Menaquinone (vitamin K2) biosynthesis: overexpression, purification, and characterization of a new isochorismate synthase from Escherichia coli

Enzymatic reconstitution of salicylate formation in promysalin biosynthesis

Promysalin is an amphipathic antibiotic isolated from Pseudomonas promysalinigenes (previously Pseudomonas putida RW10S1) which shows potent antibacterial activities against Gram-negative pathogens by inactivating succinate dehydrogenase. Based on the in-vivo studies, promysalin is hypothesized to be assembled from three building blocks: salicylic acid, proline, and myristic acid via a proposed but uncharacterized hybrid NRPS-PKS biosynthetic pathway. So far, no in-vitro biosynthetic studies have been reported for this promising antibiotic. Here, we report the first in-vitro reconstitution and biochemical characterization of two early enzymes on the pathway: PpgH, an isochorismate synthase (IS), and PpgG, an isochorismate pyruvate lyase (IPL) which are involved in the biosynthesis of salicylic acid, the polar fragment of promysalin. We also report a secondary chorismate mutase (CM) activity for PpgG. Based on our biochemical experiments, preliminary mechanistic proposals have been postulated for PpgH and PpgG. We believe this study will lay a strong foundation for elucidating the functions and mechanisms of other intriguing enzymes of the promysalin biosynthesis pathway, which may potentially unravel interesting enzyme chemistries and promote pathway engineering in the future.

Multifactorial Microvariability of the Italian Raw Milk Cheese Microbiota and Implication for Current Regulatory Scheme

Raw milk cheese manufactory is strictly regulated in Europe by the Protected Designation of Origin (PDO) quality scheme, which protects indigenous food products based on geographical and biotechnological features. This study encompassed the collection of 128 raw milk cheese samples across Italy to investigate the resident microbiome correlated to current PDO specifications. Shotgun metagenomic approaches highlighted how the microbial communities are primarily linked to each cheesemaking site and consequently to the use of site-specific Natural Whey Cultures (NWCs), defined by a multifactorial set of local environmental factors rather than solely by cheese type or geographical origin that guide the current PDO specification. Moreover, in-depth functional characterization of Cheese Community State Types (CCSTs) and comparative genomics efforts, including metagenomically assembled genomes (MAGs) of the dominant microbial taxa, revealed NWCs-related unique enzymatic profiles impacting the organoleptic features of the produced cheeses and availability of bioactive compounds to consumers, with putative health implications. Thus, these results highlighted the need for a profound rethinking of the current PDO designation with a focus on the production site-specific microbial metabolism to understand and guarantee the organoleptic features of the final product recognized as PDO. IMPORTANCE The Protected Designation of Origin (PDO) guarantees the traceability of food production processes, and that the production takes place in a well-defined restricted geographical area. Nevertheless, the organoleptic qualities of the same dairy products, i.e., cheeses under the same PDO denomination, differ between manufacturers. The final product's flavor and qualitative aspects can be related to the resident microbial population, not considered by the PDO denomination. Here, we analyzed a complete set of different Italian cheeses produced from raw milk through shotgun sequencing in order to study the variability of the different microbial profiles resident in Italian PDO cheeses. Furthermore, an in-depth functional analysis, along with a comparative genomic analysis, was performed in order to correlate the taxonomic information with the organoleptic properties of the final product. This analysis made it possible to highlight how the PDO denomination should be revisited to understand the effect that Natural Whey Cultures (NWCs), used in the traditional production of raw milk cheese and unique to each manufacturer, impacts on the organoleptic features of the final product.

The human gut microbiome of athletes: metagenomic and metabolic insights

BACKGROUND

The correlation between the physical performance of athletes and their gut microbiota has become of growing interest in the past years, since new evidences have emerged regarding the importance of the gut microbiota as a main driver of the health status of athletes. In addition, it has been postulated that the metabolic activity of the microbial population harbored by the large intestine of athletes might influence their physical performances. Here, we analyzed 418 publicly available shotgun metagenomics datasets obtained from fecal samples of healthy athletes and healthy sedentary adults.

RESULTS

This study evidenced how agonistic physical activity and related lifestyle can be associated with the modulation of the gut microbiota composition, inducing modifications of the taxonomic profiles with an enhancement of gut microbes able to produce short-fatty acid (SCFAs). In addition, our analyses revealed a correlation between specific bacterial species and high impact biological synthases (HIBSs) responsible for the generation of a range of microbially driven compounds such vitamin B12, amino acidic derivatives, and other molecules linked to cardiovascular and age-related health-risk reduction.

CONCLUSIONS

Notably, our findings show how subsist an association between competitive athletes, and modulation of the gut microbiota, and how this modulation is reflected in the potential production of microbial metabolites that can lead to beneficial effects on human physical performance and health conditions. Video Abstract.

Mycobacterial MenG: Partial Purification, Characterization, and Inhibition

Menaquinone (MK) is an essential component of the electron transport chain (ETC) in the gram-variable Mycobacterium tuberculosis and many Gram-positive pathogens. Three genes in the M. tuberculosis genome were annotated as methyltransferases involved in lipoquinone synthesis in mycobacteria. Heterologous expression of Rv0558 complemented an ubiE (the quinone C-methyltransferase involved in ubiquinone and menaquinone synthesis) deletion in Escherichia coli, and expression in a wild-type E. coli strain increased quinone C-methyltransferase specific activity by threefold. Rv0558 encodes a canonical C-methyltransferase or, more specifically, a S-adenosylmethionine/demethylmenaquinol methyltransferase. Partially purified recombinant protein catalyzed the formation of MK from demethylmenaquinone (DMK), although the activity of the recombinant protein was low and appeared to require a cofactor or intact membrane structure for activity. Membrane preparations from irradiated M. tuberculosis also showed poor activity; however, membrane preparations from wild-type Mycobacterium smegmatis showed robust, substrate-dependent activity. The apparent K_m values for demethylmenaquinone and SAM were 14 ± 5.0 and 17 ± 7.0 μM, respectively. Interestingly, addition of dithiothreitol, dithionite, NADH, or other substrates of primary dehydrogenases to reaction mixtures containing membrane preparations stimulated the activity. Thus, these observations strongly suggest that demethylmenaquinol is the actual substrate of MenG. Ro 48-8071, previously reported to inhibit mycobacterial MK synthesis and growth, inhibited Rv0558 activity with an IC₅₀ value of 5.1 ± 0.5 μM, and DG70 (GSK1733953A), first described as a respiration inhibitor in M. tuberculosis, inhibits MenG activity with an IC₅₀ value of 2.6 ± 0.6 μM.

Cry3Aa*SpyCatcher Fusion Crystals Produced in Bacteria as Scaffolds for Multienzyme Coimmobilization

The production of Cry3Aa enzyme fusion crystals in Bacillus thuringiensis provides a direct method to immobilize individual enzymes and thereby improve their stability and recyclability. Nevertheless, many reactions require multiple enzymes to produce a desired product; thus a general strategy was developed to extend our Cry3Aa technology to multienzyme coimmobilization. Here, we report the direct production of particles comprising a modified Cry3Aa (Cry3Aa*) fused to SpyCatcher002 (Cry3Aa*SpyCat2) for coimmobilization of model enzymes MenF, MenD, and MenH associated with the biosynthesis of menaquinone. The resultant coimmobilized particles showed improved reaction rates compared to free enzymes presumably due to the higher local enzyme substrate concentrations and enhanced enzyme coupling made possible by colocalization. Furthermore, coimmobilization of these enzymes on Cry3Aa*SpyCat2 led to increased thermal stability and recyclability of the overall multienzyme system. These characteristics together with its overall simplicity of production highlight the benefits of Cry3Aa*SpyCat2 crystals as a platform for enzyme coimmobilization.

Phase-Separated Multienzyme Biosynthesis

Liquid-liquid phase separation forms condensates that feature a highly concentrated liquid phase, a defined yet dynamic boundary, and dynamic exchange at and across the boundary. Phase transition drives the formation of dynamic multienzyme complexes in cells, for example, the purinosome, which forms subcellular macrobodies responsible for de novo purine biosynthesis. Here, we construct synthetic versions of multienzyme biosynthetic systems by assembling enzymes in protein condensates. A synthetic protein phase separation system using component proteins from postsynaptic density in neuronal synapses, GKAP, Shank, and Homer provides the scaffold for assembly. Three sets of guest proteins: a pair of fluorescent proteins (CFP and YFP), three sequential enzymes in menaquinone biosynthesis pathway (MenF, MenD, and MenH), and two enzymes in terpene biosynthesis pathway (Idi and IspA) are assembled via peptide-peptide interactions in the condensate. First, we discover that coassembly of CFP and YFP exhibited a broad distribution of the FRET signal within the condensate. Second, a spontaneous enrichment of the rate-limiting enzyme MenD in the condensate is sufficient to increase the 2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate production rate by 70%. Third, coassembly of both Idi and IspA in the protein condensate increases the farnesyl pyrophosphate production rate by more than 50%. Altogether, we show here that phase separation significantly accelerates the efficiency of multienzyme biocatalysis.

Transcriptome analysis of the fish pathogen Flavobacterium columnare in biofilm suggests calcium role in pathogenesis

BACKGROUND

Flavobacterium columnare is the causative agent of columnaris disease that affects cultured freshwater fishes worldwide. F. columnare easily colonizes surfaces by forming biofilm, which helps the pathogen resist antibiotic and disinfectant treatments. Previously, we had shown that increasing concentrations of calcium (Ca²⁺) promoted biofilm formation by F. columnare. The objective of this study was to further characterize the role of Ca²⁺ on biofilm formation and to compare the transcriptome profiles of planktonic and biofilm cells.

RESULTS

RNA-Seq analysis was conducted to identify genes that were differentially expressed between the following states: i) planktonic cells in control medium (P), ii) planktonic cells in calcium-enriched medium (P/Ca), and iii) biofilm cells in calcium-enriched medium (B/Ca). Overall, we identified 441 significant (FDR-adjusted p < 0.05, fold change > 2) differentially expressed genes (DEGs) between P and B/Ca samples; 112 significant DEGs between P/Ca and B/Ca samples, and 175 significant DEGs between P/Ca and P samples, corresponding to 15.87, 4.03 and 6.30% of the total protein-coding sequences, respectively. The significant DEGs fell into different functional categories including iron acquisition, oxidative stress response, extracellular protein secretion, and respiratory metabolism.

CONCLUSIONS

Our results posit Ca²⁺ as a critical signal in regulating bacterial surface adhesion and biofilm formation in F. columnare. Living in biofilm elicited a shift in several metabolic pathways that allowed the cells to cope with oxidative stress and nutrient starvation. In addition, Ca²⁺ supplementation induced the expression of putative virulence factors in F. columnare, such as extracellular protein secretion and iron acquisition.

Unraveling the Structure and Mechanism of the MST(ery) Enzymes

The menaquinone, siderophore, and tryptophan (MST) enzymes transform chorismate to generate precursor molecules for the biosynthetic pathways defined in their name. Kinetic data, both steady-state and transient-state, and X-ray crystal structures indicate that these enzymes are highly conserved both in mechanism and in structure. Because these enzymes are found in pathogens but not in humans, there is considerable interest in these enzymes as drug design targets. While great progress has been made in defining enzyme structure and mechanism, inhibitor design has lagged behind. This review provides a detailed description of the evidence that begins to unravel the mystery of how the MST enzymes work, and how that information has been used in inhibitor design.

An Open and Shut Case: The Interaction of Magnesium with MST Enzymes

The shikimate pathway of bacteria, fungi, and plants generates chorismate, which is drawn into biosynthetic pathways that form aromatic amino acids and other important metabolites, including folates, menaquinone, and siderophores. Many of the pathways initiated at this branch point transform chorismate using an MST enzyme. The MST enzymes (menaquinone, siderophore, and tryptophan biosynthetic enzymes) are structurally homologous and magnesium-dependent, and all perform similar chemical permutations to chorismate by nucleophilic addition (hydroxyl or amine) at the 2-position of the ring, inducing displacement of the 4-hydroxyl. The isomerase enzymes release isochorismate or aminodeoxychorismate as the product, while the synthase enzymes also have lyase activity that displaces pyruvate to form either salicylate or anthranilate. This has led to the hypothesis that the isomerase and lyase activities performed by the MST enzymes are functionally conserved. Here we have developed tailored pre-steady-state approaches to establish the kinetic mechanisms of the isochorismate and salicylate synthase enzymes of siderophore biosynthesis. Our data are centered on the role of magnesium ions, which inhibit the isochorismate synthase enzymes but not the salicylate synthase enzymes. Prior structural data have suggested that binding of the metal ion occludes access or egress of substrates. Our kinetic data indicate that for the production of isochorismate, a high magnesium ion concentration suppresses the rate of release of product, accounting for the observed inhibition and establishing the basis of the ordered-addition kinetic mechanism. Moreover, we show that isochorismate is channeled through the synthase reaction as an intermediate that is retained in the active site by the magnesium ion. Indeed, the lyase-active enzyme has 3 orders of magnitude higher affinity for the isochorismate complex relative to the chorismate complex. Apparent negative-feedback inhibition by ferrous ions is documented at nanomolar concentrations, which is a potentially physiologically relevant mode of regulation for siderophore biosynthesis in vivo.

Metabolic design of a platform Escherichia coli strain producing various chorismate derivatives

A synthetic metabolic pathway suitable for the production of chorismate derivatives was designed in Escherichia coli. An L-phenylalanine-overproducing E. coli strain was engineered to enhance the availability of phosphoenolpyruvate (PEP), which is a key precursor in the biosynthesis of aromatic compounds in microbes. Two major reactions converting PEP to pyruvate were inactivated. Using this modified E.coli as a base strain, we tested our system by carrying out the production of salicylate, a high-demand aromatic chemical. The titer of salicylate reached 11.5 g/L in batch culture after 48 h cultivation in a 2-liter jar fermentor, and the yield from glucose as the sole carbon source exceeded 40% (mol/mol). In this test case, we found that pyruvate was synthesized primarily via salicylate formation and the reaction converting oxaloacetate to pyruvate. In order to demonstrate the generality of our designed strain, we employed this platform for the production of each of 7 different chorismate derivatives. Each of these industrially important chemicals was successfully produced to levels of 1-3g/L in test tube-scale culture.

Biosynthesis of Menaquinone (Vitamin K2) and Ubiquinone (Coenzyme Q)

Escherichia coli and Salmonella contain the naphthoquinones menaquinone (MK; vitamin K2) and demethylmenaquinone and the benzoquinone ubiquinone (coenzyme Q; Q). Both quinones are derived from the shikimate pathway, which has been called a "metabolic tree with many branches." There are two different pathways for the biosynthesis of the naphthoquinones. The vast majority of prokaryotes, including E. coli and Salmonella, and the plants use the o-succinylbenzoate pathway, while a minority uses the futalosine pathway. The quinone nucleus of Q is derived directly from chorismate, while that of MK is derived from chorismate via isochorismate. The prenyl side chains of both quinones are from isopentenyl diphosphate formed by the 2-C-methyl-D-erythritol 4-phosphate (non-mevalonate) pathway and the methyl groups are from S-adenosylmethionine. In addition, MK biosynthesis requires 2-ketoglutarate and cofactors ATP, coenzyme A, and thiamine pyrophosphate. Despite the fact that both quinones originate from the shikimate pathway, there are important differences in their biosyntheses. The prenyl side chain in MK biosynthesis is introduced at the penultimate step, accompanied by decarboxylation, whereas in Q biosynthesis it is introduced at the second step, with retention of the carboxyl group. In MK biosynthesis, all the reactions of the pathway up to prenylation are carried out by soluble enzymes, whereas all the enzymes involved in Q biosynthesis except the first are membrane bound. In MK biosynthesis, the last step is a C-methylation; in Q biosynthesis, the last step is an O-methylation. In Q biosynthesis a second C-methylation and O-methylation take place in the middle part of the pathway. Despite the fact that Q and MK biosyntheses diverge at chorismate, the C-methylations in both pathways are carried out by the same methyltransferase.

Cascade biocatalysis by multienzyme-nanoparticle assemblies

Multienzyme complexes are of paramount importance in biosynthesis in cells. Yet, how sequential enzymes of cascade catalytic reactions synergize their activities through spatial organization remains elusive. Recent development of site-specific protein-nanoparticle conjugation techniques enables us to construct multienzyme assemblies using nanoparticles as the template. Sequential enzymes in menaquinone biosynthetic pathway were conjugated to CdSe-ZnS quantum dots (QDs, a nanosized particulate material) through metal-affinity driven self-assembly. The assemblies were characterized by electrophoretic methods, the catalytic activities were monitored by reverse-phase chromatography, and the composition of the multienzyme-QD assemblies was optimized through a progressive approach to achieve highly efficient catalytic conversion. Shorter enzyme-enzyme distance was discovered to facilitate intermediate transfer, and a fine control on the stoichiometric ratio of the assembly was found to be critical for the maximal synergy between the enzymes. Multienzyme-QD assemblies thereby provide an effective model to scrutinize the synergy of cascade enzymes in multienzyme complexes.

Expression and characterization of PhzE from P. aeruginosa PAO1: aminodeoxyisochorismate synthase involved in pyocyanin and phenazine-1-carboxylate production

PhzE from Pseudomonas aeruginosa catalyzes the first step in the biosynthesis of phenazine-1-carboxylic acid, pyocyanin, and other phenazines, which are virulence factors for Pseudomonas species. The reaction catalyzed converts chorismate into aminodeoxyisochorismate using ammonia supplied by a glutamine amidotransferase domain. It has structural and sequence homology to other chorismate-utilizing enzymes such as anthranilate synthase, isochorismate synthase, aminodeoxychorismate synthase, and salicylate synthase. Like these enzymes, it is Mg(2+) dependent and catalyzes a similar S(N)2" nucleophilic substitution reaction. PhzE catalyzes the addition of ammonia to C2 of chorismate, as does anthranilate synthase, yet unlike anthranilate synthase it does not catalyze elimination of pyruvate from enzyme-bound aminodeoxyisochorismate. Herein, the cloning of the phzE gene, high level expression of active enzyme in E. coli, purification, and kinetic characterization of the enzyme is presented, including temperature and pH dependence. Steady-state kinetics give K(chorismate)=20±4μM, K(Mg)(2+)=294±22μM, K(L-gln)=11±1mM, and k(cat)=2.2±0.2s(-1) for a random kinetic mechanism. PhzE can use NH(4)(+) as an alternative nucleophile, while Co(2+) and Mn(2+) are alternative divalent metals.

Pericyclic reactions catalyzed by chorismate-utilizing enzymes

One of the fundamental questions of enzymology is how catalytic power is derived. This review focuses on recent developments in the structure--function relationships of chorismate-utilizing enzymes involved in siderophore biosynthesis to provide insight into the biocatalysis of pericyclic reactions. Specifically, salicylate synthesis by the two-enzyme pathway in Pseudomonas aeruginosa is examined. The isochorismate-pyruvate lyase is discussed in the context of its homologues, the chorismate mutases, and the isochorismate synthase is compared to its homologues in the MST family (menaquinone, siderophore, or tryptophan biosynthesis) of enzymes. The tentative conclusion is that the activities observed cannot be reconciled by inspection of the active site participants alone. Instead, individual activities must arise from unique dynamic properties of each enzyme that are tuned to promote specific chemistries.

Inhibition of chorismate-utilising enzymes by 2-amino-4-carboxypyridine and 4-carboxypyridone and 5-carboxypyridone analogues

Several 2-amino-4-carboxypyridine, 4- and 5-carboxypyridone-based compounds were prepared and tested against three members of the chorismate-utilising enzyme family, anthranilate synthase, isochorismate synthase and salicylate synthase. Most compounds exhibited low micromolar inhibition of these three enzymes. The most potent inhibitor was a 4-carboxypyridone analogue bearing a lactate side chain on the pyridyl nitrogen which exhibited inhibition constants of 5, 91 and 54 muM against anthranilate synthase, isochorismate synthase and salicylate synthase respectively.

Structure of isochorismate synthase in complex with magnesium

The structure of the menaquinone-specific isochorismate synthase (MenF) from Escherichia coli has been refined at a resolution of 2.0 Å in complex with magnesium. The magnesium-bound structure has a well defined and organized active site which better represents the active conformation of the enzyme than the currently available structure.

The electron carrier menaquinone is one of many important bacterial metabolites that are derived from the key intermediate chorismic acid. MenF, the first enzyme in the menaquinone pathway, catalyzes the isomerization of chorismate to isochorismate. Here, an improved structure of MenF in a new crystal form is presented. The structure, solved at 2.0 Å resolution in complex with magnesium, reveals a well defined closed active site. Existing evidence suggests that the mechanism of the reaction catalyzed by MenF involves nucleophilic attack of a water molecule on the chorismate ring. The structure reveals a well defined water molecule located in an appropriate position for activation by Lys190 and attack on the substrate.

Determination of the stereochemistry of 2-succinyl-5-enolpyruvyl-6-hydroxy-3- cyclohexene-1-carboxylate, a key intermediate in menaquinone biosynthesis

The turnover product of the committed step of menaquinone biosynthesis was isolated and determined to be (1R,2S,5S,6S)-2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexene-1-carboxylate. Structural determination of this key intermediate represents a critical step to complete elucidation of the biosynthetic pathway.

Menaquinone biosynthesis in Escherichia coli: identification of 2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexene-1-carboxylate as a novel intermediate and re-evaluation of MenD activity

Menaquinone is an electron carrier in the respiratory chain of Escherichia coli during anaerobic growth. Its biosynthesis involves (1R,6R)-2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylic acid (SHCHC) as an intermediate, which is believed to be derived from isochorismate and 2-ketoglutarate by one of the biosynthetic enzymes-MenD. However, we found that the genuine MenD product is 2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexene-1-carboxylic acid (SEPHCHC), rather than SHCHC. This is supported by the following findings: (i) isochorismate consumption and SHCHC formation are not synchronized in the enzymic reaction, (ii) the rate of SHCHC formation is independent of the enzyme concentration, (iii) SHCHC is not formed in weakly acidic or neutral solutions in which the isochorismate substrate is readily consumed by MenD, and (iv) the MenD turnover product, formed under conditions disabling SHCHC formation, possesses spectroscopic characteristics consistent with the structure of SEPHCHC and spontaneously undergoes 2,5-elimination to form SHCHC and pyruvate in weakly basic solutions. Two properties of the intermediate, ultraviolet transparency and chemical instability, provide a rationale for the fact that SHCHC has been consistently mistaken as the MenD product. In accordance with these findings, MenD was rediscovered to be a highly efficient enzyme with a high second-order rate constant and should be renamed SEPHCHC synthase. Intriguingly, the enzymatic activity responsible for conversion of SEPHCHC into SHCHC appears not to associate with any of the known enzymes in menaquinone biosynthesis but is present in the crude extract of E. coli K12, suggesting that a genuine SHCHC synthase remains to be identified to fully elucidate the ubiquitous biosynthetic pathway.

Lysine 190 is the catalytic base in MenF, the menaquinone-specific isochorismate synthase from Escherichia coli: implications for an enzyme family

Menaquinone biosynthesis is initiated by the conversion of chorismate to isochorismate, a reaction that is catalyzed by the menaquinone-specific isochorismate synthase, MenF. The catalytic mechanism of MenF has been probed using a combination of structural and biochemical studies, including the 2.5 A structure of the enzyme, and Lys190 has been identified as the base that activates water for nucleophilic attack at the chorismate C2 carbon. MenF is a member of a larger family of Mg2+ dependent chorismate binding enzymes catalyzing distinct chorismate transformations. The studies reported here extend the mechanism recently proposed for this enzyme family by He et al.: He, Z., Stigers Lavoie, K. D., Bartlett, P. A., and Toney, M. D. (2004) J. Am. Chem. Soc. 126, 2378-85.

Arabidopsis Isochorismate Synthase Functional in Pathogen-induced Salicylate Biosynthesis Exhibits Properties Consistent with a Role in Diverse Stress Responses

Salicylic acid (SA) is a phytohormone best known for its role in plant defense. It is synthesized in response to diverse pathogens and responsible for the large scale transcriptional induction of defense-related genes and the establishment of systemic acquired resistance. Surprisingly, given its importance in plant defense, an understanding of the underlying enzymology is lacking. In Arabidopsis thaliana, the pathogen-induced accumulation of SA requires isochorismate synthase (AtICS1). Here, we show that AtICS1 is a plastid-localized, stromal protein using chloroplast import assays and immunolocalization. AtICS1 acts as a monofunctional isochorismate synthase (ICS), catalyzing the conversion of chorismate to isochorismate (IC) in a reaction that operates near equilibrium (K(eq) = 0.89). It does not convert chorismate directly to SA (via an IC intermediate) as does Yersinia enterocolitica Irp9. Using an irreversible coupled spectrophotometric assay, we found that AtICS1 exhibits an apparent K(m) of 41.5 mum and k(cat) = 38.7 min(-1) for chorismate. This affinity for chorismate would allow it to successfully compete with other pathogen-induced, chorismate-utilizing enzymes. Furthermore, the biochemical properties of AtICS1 indicate its activity is not regulated by light-dependent changes in stromal pH, Mg(2+), or redox and that it is remarkably active at 4 degrees C consistent with a role for SA in cold-tolerant growth. Finally, our analyses support plastidic synthesis of stress-induced SA with the requirement for one or more additional enzymes responsible for the conversion of IC to SA, because non-enzymatic conversion of IC to SA under physiological conditions was negligible.

Isochorismate synthase (PchA), the first and rate-limiting enzyme in salicylate biosynthesis of Pseudomonas aeruginosa

In Pseudomonas aeruginosa the extracellular metabolite and siderophore pyochelin is synthesized from two major precursors, chorismate and l-cysteine via salicylate as an intermediate. The regulatory role of isochorismate synthase, the first enzyme in the pyochelin biosynthetic pathway, was studied. This enzyme is encoded by pchA, the last gene in the pchDCBA operon. The PchA protein was purified to apparent electrophoretic homogeneity from a PchA-overexpressing P. aeruginosa strain. The native enzyme was a 52-kDa monomer in solution, and its activity strictly depended on Mg(2+). At pH 7.0, the optimum, a K(m) = 4.5 microm and a k(cat) = 43.1 min(-1) were determined for chorismate. No feedback inhibitors or other allosteric effectors were found. The intracellular PchA concentration critically determined the rate of salicylate formation both in vitro and in vivo. In cultures grown in iron-limiting media to high cell densities, overexpression of the pchA gene resulted in overproduction of salicylate as well as in enhanced pyochelin formation. From this work and earlier studies, it is proposed that one important factor influencing the flux through the pyochelin biosynthetic pathway is the PchA concentration, which is determined at a transcriptional level, with pyochelin acting as a positive signal and iron as a negative signal.

Salicylate biosynthesis in Pseudomonas aeruginosa. Purification and characterization of PchB, a novel bifunctional enzyme displaying isochorismate pyruvate-lyase and chorismate mutase activities

Isochorismate pyruvate-lyase (IPL), the second enzyme of pyochelin biosynthesis and the product of the pchB gene, was purified to homogeneity from Pseudomonas aeruginosa. In the reaction catalyzed by this enzyme, isochorismate --> salicylate + pyruvate, no cofactors appear to be required. At the pH optimum (pH 6.8), the enzyme displayed Michaelis-Menten kinetics, with an apparent K(m) of 12.5 microm for isochorismate and a kcat of 106 min(-1), calculated per monomer. The native enzyme behaved as a homodimer, as judged by molecular sieving chromatography, electrophoresis under nondenaturing conditions, and cross-linking experiments. PchB has approximately 20% amino acid sequence identity with AroQ-class chorismate mutases (CMs). Chorismate was shown to be converted to prephenate by purified PchB in vitro, with an apparent K(m) of 150 microm and a kcat of 7.8 min(-1). An oxabicyclic diacid transition state analog and well characterized inhibitor of CMs competitively inhibited both IPL and CM activities of PchB. Moreover, a CM-deficient Escherichia coli mutant, which is auxotrophic for phenylalanine and tyrosine, was functionally complemented by the cloned P. aeruginosa pchB gene for growth in minimal medium. A mutant form of PchB, in which isoleucine 88 was changed to threonine, had no detectable IPL activity, but retained wild-type CM activity. In conclusion, the 11.5-kDa subunit of PchB appears to contain a single active site involved in both IPL and CM activity.

Clustering of isochorismate synthase genes menF and entC and channeling of isochorismate in Escherichia coli

There are two isochorismate synthase genes entC and menF in Escherichia coli. They encode enzymes (isochorismate synthase, EC 5.4.99.6) which reversibly synthesize isochorismic acid from chorismic acid. The genes share a 24.2% identity but are differently regulated. Activity of the MenF isochorismate synthase is significantly increased under anaerobic conditions whereas the activity of the EntC isochorismate synthase is greatly stimulated during growth in an iron deficient medium. Isochorismic acid synthesized by EntC is mainly channeled into enterobactin synthesis whereas isochorismic acid synthesized by MenF is mainly channeled into menaquinone synthesis. When menF or entC were separately placed onto overexpression plasmids and the plasmids introduced into a menF(-)/entC(-) double mutant in two separate experiments, the isochorismate formed was fed into both, the menaquinone and the enterobactin pathway. Moreover, in spite of a high isochorismate synthase activity menaquinone and enterobactin formation were not fully restored, indicating that isochorismate was lost by diffusion. Thus, under these conditions channeling was not observed. We conclude that in E. coli the chromosomal position of both menF and entC in their respective clusters is a prerequisite for channeling of isochorismate in both pathways.

Biosynthesis of menaquinone (vitamin K2) and ubiquinone (coenzyme Q): a perspective on enzymatic mechanisms

The benzoquinone ubiquinone (coenzyme Q) and the naphthoquinones menaquinone (vitamin K2) and demethylmenaquinone are derived from the shikimate pathway, which has been described as a "metabolic tree with many branches." Menaquinone (MK) is considered a vitamin, but coenzyme (Q) is not; MK is an essential nutrient (it cannot be synthesized by mammals), whereas Q is not considered an essential nutrient since it can be synthesized from the amino acid tyrosine. The quinone nucleus of Q is derived directly from chorismate, whereas that of MK is derived from chorismate via isochorismate. The prenyl side chain of both quinones is derived from prenyl diphosphate, and the methyl groups are derived from S-adenosylmethionine. MK biosynthesis requires 2-ketoglutarate and the cofactors ATP, coenzyme A (CoASH), and thiamine pyrophosphate. In spite of the fact that both quinones originate from the shikimate pathway, there are important differences in their biosynthesis. In MK biosynthesis, the prenyl side chain is introduced in the next to last step, which is accompanied by loss of the carboxyl group, whereas in Q biosynthesis, the prenyl side chain is introduced at the second step, with retention of the carboxyl group. In MK biosynthesis, all the reactions of the pathway up to the prenylation (next to last step) are carried out by soluble enzymes, whereas all the enzymes involved in Q biosynthesis except the first are membrane bound. In MK biosynthesis the last step is a C-methylation; in Q biosynthesis, the last step is an O-methylation. In Q biosynthesis a second C-methylation and O-methylation take place in the middle part of the pathway. In spite of the fact that Q and MK biosynthesis diverges at chorismate, the C-methylations involved in both pathways are carried out by the same enzyme. Finally, Q biosynthesis under aerobic conditions requires molecular oxygen; anaerobic biosynthesis of Q and MK incorporates oxygen atoms derived from water. The current status of the pathways with particular emphasis on the reaction mechanisms, is discussed in this review.

Interim report on genomics of Escherichia coli

We present a summary of recent progress in understanding Escherichia coli K-12 gene and protein functions. New information has come both from classical biological experimentation and from using the analytical tools of functional genomics. The content of the E. coli genome can clearly be seen to contain elements acquired by horizontal transfer. Nevertheless, there is probably a large, stable core of >3500 genes that are shared among all E. coli strains. The gene-enzyme relationship is examined, and, in many cases, it exhibits complexity beyond a simple one-to-one relationship. Also, the E. coli genome can now be seen to contain many multiple enzymes that carry out the same or closely similar reactions. Some are similar in sequence and may share common ancestry; some are not. We discuss the concept of a minimal genome as being variable among organisms and obligatorily linked to their life styles and defined environmental conditions. We also address classification of functions of gene products and avenues of insight into the history of protein evolution.

Characterization of hydroxylaminobenzene mutase from pNBZ139 cloned from Pseudomonas pseudoalcaligenes JS45. A highly associated SDS-stable enzyme catalyzing an intramolecular transfer of hydroxy groups

Hydroxylaminobenzene mutase is the enzyme that converts intermediates formed during initial steps in the degradation of nitrobenzene to a novel ring-fission lower pathway in Pseudomonas pseudoalcaligenes JS45. The mutase catalyzes a rearrangement of hydroxylaminobenzene to 2-aminophenol. The mechanism of the reactions and the properties of the enzymes are unknown. In crude extracts, the hydroxylaminobenzene mutase was stable at SDS concentrations as high as 2%. A procedure including Hitrap-SP, Hitrap-Q and Cu(II)-chelating chromatography was used to partially purify the enzyme from an Escherichia coli clone. The partially purified enzyme was eluted in the void volume of a Superose-12 gel-filtration column even in the presence of 0.05% SDS in 25 mM Tris/HCl buffer, which indicated that it was highly associated. When the enzymatic conversion of hydroxylaminobenzene to 2-aminophenol was carried out in 18O-labeled water, the product did not contain 18O, as determined by GC-MS. The results indicate that the reaction proceeded by intramolecular transfer of the hydroxy group from the nitrogen to the C-2 position of the ring. The mechanism is clearly different from the intermolecular transfer of the hydroxy group in the non-enzymatic Bamberger rearrangement of hydroxylaminobenzene to 4-aminophenol and in the enzymatic hydroxymutation of chorismate to isochorismate.

Purification and cDNA cloning of isochorismate synthase from elicited cell cultures of Catharanthus roseus

Isochorismate is an important metabolite formed at the end of the shikimate pathway, which is involved in the synthesis of both primary and secondary metabolites. It is synthesized from chorismate in a reaction catalyzed by the enzyme isochorismate synthase (ICS; EC 5.4.99.6). We have purified ICS to homogeneity from elicited Catharanthus roseus cell cultures. Two isoforms with an apparent molecular mass of 64 kD were purified and characterized. The Km values for chorismate were 558 and 319 microM for isoforms I and II, respectively. The isoforms were not inhibited by aromatic amino acids and required Mg2+ for enzyme activity. Polymerase chain reaction on a cDNA library from elicited C. roseus cells with a degenerated primer based on the sequence of an internal peptide from isoform II resulted in an amplification product that was used to screen the cDNA library. This led to the first isolation, to our knowledge, of a plant ICS cDNA. The cDNA encodes a protein of 64 kD with an N-terminal chloroplast-targeting signal. The deduced amino acid sequence shares homology with bacterial ICS and also with anthranilate synthases from plants. Southern analysis indicates the existence of only one ICS gene in C. roseus.

The role of isochorismate hydroxymutase genes entC and menF in enterobactin and menaquinone biosynthesis in Escherichia coli

Klebsiella pneumoniae 62-1, a triple mutant impaired in aromatic amino acid biosynthesis (Phe-, Tyr-, Trp-), excretes chorismic acid into the culture broth. When transformed with plasmids harbouring Escherichia coli genes entC or menF the mutant excretes a mixture of both chorismic and isochorismic acid indicating that not only entC but also menF encodes an isochorismate hydroxymutase (isochorismate synthase, EC 5.4.99.6) enzyme. These enzymes catalyze the first step in enterobactin or menaquinone biosynthesis, respectively. Although both gene products (EntC and MenF) catalyze the same reaction, they play distinct roles in the biosynthesis of menaquinone (MK8) and enterobactin. An E. coli mutant (PBB7) with an intact menF but a disrupted entC produced menaquinone (MK8) but no enterobactin, whereas a mutant (PBB9) with an intact entC but a disrupted menF produced enterobactin and only a trace of menaquinone (MK8). When both menF and entC were disrupted (mutant PBB8) neither menaquinone (MK8) nor enterobactin was detectable. Our previous assumption that entC is responsible for both menaquinone and enterobactin biosynthesis is inconsistent with these mutant studies and has to be revised. The presence in the promoter region of menF of a putative cAMP receptor protein binding site indicates that menF is regulated differently from entC. The menF gene was overexpressed as a fusion gene and its product (6xHis-tagged MenF) isolated. The enzyme catalyzed the formation of isochorismic from chorismic acid and as opposed to a previous publication also the reverse reaction. The enzyme was characterized and its kinetic data determined.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.