There is significant progress toward understanding catalysis throughout the essential MEP pathway to isoprenoids in human pathogens; however, little is known about pathway regulation. The present study begins by testing the hypothesis that isoprenoid biosynthesis is regulated via feedback inhibition of the fifth enzyme cyclodiphosphate IspF by downstream isoprenoid diphosphates. Here, we demonstrate recombinant E. coli IspF is not inhibited by downstream metabolites and isopentenyl diphosphate (IDP), dimethylallyl diphosphate (DMADP), geranyl diphosphate (GDP) and farnesyl diphosphate (FDP) under standard assay conditions. However, 2C-methyl-d-erythritol 4-phosphate (MEP), the product of reductoisomerase IspC and first committed MEP pathway intermediate, activates and sustains this enhanced IspF activity, and the IspF-MEP complex is inhibited by FDP. We further show that the methylerythritol scaffold itself, which is unique to this pathway, drives the activation and stabilization of active IspF. Our results suggest a novel feed-forward regulatory mechanism for 2Cmethyl-d-erythritol 2,4-cyclodiphosphate (MEcDP) production and support an isoprenoid biosynthesis regulatory mechanism via feedback inhibition of the IspF-MEP complex by FDP. The results have important implications for development of inhibitors against the IspF-MEP complex, which may be the physiologically relevant form of the enzyme.
cyclodiphosphate synthase; IspF; methylerythritol phosphate; MEP pathway regulation
The ability to assemble multiple fragments of DNA into a plasmid in a single step is invaluable to studies in metabolic engineering and synthetic biology. Using phosphorothioate chemistry for high efficiency and site specific cleavage of sequences, a novel ligase independent cloning method (cross-lapping in vitro assembly, CLIVA) was systematically and rationally optimized in E. coli. A series of 16 constructs combinatorially expressing genes encoding enzymes in the 1-deoxy-D-xylulose 5-phosphate (DXP) pathway were assembled using multiple DNA modules. A plasmid (21.6 kb) containing 16 pathway genes, was successfully assembled from 7 modules with high efficiency (2.0 x 103 cfu/ µg input DNA) within 2 days. Overexpressions of these constructs revealed the unanticipated inhibitory effects of certain combinations of genes on the production of amorphadiene. Interestingly, the inhibitory effects were correlated to the increase in the accumulation of intracellular methylerythritol cyclodiphosphate (MEC), an intermediate metabolite in the DXP pathway. The overexpression of the iron sulfur cluster operon was found to modestly increase the production of amorphadiene. This study demonstrated the utility of CLIVA in the assembly of multiple fragments of DNA into a plasmid which enabled the rapid exploration of biological pathways.
The biosynthesis of isopentenyl diphosphate (IPP) from either the mevalonate (MVA) or the 1-deoxy-d-xylulose 5-phosphate (DXP) pathway provides the key metabolite for primary and secondary isoprenoid biosynthesis. Isoprenoid metabolism plays crucial roles in membrane stability, steroid biosynthesis, vitamin production, protein localization, defense and communication, photoprotection, sugar transport, and glycoprotein biosynthesis. Recently, an alternative branch of the MVA pathway was discovered in the archaeon Methanocaldococcus jannaschii involving a small molecule kinase, isopentenyl phosphate kinase (IPK). IPK belongs to the amino acid kinase (AAK) superfamily. In vitro, IPK phosphorylates isopentenyl monophosphate (IP) in an ATP and Mg2+-dependent reaction producing IPP. Here, we describe crystal structures of IPK from M. jannaschii refined to nominal resolutions of 2.0−2.8 Å. Notably, an active site histidine residue (His60) forms a hydrogen bond with the terminal phosphate of both substrate and product. This His residue serves as a marker for a subset of the AAK family that catalyzes phosphorylation of phosphate or phosphonate functional groups; the larger family includes carboxyl-directed kinases, which lack this active site residue. Using steady-state kinetic analysis of H60A, H60N, and H60Q mutants, the protonated form of the Nε2 nitrogen of His60 was shown to be essential for catalysis, most likely through hydrogen bond stabilization of the transition state accompanying transphosphorylation. Moreover, the structures served as the starting point for the engineering of IPK mutants capable of the chemoenzymatic synthesis of longer chain isoprenoid diphosphates from monophosphate precursors.
Heterologous microbial production of rare plant terpenoids of medicinal or industrial interest is attracting more and more attention but terpenoid yields are still low. Escherichia coli and Saccharomyces cerevisiae are the most widely used heterologous hosts; a direct comparison of both hosts based on experimental data is difficult though. Hence, the terpenoid pathways of E. coli (via 1-deoxy-D-xylulose 5-phosphate, DXP) and S. cerevisiae (via mevalonate, MVA), the impact of the respective hosts metabolism as well as the impact of different carbon sources were compared in silico by means of elementary mode analysis. The focus was set on the yield of isopentenyl diphosphate (IPP), the general terpenoid precursor, to identify new metabolic engineering strategies for an enhanced terpenoid yield.
Starting from the respective precursor metabolites of the terpenoid pathways (pyruvate and glyceraldehyde-3-phosphate for the DXP pathway and acetyl-CoA for the MVA pathway) and considering only carbon stoichiometry, the two terpenoid pathways are identical with respect to carbon yield. However, with glucose as substrate, the MVA pathway has a lower potential to supply terpenoids in high yields than the DXP pathway if the formation of the required precursors is taken into account, due to the carbon loss in the formation of acetyl-CoA. This maximum yield is further reduced in both hosts when the required energy and reduction equivalents are considered. Moreover, the choice of carbon source (glucose, xylose, ethanol or glycerol) has an effect on terpenoid yield with non-fermentable carbon sources being more promising. Both hosts have deficiencies in energy and redox equivalents for high yield terpenoid production leading to new overexpression strategies (heterologous enzymes/pathways) for an enhanced terpenoid yield. Finally, several knockout strategies are identified using constrained minimal cut sets enforcing a coupling of growth to a terpenoid yield which is higher than any yield published in scientific literature so far.
This study provides for the first time a comprehensive and detailed in silico comparison of the most prominent heterologous hosts E. coli and S. cerevisiae as terpenoid factories giving an overview on several promising metabolic engineering strategies paving the way for an enhanced terpenoid yield.
Terpenoids; Isoprenoids; In silico; Elementary mode analysis; Constrained minimal cut sets; Metabolic engineering; Escherichia coli; Saccharomyces cerevisiae
Isoprenoids are a large and diverse class of compounds that includes many high value natural products and are thus in great demand. To meet the increasing demand for isoprenoid compounds, metabolic engineering of microbes has been used to produce isoprenoids in an economical and sustainable manner. To achieve high isoprenoid yields using this technology, the availability of metabolic precursors feeding the deoxyxylulose phosphate (DXP) pathway, responsible for isoprenoid biosynthesis, has to be optimized. In this study, phosphoenolpyruvate, a vital DXP pathway precursor, was enriched by deleting the genes encoding the carbohydrate phosphotransferase system (PTS) in E. coli. Production of lycopene (a C40 isoprenoid) was maximized by optimizing growth medium and culture conditions. In optimized conditions, the lycopene yield from PTS mutant was seven fold higher than that obtained from the wild type strain. This resulted in the highest reported specific yield of lycopene produced from the DXP pathway in E. coli to date (20,000 µg/g dry cell weight). Both the copy number of the plasmid encoding the lycopene biosynthetic genes and the expression were found to be increased in the optimized media. Deletion of PTS together with a similar optimization strategy was also successful in enhancing the production of amorpha-1,4-diene, a distinct C15 isoprenoid, suggesting that the approaches developed herein can be generally applied to optimize production of other isoprenoids.
Isoprenoids constitute the largest class of natural products with greater than 55,000 identified members. They play essential roles in maintaining proper cellular function leading to maintenance of human health, plant defense mechanisms against predators, and are often exploited for their beneficial properties in the pharmaceutical and nutraceutical industries. Most impressively, all known isoprenoids are derived from one of two C5-precursors, isopentenyl diphosphate (IPP) or dimethylallyl diphosphate (DMAPP). In order to study the enzyme transformations leading to the extensive structural diversity found within this class of compounds there must be access to the substrates. Sometimes, intermediates within a biological pathway can be isolated and used directly to study enzyme/pathway function. However, the primary route to most of the isoprenoid intermediates is through chemical catalysis. As such, this review provides the first exhaustive examination of synthetic routes to isoprenoid and isoprenoid precursors with particular emphasis on the syntheses of intermediates found as part of the 2C-methylerythritol 4-phosphate (MEP) pathway. In addition, representative syntheses are presented for the monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), triterpenes (C30) and tetraterpenes (C40). Finally, in some instances, the synthetic routes to substrate analogs found both within the MEP pathway and downstream isoprenoids are examined.
Enzyme mechanism; isoprenoids; terpenes; MEP pathway.
Isoprenoid biosynthesis is essential for survival of all living organisms. More than 50,000 unique isoprenoids occur naturally, with each constructed from two simple five-carbon precursors: isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). Two pathways for the biosynthesis of IPP and DMAPP are found in nature. Humans exclusively use the mevalonate (MVA) pathway, while most bacteria, including all Gram-negative and many Gram-positive species, use the unrelated methylerythritol phosphate (MEP) pathway. Here we report the development of a novel, whole-cell phenotypic screening platform to identify compounds that selectively inhibit the MEP pathway. Strains of Salmonella enterica serovar Typhimurium were engineered to have separately inducible MEP (native) and MVA (nonnative) pathways. These strains, RMC26 and CT31-7d, were then used to differentiate MVA pathway- and MEP pathway-specific perturbation. Compounds that inhibit MEP pathway-dependent bacterial growth but leave MVA-dependent growth unaffected represent MEP pathway-selective antibacterials. This screening platform offers three significant results. First, the compound is antibacterial and is therefore cell permeant, enabling access to the intracellular target. Second, the compound inhibits one or more MEP pathway enzymes. Third, the MVA pathway is unaffected, suggesting selectivity for targeting the bacterial versus host pathway. The cell lines also display increased sensitivity to two reported MEP pathway-specific inhibitors, further biasing the platform toward inhibitors selective for the MEP pathway. We demonstrate development of a robust, high-throughput screening platform that combines phenotypic and target-based screening that can identify MEP pathway-selective antibacterials simply by monitoring optical density as the readout for cell growth/inhibition.
Isoprenoids are one of the largest classes of natural products and all of them are constructed from two precursors, isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). For decades, the mevalonic acid (MVA) pathway was proposed to be the only IPP and DMAPP biosynthetic pathway. This review summarizes the newly discovered IPP and DMAPP production pathways since late 1990s, their distribution among different kingdoms, and their roles in secondary metabolite production. These new IPP and DMAPP production pathways include the methylerythritol phosphate (MEP) pathway, a modified MVA pathway, and the 5-Methylthioadenosine shunt pathway. Relative to the studies on the MVA pathway, information on the MEP pathway regulation is limited and the mechanistic details of several of its novel transformations remain to be addressed. Current status on both MEP pathway regulation and mechanistic issues are also presented.
isoprenoids; MVA; MEP; methylthioadenosine; regulation; biosynthesis
The biogenesis of isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) is accomplished by the methylerythritol phosphate (MEP) pathway in plants, bacteria and parasites, making it a potential target for the development of anti-infective agents and herbicides. The biosynthetic enzymes comprising this pathway catalyze intriguing chemical transformations on diphosphate scaffolds, offering an opportunity to generate novel analogs in this synthetically challenging compound class. Such a biosynthetic approach to generating new diphosphate analogs may involve transformation through discrete diphosphate species, presenting unique challenges in structure determination and characterization of unnatural enzyme-generated diphosphate products produced in tandem. We have developed 1H–31P–31P correlation NMR spectroscopy techniques for the direct characterization of crude MEP pathway enzyme products at low concentrations (200 μM to 5 mM) on a room temperature (non-cryogenic) NMR probe. Coupling the 100% natural abundance of the 31P nucleus with the high intrinsic sensitivity of proton NMR, 1H–31P–31P correlation spectroscopy is particularly useful for characterization of unnatural diphosphate enzyme products in the MEP pathway. As proof of principle, we demonstrate the rapid characterization of natural enzyme products of the enzymes IspD, E and F in tandem enzyme incubations. In addition, we have characterized several unnatural enzyme products using this technique, including new products of cytidyltransferase IspD bearing erythritol, glycerol and ribose components. The results of this study indicate that IspD may be a useful biocatalyst and highlight 1H–31P–31P correlation spectroscopy as a valuable tool for the characterization of other unnatural products in non-mammalian isoprenoid biosynthesis.
The unique methylerythritol phosphate (MEP) pathway for isoprenoid biosynthesis is essential in most bacterial pathogens. The first enzyme in this pathway, 1-deoxy-D-xylulose 5-phosphate (DXP) synthase, catalyzes a distinct thiamin diphosphate (ThDP)-dependent reaction to form DXP from D-glyceraldehyde 3-phosphate (D-GAP) and pyruvate and represents a potential anti-infective drug target. We have previously demonstrated that the unnatural bisubstrate analog, butylacetylphosphonate (BAP), exhibits selective inhibition of Escherichia coli DXP synthase over mammalian ThDP-dependent enzymes. Here, we report the selective inhibition by BAP against recombinant DXP synthase homologs from Mycobacterium tuberculosis, Yersinia pestis, and Salmonella enterica. We also demonstrate antimicrobial activity of BAP against both Gram-negative and Gram-positive strains (including E. coli, S. enterica, Bacillus anthracis), and several clinically isolated pathogens. Our results suggest a mechanism of action involving inhibition of DXP synthase and show that BAP acts synergistically with established antimicrobial agents, highlighting a potential strategy to combat emerging resistance in bacterial pathogens.
DXP synthase; isoprenoid biosynthesis; selective inhibitor; thiamin diphosphate
In cyanobacteria many compounds, including chlorophylls, carotenoids, and hopanoids, are synthesized from the isoprenoid precursors isopentenyl diphosphate (IPP) and dimethylallyl diphosphate. Isoprenoid biosynthesis in extracts of the cyanobacterium Synechocystis strain PCC 6803 grown under photosynthetic conditions, stimulated by pentose phosphate cycle substrates, does not appear to require methylerythritol phosphate pathway intermediates. The sll1556 gene, distantly related to type 2 IPP isomerase genes, was disrupted by insertion of a Kanr cassette. The mutant was fully viable under photosynthetic conditions although impaired in the utilization of pentose phosphate cycle substrates. Compared to the parental strain the Δsll1556 mutant (i) is deficient in isoprenoid biosynthesis in vitro with substrates including glyceraldehyde-3-phosphate, fructose-6-phosphate, and glucose-6-phosphate; (ii) has smaller cells (diameter ca. 13% less); (iii) has fewer thylakoids (ca. 30% less); and (iv) has a more extensive fibrous outer wall layer. Isoprenoid biosynthesis is restored with pentose phosphate cycle substrates plus the recombinant Sll1556 protein in the Δsll1556 supernatant fraction. IPP isomerase activity could not be demonstrated for the purified Sll1556 protein under our in vitro conditions. The reduction of thylakoid area and the effect on outer wall layer components are consistent with an impairment of isoprenoid biosynthesis in the mutant, possibly via hopanoid biosynthesis. Our findings are consistent with an alternate metabolic shunt for biosynthesis of isoprenoids.
A functional 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway is required for isoprenoid biosynthesis and hence survival in Escherichia coli and most other bacteria. In the first two steps of the pathway, MEP is produced from the central metabolic intermediates pyruvate and glyceraldehyde 3-phosphate via 1-deoxy-D-xylulose 5-phosphate (DXP) by the activity of the enzymes DXP synthase (DXS) and DXP reductoisomerase (DXR). Because the MEP pathway is absent from humans, it was proposed as a promising new target to develop new antibiotics. However, the lethal phenotype caused by the deletion of DXS or DXR was found to be suppressed with a relatively high efficiency by unidentified mutations. Here we report that several mutations in the unrelated genes aceE and ribB rescue growth of DXS-defective mutants because the encoded enzymes allowed the production of sufficient DXP in vivo. Together, this work unveils the diversity of mechanisms that can evolve in bacteria to circumvent a blockage of the first step of the MEP pathway.
Recombinant proteins are routinely overexpressed in metabolic engineering. It is well known that some over-expressed heterologous recombinant enzymes are insoluble with little or no enzymatic activity. This study examined the solubility of over-expressed homologous enzymes of the deoxyxylulose phosphate pathway (DXP) and the impact of inclusion body formation on metabolic engineering of microbes.
Four enzymes of this pathway (DXS, ISPG, ISPH and ISPA), but not all, were highly insoluble, regardless of the expression systems used. Insoluble dxs (the committed enzyme of DXP pathway) was found to be inactive. Expressions of fusion tags did not significantly improve the solubility of dxs. However, hypertonic media containing sorbitol, an osmolyte, successfully doubled the solubility of dxs, with the concomitant improvement in microbial production of the metabolite, DXP. Similarly, sorbitol significantly improved the production of soluble and functional ERG12, the committed enzyme in the mevalonate pathway.
This study demonstrated the unanticipated findings that some over-expressed homologous enzymes of the DXP pathway were highly insoluble, forming inclusion bodies, which affected metabolite formation. Sorbitol was found to increase both the solubility and function of some of these over-expressed enzymes, a strategy to increase the production of secondary metabolites.
Isoprenoids; Protein solubility; Deoxyxylulose phosphate pathway; Activity analysis; Metabolic engineering
1-Deoxy-d-xylulose 5-phosphate (DXP) synthase catalyzes the first step in the non-mammalian isoprenoid biosynthetic pathway to form DXP from pyruvate and d-glyceraldehyde 3-phosphate (d-GAP) in a thiamin diphosphate-dependent manner. Its unique structure and mechanism distinguish DXP synthase from its homologs, suggesting it should be pursued as an anti-infective drug target. However, few reports describe development of selective inhibitors of this enzyme. Here, we reveal a function of DXP synthase that catalyzes C-N bond formation and exploit aromatic nitroso substrates as active site probes. Substrate specificity studies reveal high affinity of DXP synthase for aromatic nitroso substrates compared to the related ThDP-dependent enzyme Pyruvate Dehydrogenase (PDH). Results from inhibition and mutagenesis studies indicate nitroso substrates bind to E. coli DXP synthase in a manner distinct from d-GAP. Our results suggest that incorporation of aryl acceptor substrate mimics into unnatural bisubstrate analogs will impart selectivity to DXP synthase inhibitors. As proof of concept, we show selective inhibition of DXP synthase by benzylacetylphosphonate (BnAP).
enzyme inhibitors; kinetics; isoprenoid biosynthesis; substrate specificity; 1-Deoxy-d-xylulose 5-phosphate (DXP) synthase
The methylerythritol phosphate (MEP) pathway found in many bacteria governs the synthesis of isoprenoids, which are crucial lipid precursors for vital cell components such as ubiquinone. Because mammals synthesize isoprenoids via an alternate pathway, the bacterial MEP pathway is an attractive target for novel antibiotic development, necessitated by emerging antibiotic resistance as well as biodefense concerns. The first committed step in the MEP pathway is the reduction and isomerization of 1-deoxy-D-xylulose-5-phosphate (DXP) to methylerythritol phosphate (MEP), catalyzed by MEP synthase. To facilitate drug development, we cloned, expressed, purified, and characterized MEP synthase from Yersinia pestis. Enzyme assays indicate apparent kinetic constants of KMDXP = 252 µM and KMNADPH = 13 µM, IC50 values for fosmidomycin and FR900098 of 710 nM and 231 nM respectively, and Ki values for fosmidomycin and FR900098 of 251 nM and 101 nM respectively. To ascertain if the Y. pestis MEP synthase was amenable to a high-throughput screening campaign, the Z-factor was determined (0.9) then the purified enzyme was screened against a pilot scale library containing rationally designed fosmidomycin analogs and natural product extracts. Several hit molecules were obtained, most notably a natural product allosteric affector of MEP synthase and a rationally designed bisubstrate derivative of FR900098 (able to associate with both the NADPH and DXP binding sites in MEP synthase). It is particularly noteworthy that allosteric regulation of MEP synthase has not been described previously. Thus, our discovery implicates an alternative site (and new chemical space) for rational drug development.
Retinoids are lipophilic isoprenoids composed of a cyclic group and a linear chain with a hydrophilic end group. These compounds include retinol, retinal, retinoic acid, retinyl esters, and various derivatives of these structures. Retinoids are used as cosmetic agents and effective pharmaceuticals for skin diseases. Retinal, an immediate precursor of retinoids, is derived by β-carotene 15,15'-mono(di)oxygenase (BCM(D)O) from β-carotene, which is synthesized from the isoprenoid building blocks isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). Retinoids are chemically unstable and biologically degraded via retinoic acid. Although extensive studies have been performed on the microbial production of carotenoids, retinoid production using microbial metabolic engineering has not been reported. Here, we report retinoid production using engineered Escherichia coli that express exogenous BCM(D)O and the mevalonate (MVA) pathway for the building blocks synthesis in combination with a two-phase culture system using a dodecane overlay.
Among the BCM(D)O tested in E. coli, the synthetic retinoid synthesis protein (SR), based on bacteriorhodopsin-related protein-like homolog (Blh) of the uncultured marine bacteria 66A03, showed the highest β-carotene cleavage activity with no residual intracellular β-carotene. By introducing the exogenous MVA pathway, 8.7 mg/L of retinal was produced, which is 4-fold higher production than that of augmenting the MEP pathway (dxs overexpression). There was a large gap between retinal production and β-carotene consumption using the exogenous MVA pathway; therefore, the retinal derivatives were analyzed. The derivatives, except for retinoic acid, that formed were identified, and the levels of retinal, retinol, and retinyl acetate were measured. Amounts as high as 95 mg/L retinoids were obtained from engineered E. coli DH5α harboring the synthetic SR gene and the exogenous MVA pathway in addition to dxs overexpression, which were cultured at 29°C for 72 hours with 2YT medium containing 2.0% (w/v) glycerol as the main carbon source. However, a significant level of intracellular degradation of the retinoids was also observed in the culture. To prevent degradation of the intracellular retinoids through in situ extraction from the cells, a two-phase culture system with dodecane was used. The highest level of retinoid production (136 mg/L) was obtained after 72 hours with 5 mL of dodecane overlaid on a 5 mL culture.
In this study, we successfully produced 136 mg/L retinoids, which were composed of 67 mg/L retinal, 54 mg/L retinol, and 15 mg/L retinyl acetate, using a two-phase culture system with dodecane, which produced 68-fold more retinoids than the initial level of production (2.2 mg/L). Our results demonstrate the potential use of E. coli as a promising microbial cell factory for retinoid production.
The isopentenols, including isoprenol and prenol, are excellent alternative fuels. However, they are not compounds largely accumulated in natural organism. The need for the next generation of biofuels with better physical and chemical properties impels us to develop biosynthetic routes for the production of isoprenol and prenol from renewable sugar. In this study, we use the heterogenous mevalonate-dependent (MVA) isoprenoid pathway for the synthesis of isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) intermediates, and then convert IPP and DMAPP to isoprenol and prenol, respectively.
A mevalonate titer of 1.7 g/L was obtained by constructing an efficient MVA upper pathway in engineered E. coli. Different phosphatases and pyrophosphatases were investigated for their abilities in hydrolyzing the IPP and DMAPP. Consequently, ADP-ribose pyrophosphatase was found to be an efficient IPP and DMAPP hydrolase. Moreover, ADP-ribose pyrophosphatase from Bacillus subtilis (BsNudF) exhibited a equivalent substrate specificity towards IPP and DMAPP, while ADP-ribose pyrophosphatase from E. coli (EcNudF) presented a high substrate preference for DMAPP. Without the expression of any phosphatases or pyrophosphatases, a background level of isopentenols was synthesized. When the endogenous pyrophosphatase genes (EcNudF and yggV) that were capable of enhancing the hydrolyzation of the IPP and DMAPP were knocked out, the background level of isopentenols was still obtained. Maybe the synthesized IPP and DMAPP were hydrolyzed by some unknown hydrolases of E. coli. Finally, 1.3 g/L single isoprenol was obtained by blocking the conversion of IPP to DMAPP and employing the BsNudF, and 0.2 g/L ~80% prenol was produced by employing the EcNudF. A maximal yield of 12% was achieved in both isoprenol and prenol producing strains.
To the best of our knowledge, this is the first successful report on high-specificity production of isoprenol and prenol by microbial fermentation. Over 1.3 g/L isoprenol achieved in shake-flask experiments represents a quite encouraging titer of higher alcohols. In addition, the substrate specificities of ADP-ribose pyrophosphatases were determined and successfully applied for the high-specificity synthesis of isoprenol and prenol. Altogether, this work presents a promising strategy for high-specificity production of two excellent biofuels, isoprenol and prenol.
Isoprenol; Prenol; Metabolic engineering; Escherichia coli; Biofuel
Isoprenoid compounds constitute an immensely diverse group of acyclic, monocyclic and polycyclic compounds that play important roles in all living organisms. Despite the diversity of their structures, this plethora of natural products arises from only two 5-carbon precursors, isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). This review will discuss the enzymes in the mevalonate (MVA) and methylerythritol phosphate (MEP) biosynthetic pathways leading to IPP and DMAPP with a particular focus on MEP synthase (DXR) and IPP isomerase (IDI), which are potential targets for the development of antibiotic compounds. DXR is the second enzyme in the MEP pathway and the only one for which inhibitors with antimicrobial activity at pharmaceutically relevant concentrations are known. All of the published DXR inhibitors are fosmidomycin analogues, except for a few bisphosphonates with moderate inhibitory activity. These far, there are no other candidates that target DXR. IDI was first identified and characterised over 40 years ago (IDI-1) and a second convergently evolved isoform (IDI-2) was discovered in 2001. IDI-1 is a metalloprotein found in Eukarya and many species of Bacteria. Its mechanism has been extensively studied. In contrast, IDI-2 requires reduced flavin mononucleotide as a cofactor. The mechanism of action for IDI-2 is less well defined. This review will describe how lead inhibitors are being improved by structure-based drug design and enzymatic assays against DXR to lead to new drug families and how mechanistic probes are being used to address questions about the mechanisms of the isomerases.
DXR; IDI; isomerase; isopentenyl; isoprenoid; MEP; mevalonate; MVA; reductoisomerase
Isoprenoids are a diverse group of molecules found in all organisms, where they perform such important biological functions as hormone signaling (e.g., steroids) in mammals, antioxidation (e.g., carotenoids) in plants, electron transport (e.g., ubiquinone), and cell wall biosynthesis intermediates in bacteria. All isoprenoids are synthesized by the consecutive condensation of the five-carbon monomer isopentenyl diphosphate (IPP) to its isomer, dimethylallyl diphosphate (DMAPP). The biosynthetic pathway for the formation of IPP from acetyl-CoA (i.e., the mevalonate pathway) had been established mainly in mice and the budding yeast Saccharomyces cerevisiae. Curiously, most prokaryotic microorganisms lack homologs of the genes in the mevalonate pathway, even though IPP and DMAPP are essential for isoprenoid biosynthesis in bacteria. This observation provided an impetus to search for an alternative pathway to synthesize IPP and DMAPP, ultimately leading to the discovery of the mevalonate-independent 2-C-methyl-d-erythritol 4-phosphate pathway. This review article focuses on our significant contributions to a comprehensive understanding of the biosynthesis of IPP and DMAPP.
biosynthesis; inhibitor; isoprenoid; MEP pathway; mevalonate pathway; terpenoid
The final step of the methylerythritol phosphate isoprenoid biosynthesis pathway is catalysed by the iron–sulphur enzyme IspH, producing the universal precursors of terpenes: isopentenyl diphosphate and dimethylallyl diphosphate. Here we report an unforeseen reaction discovered during the investigation of the interaction of IspH with acetylene inhibitors by X-ray crystallography, Mößbauer, and nuclear magnetic resonance spectroscopy. In addition to its role as a 2H+/2e− reductase, IspH can hydrate acetylenes to aldehydes and ketones via anti-Markovnikov/Markovnikov addition. The reactions only occur with the oxidised protein and proceed via η1-O-enolate intermediates. One of these is characterized crystallographically and contains a C4 ligand oxygen bound to the unique, fourth iron in the 4Fe-4S cluster: this intermediate subsequently hydrolyzes to produce an aldehyde product. This unexpected side to IspH reactivity is of interest in the context of the mechanism of action of other acetylene hydratases, as well as in the design of antiinfectives targeting IspH.
The role of peroxisomes in isoprenoid metabolism, especially in plants, has been questioned in several reports. A recent study of Sapir-Mir et al.1 revealed that the two isoforms of isopentenyl diphosphate (IPP) isomerase, catalyzing the isomerisation of IPP to dimethylallyl diphosphate (DMAPP) are found in the peroxisome. In this addendum, we provide additional data describing the peroxisomal localization of 5-phosphomevalonate kinase and mevalonate 5-diphosphate decarboxylase, the last two enzymes of the mevalonic acid pathway leading to IPP.2 This finding was reinforced in our latest report showing that a short isoform of farnesyl diphosphate, using IPP and DMAPP as substrates, is also targeted to the organelle.3 Therefore, the classical sequestration of isoprenoid biosynthesis between plastids and cytosol/ER can be revisited by including the peroxisome as an additional isoprenoid biosynthetic compartment within plant cells.
5-phosphomevalonate kinase; Arabidopsis thaliana; Catharanthus roseus; farnesyl diphosphate synthase; isoprenoid; mevalonate 5-diphosphate decarboxylase; mevalonic acid pathway; peroxisome
Many pathogenic bacteria utilize the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway for the biosynthesis of isopentenyl diphosphate and dimethylallyl diphosphate, two major building blocks of isoprenoid compounds. The fifth enzyme in the MEP pathway, 2-C-methyl-D-erythritol 2,4-cyclodiphosphate (ME-CPP) synthase (IspF), catalyzes the conversion of 4-diphosphocytidyl-2-C-methyl-D-erythritol 2-phosphate (CDP-ME2P) to ME-CPP with a corresponding release of cytidine 5-monophosphate (CMP). Since there is no ortholog of IspF in human cells IspF is of interest as a potential drug target. However, study of IspF has been hindered by a lack of enantiopure CDP-ME2P. Herein, we report the first synthesis of enantiomerically pure CDP-ME2P from commercially available D-arabinose. Cloned, expressed, and purified M. tuberculosis IspF was able to utilize the synthetic CDP-ME2P as a substrate, a result confirmed by mass spectrometry. A convenient, sensitive, in vitro IspF assay was developed by coupling the CMP released during production of ME-CPP to mononucleotide kinase, which can be used for high throughput screening.
β-carotene is a carotenoid compound that has been widely used not only in the industrial production of pharmaceuticals but also as nutraceuticals, animal feed additives, functional cosmetics, and food colorants. Currently, more than 90% of commercial β-carotene is produced by chemical synthesis. Due to the growing public concern over food safety, the use of chemically synthesized β-carotene as food additives or functional cosmetic agents has been severely controlled in recent years. This has reignited the enthusiasm for seeking natural β-carotene in large-scale fermentative production by microorganisms.
To increase β-carotene production by improving the isopentenyl pyrophosphate (IPP) and geranyl diphospate (GPP) concentration in the cell, the optimized MEP (methylerythritol 4-phosphate) pathway containing 1-deoxy-D-xylulose-5-phosphate synthase (DXS) and isopentenyl pyrophosphate isomerase (FNI) from Bacillus subtilis, geranyl diphosphate synthase (GPPS2) from Abies grandis have been co-expressed in an engineered E. coli strain. To further enhance the production of β-carotene, the hybrid MVA (mevalonate) pathway has been introduced into an engineered E. coli strain, co-expressed with the optimized MEP pathway and GPPS2. The final genetically modified strain, YJM49, can accumulate 122.4±6.2 mg/L β-carotene in flask culture, approximately 113-fold and 1.7 times greater than strain YJM39, which carries the native MEP pathway, and YJM45, which harbors the MVA pathway and the native MEP pathway, respectively. Subsequently, the fermentation process was optimized to enhance β-carotene production with a maximum titer of 256.8±10.4 mg/L. Finally, the fed-batch fermentation of β-carotene was evaluated using the optimized culture conditions. After induction for 56 h, the final engineered strain YJM49 accumulated 3.2 g/L β-carotene with a volumetric productivity of 0.37 mg/(L · h · OD600) in aerobic fed-batch fermentation, and the conversion efficiency of glycerol to β-carotene (gram to gram) reached 2.76%.
In this paper, by using metabolic engineering techniques, the more efficient biosynthetic pathway of β-carotene was successfully assembled in E. coli BL21(DE3) with the optimized MEP (methylerythritol 4-phosphate) pathway, the gene for GPPS2 from Abies grandis, the hybrid MVA (mevalonate) pathway and β-carotene synthesis genes from Erwinia herbicola.
Electronic supplementary material
The online version of this article (doi:10.1186/s12934-014-0160-x) contains supplementary material, which is available to authorized users.
β-carotene; MEP pathway; MVA pathway; E. coli
Engineering biosynthetic pathways in heterologous microbial host organisms offers an elegant approach to pathway elucidation via the incorporation of putative biosynthetic enzymes and characterization of resulting novel metabolites. Our previous work in Escherichia coli demonstrated the feasibility of a facile modular approach to engineering the production of labdane-related diterpene (20 carbon) natural products. However, yield was limited (<0.1 mg/L), presumably due to reliance on endogenous production of the isoprenoid precursors dimethylallyl diphosphate and isopentenyl diphosphate. Here, we report incorporation of either a heterologous mevalonate pathway (MEV) or enhancement of the endogenous methyl erythritol phosphate pathway (MEP) with our modular metabolic engineering system. With MEP pathway enhancement, it was found that pyruvate supplementation of rich media and simultaneous overexpression of three genes (idi, dxs, and dxr) resulted in the greatest increase in diterpene yield, indicating distributed metabolic control within this pathway. Incorporation of a heterologous MEV pathway in bioreactor grown cultures resulted in significantly higher yields than MEP pathway enhancement. We have established suitable growth conditions for diterpene production levels ranging from 10 to >100 mg/L of E. coli culture. These amounts are sufficient for nuclear magnetic resonance analyses, enabling characterization of enzymatic products and hence, pathway elucidation. Furthermore, these results represent an up to >1,000-fold improvement in diterpene production from our facile, modular platform, with MEP pathway enhancement offering a cost effective alternative with reasonable yield. Finally, we reiterate here that this modular approach is expandable and should be easily adaptable to the production of any terpenoid natural product.
Electronic supplementary material
The online version of this article (doi:10.1007/s00253-009-2219-x) contains supplementary material, which is available to authorized users.
Terpenoid; Natural products biosynthesis; Metabolic engineering; Isoprenoid
Isoprenoid compounds are ubiquitous in nature, participating in important biological phenomena such as signal transduction, aerobic cellular respiration, photosynthesis, insect communication, and many others. They are derived from the 5-carbon isoprenoid substrates isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). In Archaea and Eukarya, these building blocks are synthesized via the mevalonate pathway. However, the genes required to convert mevalonate phosphate (MP) to IPP are missing in several species of Archaea. An enzyme with isopentenyl phosphate kinase (IPK) activity was recently discovered in Methanocaldococcus jannaschii (MJ), suggesting a departure from the classical sequence of converting MP to IPP. We have determined the high-resolution crystal structures of isopentenyl phosphate kinases in complex with both substrates and products from Thermoplasma acidophilum (THA), as well as the IPK from Methanothermobacter thermautotrophicus (MTH), by means of single-wavelength anomalous diffraction (SAD) and molecular replacement. A histidine residue (His50) in THA IPK makes a hydrogen bond with the terminal phosphates of IP and IPP, poising these molecules for phosphoryl transfer through an in-line geometry. Moreover, a lysine residue (Lys14) makes hydrogen bonds with non-bridging oxygen atoms at Pα and Pγ and with the Pβ- Pγ bridging oxygen atom in ATP. These interactions suggest a transition state-stabilizing role for this residue. Lys14 is a part of a newly discovered “lysine triangle” catalytic motif in IPK’s that also includes Lys5 and Lys205. Moreover, His50, Lys5, Lys14, and Lys205 are conserved in all IPK’s and can therefore serve as fingerprints for identifying new homologues.