis of Membrane Lipids

The pathways in Escherichia coli and (largely by analogy) S. enterica remain the paradigm of bacterial lipid synthetic pathways, although recently considerable diversity among bacteria in the specific areas of lipid synthesis has been demonstrated. The structural biology of the fatty acid synthetic proteins is essentially complete. However, the membrane-bound enzymes of phospholipid synthesis remain recalcitrant to structural analyses. Recent advances in genetic technology have allowed the essentialgenes of lipid synthesis to be tested with rigor, and as expected most genes are essential under standard growth conditions. Conditionally lethal mutants are available in numerous genes, which facilitates physiological analyses. The array of genetic constructs facilitates analysis of the functions of genes from other organisms. Advances in mass spectroscopy have allowed very accurate and detailed analyses of lipid compositions as well as detection of the interactions of lipid biosynthetic proteins with one another and with proteins outside the lipid pathway. The combination of these advances has resulted in use of E. coli and S. enterica for discovery of new antimicrobials targeted to lipid synthesis and in deciphering the molecular actions of known antimicrobials. Finally,roles for bacterial fatty acids other than as membrane lipid structural components have been uncovered. For example, fatty acid synthesis plays major roles in the synthesis of the essential enzyme cofactors, biotin and lipoic acid. Although other roles for bacterial fatty acids, such as synthesis of acyl-homoserine quorum-sensing molecules, are not native to E. coli introduction of the relevant gene(s) synthesis of these foreign molecules readily proceeds and the sophisticated tools available can used to decipher the mechanisms of synthesis of these molecules.

Enhancing flavonoid production by systematically tuning the central metabolic pathways based on a CRISPR interference system in Escherichia coli

The limited supply of intracellular malonyl-CoA in Escherichia coli impedes the biological synthesis of polyketides, flavonoids and biofuels. Here, a clustered regularly interspaced short palindromic repeats (CRISPR) interference system was constructed for fine-tuning central metabolic pathways to efficiently channel carbon flux toward malonyl-CoA. Using synthetic sgRNA to silence candidate genes, genes that could increase the intracellular malonyl-CoA level by over 223% were used as target genes. The efficiencies of repression of these genes were tuned to achieve appropriate levels so that the intracellular malonyl-CoA level was enhanced without significantly altering final biomass accumulation (the final OD600 decreased by less than 10%). Based on the results, multiple gene repressing was successful in approaching the limit of the amount of malonyl-CoA needed to produce the plant-specific secondary metabolite (2S)-naringenin. By coupling the genetic modifications to cell growth, the combined effects of these genetic perturbations increased the final (2S)-naringenin titer to 421.6 mg/L, which was 7.4-fold higher than the control strain. The strategy described here could be used to characterize genes that are essential for cell growth and to develop E. coli as a well-organized cell factory for producing other important products that require malonyl-CoA as a precursor.

Ralstonia solanacearum RSp0194 Encodes a Novel 3-Keto-Acyl Carrier Protein Synthase III

Fatty acid synthesis (FAS), a primary metabolic pathway, is essential for survival of bacteria. Ralstonia solanacearum, a β-proteobacteria member, causes a bacterial wilt affecting more than 200 plant species, including many economically important plants. However, thus far, the fatty acid biosynthesis pathway of R. solanacearum has not been well studied. In this study, we characterized two forms of 3-keto-ACP synthase III, RsFabH and RsFabW, in R. solanacearum. RsFabH, the homologue of Escherichia coli FabH, encoded by the chromosomal RSc1050 gene, catalyzes the condensation of acetyl-CoA with malonyl-ACP in the initiation steps of fatty acid biosynthesis in vitro. The RsfabH mutant lost de novo fatty acid synthetic ability, and grows in medium containing free fatty acids. RsFabW, a homologue of Pseudomonas aeruginosa PA3286, encoded by a megaplasmid gene, RSp0194, condenses acyl-CoA (C₂-CoA to C₁₀-CoA) with malonyl-ACP to produce 3-keto-acyl-ACP in vitro. Although the RsfabW mutant was viable, RsfabW was responsible for RsfabH mutant growth on medium containing free fatty acids. Our results also showed that RsFabW could condense acyl-ACP (C₄-ACP to C₈-ACP) with malonyl-ACP, to produce 3-keto-acyl-ACP in vitro, which implies that RsFabW plays a special role in fatty acid synthesis of R. solanacearum. All of these data confirm that R. solanacearum not only utilizes acetyl-CoA, but also, utilizes medium-chain acyl-CoAs or acyl-ACPs as primers to initiate fatty acid synthesis.

Using modern tools to probe the structure-function relationship of fatty acid synthases

Fatty acid biosynthesis is essential to life and represents one of the most conserved pathways in nature, preserving the same handful of chemical reactions across all species. Recent interest in the molecular details of the de novo fatty acid synthase (FAS) has been heightened by demand for renewable fuels and the emergence of multidrug-resistant bacterial strains. Central to FAS is the acyl carrier protein (ACP), a protein chaperone that shuttles the growing acyl chain between catalytic enzymes within the FAS. Human efforts to alter fatty acid biosynthesis for oil production, chemical feedstock, or antimicrobial purposes has been met with limited success, due in part to a lack of detailed molecular information behind the ACP-partner protein interactions inherent to the pathway. This review will focus on recently developed tools for the modification of ACP and analysis of protein-protein interactions, such as mechanism-based crosslinking, and the studies exploiting them. Discussion specific to each enzymatic domain will focus first on mechanism and known inhibitors, followed by available structures and known interactions with ACP. Although significant unknowns remain, new understandings of the intricacies of FAS point to future advances in manipulating this complex molecular factory.

Validation of RetroPath, a computer-aided design tool for metabolic pathway engineering

Metabolic engineering has succeeded in biosynthesis of numerous commodity or high value compounds. However, the choice of pathways and enzymes used for production was many times made ad hoc, or required expert knowledge of the specific biochemical reactions. In order to rationalize the process of engineering producer strains, we developed the computer-aided design (CAD) tool RetroPath that explores and enumerates metabolic pathways connecting the endogenous metabolites of a chassis cell to the target compound. To experimentally validate our tool, we constructed 12 top-ranked enzyme combinations producing the flavonoid pinocembrin, four of which displayed significant yields. Namely, our tool queried the enzymes found in metabolic databases based on their annotated and predicted activities. Next, it ranked pathways based on the predicted efficiency of the available enzymes, the toxicity of the intermediate metabolites and the calculated maximum product flux. To implement the top-ranking pathway, our procedure narrowed down a list of nine million possible enzyme combinations to 12, a number easily assembled and tested. One round of metabolic network optimization based on RetroPath output further increased pinocembrin titers 17-fold. In total, 12 out of the 13 enzymes tested in this work displayed a relative performance that was in accordance with its predicted score. These results validate the ranking function of our CAD tool, and open the way to its utilization in the biosynthesis of novel compounds.

Computer-aided design for metabolic engineering

The development and application of biotechnology-based strategies has had a great socio-economical impact and is likely to play a crucial role in the foundation of more sustainable and efficient industrial processes. Within biotechnology, metabolic engineering aims at the directed improvement of cellular properties, often with the goal of synthesizing a target chemical compound. The use of computer-aided design (CAD) tools, along with the continuously emerging advanced genetic engineering techniques have allowed metabolic engineering to broaden and streamline the process of heterologous compound-production. In this work, we review the CAD tools available for metabolic engineering with an emphasis, on retrosynthesis methodologies. Recent advances in genetic engineering strategies for pathway implementation and optimization are also reviewed as well as a range of bionalytical tools to validate in silico predictions. A case study applying retrosynthesis is presented as an experimental verification of the output from Retropath, the first complete automated computational pipeline applicable to metabolic engineering. Applying this CAD pipeline, together with genetic reassembly and optimization of culture conditions led to improved production of the plant flavonoid pinocembrin. Coupling CAD tools with advanced genetic engineering strategies and bioprocess optimization is crucial for enhanced product yields and will be of great value for the development of non-natural products through sustainable biotechnological processes.

Cerulenin inhibits unsaturated fatty acids synthesis in Bacillus subtilis by modifying the input signal of DesK thermosensor

Bacillus subtilis responds to a sudden decrease in temperature by transiently inducing the expression of the des gene encoding for a lipid desaturase, Δ5-Des, which introduces a double bond into the acyl chain of preexisting membrane phospholipids. This Δ5-Des-mediated membrane remodeling is controlled by the cold-sensor DesK. After cooling, DesK activates the response regulator DesR, which induces transcription of des. We show that inhibition of fatty acid synthesis by the addition of cerulenin, a potent and specific inhibitor of the type II fatty acid synthase, results in increased levels of short-chain fatty acids (FA) in membrane phospholipids that lead to inhibition of the transmembrane-input thermal control of DesK. Furthermore, reduction of phospholipid synthesis by conditional inactivation of the PlsC acyltransferase causes significantly elevated incorporation of long-chain FA and constitutive upregulation of the des gene. Thus, we provide in vivo evidence that the thickness of the hydrophobic core of the lipid bilayer serves as one of the stimulus sensed by the membrane spanning region of DesK.

Fatty acid synthesis in Escherichia coli and its applications towards the production of fatty acid based biofuels

The idea of renewable and regenerative resources has inspired research for more than a hundred years. Ideally, the only spent energy will replenish itself, like plant material, sunlight, thermal energy or wind. Biodiesel or ethanol are examples, since their production relies mainly on plant material. However, it has become apparent that crop derived biofuels will not be sufficient to satisfy future energy demands. Thus, especially in the last decade a lot of research has focused on the production of next generation biofuels. A major subject of these investigations has been the microbial fatty acid biosynthesis with the aim to produce fatty acids or derivatives for substitution of diesel. As an industrially important organism and with the best studied microbial fatty acid biosynthesis, Escherichia coli has been chosen as producer in many of these studies and several reviews have been published in the fields of E. coli fatty acid biosynthesis or biofuels. However, most reviews discuss only one of these topics in detail, despite the fact, that a profound understanding of the involved enzymes and their regulation is necessary for efficient genetic engineering of the entire pathway. The first part of this review aims at summarizing the knowledge about fatty acid biosynthesis of E. coli and its regulation, and it provides the connection towards the production of fatty acids and related biofuels. The second part gives an overview about the achievements by genetic engineering of the fatty acid biosynthesis towards the production of next generation biofuels. Finally, the actual importance and potential of fatty acid-based biofuels will be discussed.

Bacterial lipids: metabolism and membrane homeostasis

Membrane lipid homeostasis is a vital facet of bacterial cell physiology. For decades, research in bacterial lipid synthesis was largely confined to the Escherichia coli model system. This basic research provided a blueprint for the biochemistry of lipid metabolism that has largely defined the individual steps in bacterial fatty acid and phospholipids synthesis. The advent of genomic sequencing has revealed a surprising amount of diversity in the genes, enzymes and genetic organization of the components responsible for bacterial lipid synthesis. Although the chemical steps in fatty acid synthesis are largely conserved in bacteria, there are surprising differences in the structure and cofactor requirements for the enzymes that perform these reactions in Gram-positive and Gram-negative bacteria. This review summarizes how the explosion of new information on the diversity of biochemical and genetic regulatory mechanisms has impacted our understanding of bacterial lipid homeostasis. The potential and problems of developing therapeutics that block pathogen phospholipid synthesis are explored and evaluated. The study of bacterial lipid metabolism continues to be a rich source for new biochemistry that underlies the variety and adaptability of bacterial life styles.

Improved triclosan delivery by a novel silica-based nanocomposite

In this study, we report on the design, synthesis, and full characterization of a covalently-linked, triclosan silica-based nanoparticles (T-SNPs), coated with a polyaminated shell (NH2 -T-SNPs). Various techniques are used to elucidate and rationalize the potential biological mechanism of action of these novel nanoparticles. NH2 -T-SNPs are found to be potently bactericidal with no detectable lag time for the antimicrobial activity against E. coli and S. aureus. In this context, we also prove that triclosan is the chemical agent that mediated the bactericidal activity of these chemically-modified NPs. The obtained experimental data allows us to pinpoint the actual minimal bactericidal concentrations (MBCs) of triclosan-bound NPs by quantifying intracellular triclosan concentrations. Furthermore, we conduct preliminary cytotoxicity studies, which show that triclosan bound NPs are less cytotoxic (2000 fold) in vitro compared to free-triclosan when tested with various human and mammalian cell lines. Taken together, our results further support the characterization and development of these new nanoscale materials for various biomedical applications.

Feedback regulation of plastidic acetyl-CoA carboxylase by 18:1-acyl carrier protein in Brassica napus

Plant seed oil represents a major renewable source of reduced carbon, but little is known about the biochemical regulation of its synthesis. The goal of this research was to identify potential feedback regulation of fatty acid biosynthesis in Brassica napus embryo-derived cell cultures and to characterize both the feedback signals and enzymatic targets of the inhibition. Fatty acids delivered via Tween esters rapidly reduced the rate of fatty acid synthesis in a dose-dependent and reversible manner, demonstrating the existence of feedback inhibition in an oil-accumulating tissue. Tween feeding did not affect fatty acid elongation in the cytosol or the incorporation of radiolabeled malonate into nascent fatty acids, which together pinpoint plastidic acetyl-CoA carboxylase (ACCase) as the enzymatic target of feedback inhibition. To identify the signal responsible for feedback, a variety of Tween esters were tested for their effects on the rate of fatty acid synthesis. Maximum inhibition was achieved upon feeding oleic acid (18:1) Tween esters that resulted in the intracellular accumulation of 18:1 free fatty acid, 18:1-CoA, and 18:1-acyl-carrier protein (ACP). Direct, saturable inhibition of ACCase enzyme activity was observed in culture extracts and in extracts of developing canola seeds supplemented with 18:1-ACP at physiological concentrations. A mechanism for feedback inhibition is proposed in which reduced demand for de novo fatty acids results in the accumulation of 18:1-ACP, which directly inhibits plastidic ACCase, leading to reduced fatty acid synthesis. Defining this mechanism presents an opportunity for mitigating feedback inhibition of fatty acid synthesis in crop plants to increase oil yield.

Renewable energy from Cyanobacteria: energy production optimization by metabolic pathway engineering

The need to develop and improve sustainable energy resources is of eminent importance due to the finite nature of our fossil fuels. This review paper deals with a third generation renewable energy resource which does not compete with our food resources, cyanobacteria. We discuss the current state of the art in developing different types of bioenergy (ethanol, biodiesel, hydrogen, etc.) from cyanobacteria. The major important biochemical pathways in cyanobacteria are highlighted, and the possibility to influence these pathways to improve the production of specific types of energy forms the major part of this review.

FasR, a novel class of transcriptional regulator, governs the activation of fatty acid biosynthesis genes in Streptomyces coelicolor

Membrane lipid homeostasis is essential for bacterial survival and adaptation to different environments. The regulation of fatty acid biosynthesis is therefore crucial for maintaining the correct composition and biophysical properties of cell membranes. This regulation implicates a biochemical control of key enzymes and a transcriptional regulation of genes involved in lipid metabolism. In Streptomyces coelicolor we found that control of lipid homeostasis is accomplished, at least in part, through the transcriptional regulation of fatty acid biosynthetic genes. A novel transcription factor, FasR (SCO2386), controls expression of fabDHPF operon and lies immediately upstream of fabD, in a cluster of genes that is highly conserved within actinomycetes. Disruption of fasR resulted in a mutant strain, with severe growth defects and a delay in the timing of morphological and physiological differentiation. Expression of fab genes was downregulated in the fasR mutant, indicating a role for this transcription factor as an activator. Consequently, the mutant showed a significant drop in fatty acid synthase activity and triacylglyceride accumulation. FasR binds specifically to a DNA sequence containing fabDHPF promoter region, both in vivo and in vitro. These data provide the first example of positive regulation of genes encoding core proteins of saturated fatty acid synthase complex.

A novel role of malonyl-ACP in lipid homeostasis

The FapR protein of Bacillus subtilis has been shown to play an important role in membrane lipid homeostasis. FapR acts as a repressor of many genes involved in fatty acid and phospholipid metabolism (the fap regulon). FapR binding to DNA is antagonized by malonyl-CoA, and thus FapR acts as a sensor of the status of fatty acid biosynthesis. However, malonyl-CoA is utilized for fatty acid synthesis only following its conversion to malonyl-ACP, which plays a central role in the initiation and elongation cycles carried out by the type II fatty acid synthase. Using in vitro transcription studies and isothermal titration calorimetry, we show here that malonyl-ACP binds FapR, disrupting the repressor-operator complex with an affinity similar to that of its precursor malonyl-CoA. NMR experiments reveal that there is no protein-protein recognition between ACP and FapR. These findings are consistent with the crystal structure of malonyl-ACP, which shows that the malonyl-phosphopantetheine moiety protrudes away from the protein core and thus can act as an effector ligand. Therefore, FapR regulates the expression of the fap regulon in response to the composition of the malonyl-phosphopantetheine pool. This mechanism ensures that fatty acid biosynthesis in B. subtilis is finely regulated at the transcriptional level by sensing the concentrations of the two first intermediates (malonyl-CoA and malonyl-ACP) in order to balance the production of membrane phospholipids.

PqsD is responsible for the synthesis of 2,4-dihydroxyquinoline, an extracellular metabolite produced by Pseudomonas aeruginosa

2,4-Dihydroxyquinoline (DHQ) is an abundant extracellular metabolite of the opportunistic pathogen Pseudomonas aeruginosa that is secreted into growth medium in stationary phase to concentrations comparable with those of the Pseudomonas quinolone signal. Using a combination of biochemical and genetic approaches, we show that PqsD, a condensing enzyme in the pqs operon that is essential for Pseudomonas quinolone signal synthesis, accounts for DHQ formation in vivo. First, the anthraniloyl moiety is transferred to the active-site Cys of PqsD to form an anthraniloyl-PqsD intermediate, which then condenses with either malonyl-CoA or malonyl-acyl carrier protein to produce 3-(2-aminophenyl)-3-oxopropanoyl-CoA. This short-lived intermediate undergoes an intramolecular rearrangement to form DHQ. DHQ was produced by Escherichia coli coexpressing PqsA and PqsD, illustrating that these two proteins are the only factors necessary for DHQ synthesis. Thus, PqsD is responsible for the production of DHQ in P. aeruginosa.

Application of functional genomics to pathway optimization for increased isoprenoid production

Producing complex chemicals using synthetic metabolic pathways in microbial hosts can have many advantages over chemical synthesis but is often complicated by deleterious interactions between pathway intermediates and the host cell metabolism. With the maturation of functional genomic analysis, it is now technically feasible to identify modes of toxicity associated with the accumulation of foreign molecules in the engineered bacterium. Previously, Escherichia coli was engineered to produce large quantities of isoprenoids by creating a mevalonate-based isopentenyl pyrophosphate biosynthetic pathway (V. J. J. Martin et al., Nat. Biotechnol. 21:796-802, 2003). The engineered E. coli strain produced high levels of isoprenoids, but further optimization led to an imbalance in carbon flux and the accumulation of the pathway intermediate 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA), which proved to be cytotoxic to E. coli. Using both DNA microarray analysis and targeted metabolite profiling, we have studied E. coli strains inhibited by the intracellular accumulation of HMG-CoA. Our results indicate that HMG-CoA inhibits fatty acid biosynthesis in the microbial host, leading to generalized membrane stress. The cytotoxic effects of HMG-CoA accumulation can be counteracted by the addition of palmitic acid (16:0) and, to a lesser extent, oleic acid (cis-Delta(9)-18:1) in the growth medium. This work demonstrates the utility of using transcriptomic and metabolomic methods to optimize synthetic biological systems.

Strain improvement of recombinant Escherichia coli for efficient production of plant flavonoids

Plant flavonoid polyphenols continue to find increasing pharmaceutical and nutraceutical applications; however their isolation, especially of pure compounds, from plant material remains an underlying challenge. In the past Escherichia coli, one of the most well-characterized microorganisms, has been utilized as a recombinant host for protein expression and heterologous biosynthesis of small molecules. However, in many cases the expressed protein activities and biosynthetic efficiency are greatly limited by the host cellular properties, such as precursor and cofactor availability and protein or product tolerance. In the present work, we developed E. coli strains capable of high-level flavonoid synthesis through traditional metabolic engineering techniques. In addition to grafting the plant biosynthetic pathways, the methods included engineering of an alternative carbon assimilation pathway and the inhibition of competitive reaction pathways in order to increase intracellular flavonoid backbone precursors and cofactors. With this strategy, we report the production of plant-specific flavanones up to 700 mg/L and anthocyanins up to 113 mg/L from phenylpropanoic acid and flavan-3-ol precursors, respectively. These results demonstrated the efficient and scalable production of plant flavonoids from E. coli for pharmaceutical and nutraceutical applications.

A substitutive substrate for measurements of beta-ketoacyl reductases in two fatty acid synthase systems

Bacterial beta-ketoacyl-ACP reductase (FabG) and the beta-ketoacyl reductase domain in mammalian fatty acid synthase (FAS) have the same function and both are rendered as the novel targets for drugs. Herein we developed a convenient method, using an available compound ethyl acetoacetate (EAA) as the substitutive substrate, to measure their activities by monitoring decrease of NADPH absorbance at 340 nm. In addition to the result, ethyl 3-hydroxybutyrate (EHB) was detected by HPLC analysis in the reaction system, indicating that EAA worked effectively as the substrate of FabG and FAS since its beta-keto group was reduced. Then, the detailed kinetic characteristics, such as optimal ionic strength, pH value and temperature, and kinetic parameters, for FabG and FAS with this substitutive substrate were determined. The Km and kcat values of FabG obtained for EAA were 127 mM and 0.30 s(-1), while those of this enzyme for NADPH were 10.0 microM and 0.59 s(-1), respectively. The corresponding Km and kcat values of FAS were 126 mM and 4.63 s(-1) for EAA; 8.7 microM and 4.09 s(-1) for NADPH. Additionally, the inhibitory kinetics of FabG and FAS, by a known inhibitor EGCG, was also studied.

Regulation of fatty acid metabolism in bacteria

In Escherichia coli, the main player in transcription regulation of fatty acid metabolism is the FadR protein, which is involved in negative regulation of fatty acid degradation and in positive and negative regulation of the cellular processes related to it, as well as in positive regulation of the biosynthesis of unsaturated fatty acids in a concerted manner with negative regulation of FabR. On the other hand, Bacillus subtilis possesses two global transcriptional regulators, FadR (YsiA) and FapR. B. subtilis FadR represses fatty acid degradation, whereas FapR represses almost all the processes in the biosynthesis of saturated fatty acids and phospholipids. Furthermore, Streptococcus pneumoniae FabT represses the genes of fatty acid biosynthesis that are clustered in its genome. Long-chain acyl-CoAs appear to be metabolic signals for fatty acid degradation by bacteria in general, and antagonize the FadR protein from either E. coli or B. subtilis. However, malonyl-CoA is a metabolic signal for fatty acid and phospholipid biosynthesis by Gram-positive low-GC bacteria, and it antagonizes FapR. These would be the primary aspects for understanding the elaborate and complex regulation of fatty acid metabolism in bacteria to maintain membrane lipid homeostasis.

Involvement of the YneS/YgiH and PlsX proteins in phospholipid biosynthesis in both Bacillus subtilis and Escherichia coli

Background

Phospholipid biosynthesis commences with the acylation of glycerol-3-phosphate (G3P) to form 1-acyl-G3P. This step is catalyzed by the PlsB protein in Escherichia coli. The gene encoding this protein has not been identified, however, in the majority of bacterial genome sequences, including that of Bacillus subtilis. Recently, a new two-step pathway catalyzed by PlsX and PlsY proteins for the initiation of phospholipid formation in Streptococcus pneumoniae has been reported.

Results

In B. subtilis, 271 genes have been reported to be indispensable, when inactivated singly, for growth in LB medium. Among these, 11 genes encode proteins with unknown functions. As part of a genetic study to identify the functions of these genes, we show here that the B. subtilis ortholog of S. pneumoniae PlsY, YneS, is required for G3P acyltransferase activity, together with PlsX. The B. subtilis genome lacks plsB, and we show in vivo that the PlsX/Y pathway is indeed essential for the growth of bacteria lacking plsB. Interestingly, in addition to plsB, E. coli possesses plsX and the plsY ortholog, ygiH. We therefore explored the functional relationship between PlsB, PlsX and YgiH in E. coli, and found that plsB is essential for E. coli growth, indicating that PlsB plays an important role in 1-acyl-G3P synthesis in E. coli. We also found, however, that the simultaneous inactivation of plsX and ygiH was impossible, revealing important roles for PlsX and YgiH in E. coli growth.

Conclusion

Both plsX and yneS are essential for 1-acyl-G3P synthesis in B. subtilis, in agreement with recent reports on their biochemical functions. In E. coli, PlsB plays a principal role in 1-acyl-G3P synthesis and is also essential for bacterial growth. PlsX and YgiH also, however, play important roles in E. coli growth, possibly by regulating the intracellular concentration of acyl-ACP. These proteins are therefore important targets for development of new antibacterial agents.

Balancing a heterologous mevalonate pathway for improved isoprenoid production in Escherichia coli

Engineering biosynthetic pathways in microbes for the production of complex chemicals and pharmaceuticals is an attractive alternative to chemical synthesis. However, in transferring large pathways to alternate hosts and manipulating expression levels, the native regulation of carbon flux through the pathway may be lost leading to imbalances in the pathways. Previously, Escherichia coli was engineered to produce large quantities of isoprenoids by creating a mevalonate-based isopentenyl pyrophosphate biosynthetic pathway [Martin, V.J., Pitera, D.J., Withers, S.T., Newman, J.D., Keasling, J.D., 2003. Engineering a mevalonate pathway in Escherichia coli for production of terpenoids. Nat. Biotechnol. 21, 796-802]. The strain produces high levels of isoprenoids, but upon further investigation we discovered that the accumulation of pathway intermediates limited flux and that high-level expression of the mevalonate pathway enzymes inhibited cell growth. Gene titration studies and metabolite profiling using liquid chromatography-mass spectrometry linked the growth inhibition phenotype with the accumulation of the pathway intermediate 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG-CoA). Such an accumulation implies that the activity of HMG-CoA reductase was insufficient to balance flux in the engineered pathway. By modulating HMG-CoA reductase production, we eliminated the pathway bottleneck and increased mevalonate production. These results demonstrate that balancing carbon flux through the heterologous pathway is a key determinant in optimizing isoprenoid biosynthesis in microbial hosts.

Inactivation mechanism of the β-ketoacyl-[acyl carrier protein] reductase of bacterial type-II fatty acid synthase by epigallocatechin gallate

Epigallocatechin gallate (EGCG), a major compound from green tea, reversibly inhibits β-ketoacyl-[acyl carrier protein] reductase (FabG) from Escherichia coli. In this study, we found that EGCG exhibited an atypical time-dependent inhibition of FabG, which possibly resulted from the EGCG-induced aggregation of FabG. It was observed that FabG inactivation and aggregation occurred nearly simultaneously, with a lag time that decreased with increasing EGCG concentration. These results suggest that some chemical reactions, required for aggregation and inactivation, occurred during the lag time. Since EGC was detected by HPLC after the incubation of EGCG with FabG, EGCG probably covalently modified FabG. These further results showed that 1 tetramer of FabG must be modified by several, possibly 4, EGCG molecules before the formation of FabG aggregates. FabG aggregation was a first-order reaction independent of protein concentration. Due to an initial lag time, the first-order rate of aggregation gradually increased, reaching a maximal and constant value. The effect of increasing concentration of EGCG on the first-order rate constant for aggregation indicated that EGCG bound to FabG by affinity labeling. Based on the results, we propose a mechanism for the interaction of EGCG with FabG:EGCG first binds reversibly to each subunit of FabG, followed by covalent modification and then aggregation of the 4 EGCG-modified subunits.

Inhibitory activity of chlorogenic acid on enzymes involved in the fatty acid synthesis in animals and bacteria

It was found that chlorogenic acid inhibited in vitro animal fatty acid synthase (FAS I) and the ss-ketoacyl-ACP reductase (FabG) from Escherichia coli in a concentration-dependent manner with respective IC50 of 94.8 and 88.1 microM. The results of Lineweaver-Burk plots indicated that chlorogenic acid inhibited competitively the binding of NADPH to FAS I, while left those of acetyl-CoA and malonyl-CoA unaffected. Further kinetic studies showed that chlorogenic acid blocked the activity of FAS I mainly by inhibiting the ss-ketoacyl reductase domain, which catalyzed the same reaction as that done by FabG in the fatty acid synthesis. The ss-ketoacyl reduction reactions accomplished by both FAS I and FabG required nucleotide cofactor, NADPH. Furthermore, the Lineweaver-Burk and Yonetani-Theorell analyses implicated that chlorogenic acid filled competitively in the binding-pocket of NADPH in the ss-ketoacyl reductase domain of FAS I. The similar results were also obtained from the inhibition of FabG by chlorogenic acid. As observed in these results, the inhibitions of FAS I and FabG by chlorogenic acid were highly related to the interference of the inhibitor with NADPH, which was possibly due to the similarity between chlorogenic acid and some portion of NADPH, maybe the section consisting of the two ribose groups.

Transcriptional control of membrane lipid synthesis in bacteria

Fatty acid synthesis is coordinately regulated with phospholipid, macromolecular synthesis and growth as part of the response to changes in the environment. Many of these processes are rapid responses of the integrated biochemical network and do not involve changes in gene expression. An important recent development is the identification and characterization of transcription factors that modify pathway activity by either altering the expression levels of a few important genes or controlling a global adjustment in the expression of the entire pathway. For most of these transcription factors the signaling molecules controlling their activities are still poorly defined.

β-Ketoacyl-Acyl Carrier Protein Synthase III (FabH) Is Essential for Bacterial Fatty Acid Synthesis

beta-Ketoacyl-acyl carrier protein (ACP) synthase III (KAS III, also called acetoacetyl-ACP synthase) encoded by the fabH gene is thought to catalyze the first elongation reaction (Claisen condensation) of type II fatty acid synthesis in bacteria and plant plastids. However, direct in vivo evidence that KAS III catalyzes an essential reaction is lacking, because no mutant organism deficient in this activity has been isolated. We report the first bacterial strain lacking KAS III, a fabH mutant constructed in the Gram-positive bacterium Lactococcus lactis subspecies lactis IL1403. The mutant strain carries an in-frame deletion of the KAS III active site region and was isolated by gene replacement using a medium supplemented with a source of saturated and unsaturated long-chain fatty acids. The mutant strain is devoid of KAS III activity and fails to grow in the absence of supplementation with exogenous long-chain fatty acids demonstrating that KAS III plays an essential role in cellular metabolism. However, the L. lactis fabH deletion mutant requires only long-chain unsaturated fatty acids for growth, a source of long-chain saturated fatty acids is not required. Because both saturated and unsaturated fatty acids are required for growth when fatty acid synthesis is blocked by biotin starvation (which prevents the synthesis of malonyl-CoA), another pathway for saturated fatty acid synthesis must remain in the fabH deletion strain. Indeed, incorporation of [1-14C]acetate into fatty acids in vivo showed that the fabH mutant retained about 10% of the fatty acid synthetic ability of the wild-type strain and that this residual synthetic capacity was preferentially diverted to the saturated branch of the pathway. Moreover, mass spectrometry showed that the fabH mutant retained low levels of palmitic acid upon fatty acid starvation. Derivatives of the fabH deletion mutant strain were isolated that were octanoic acid auxotrophs consistent with biochemical studies indicating that the major role of FabH is production of short-chain fatty acid primers. We also confirmed the essentiality of FabH in Escherichia coli by use of a plasmid-based gene insertion/deletion system. Together these results provide the first genetic evidence demonstrating that FabH conducts the major condensation reaction in the initiation of type II fatty acid biosynthesis in both Gram-positive and Gram-negative bacteria.

FapR, a bacterial transcription factor involved in global regulation of membrane lipid biosynthesis

Bacterial cells exert exquisite control over the biosynthesis of their membrane lipids, but the mechanisms are obscure. We describe the identification and purification from Bacillus subtilis of a transcription factor, FapR, that controls the expression of many genes involved in fatty acid and phospholipid metabolism (the fap regulon). Expression of this fap regulon is influenced by antibiotics that specifically inhibit the fatty acid biosynthetic pathway. We show that FapR negatively regulates fap expression and that the effects of antibiotics on fap expression are mediated by FapR. We further show that decreasing the cellular levels of malonyl-CoA, an essential molecule for fatty acid elongation, inhibits expression of the fap regulon and that this effect is FapR dependent. Our results indicate that control of FapR by the cellular pools of malonyl-CoA provides a mechanism for sensing the status of fatty acid biosynthesis and to adjust the expression of the fap regulon accordingly.