The in vitro Conversion of Chorismate to Isochorismate Catalyzed by the Escherichia coli entC Gene Product

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.

Total (bio)synthesis: strategies of nature and of chemists

The biosynthetic pathways to a number of natural products have been reconstituted in vitro using purified enzymes. Many of these molecules have also been synthesized by organic chemists. Here we compare the strategies used by nature and by chemists to reveal the underlying logic and success of each total synthetic approach for some exemplary molecules with diverse biosynthetic origins.

DegS is necessary for virulence and is among extraintestinal Escherichia coli genes induced in murine peritonitis

Extraintestinal Escherichia coli strains cause meningitis, sepsis, urinary tract infection, and other infections outside the bowel. We examined here extraintestinal E. coli strain CFT073 by differential fluorescence induction. Pools of CFT073 clones carrying a CFT073 genomic fragment library in a promoterless gfp vector were inoculated intraperitoneally into mice; bacteria were recovered by lavage 6 h later and then subjected to fluorescence-activated cell sorting. Eleven promoters were found to be active in the mouse but not in Luria-Bertani (LB) broth culture. Three are linked to genes for enterobactin, aerobactin, and yersiniabactin. Three others are linked to the metabolic genes metA, gltB, and sucA, and another was linked to iha, a possible adhesin. Three lie before open reading frames of unknown function. One promoter is associated with degS, an inner membrane protease. Mutants of the in vivo-induced loci were tested in competition with the wild type in mouse peritonitis. Of the mutants tested, only CFT073 degS was found to be attenuated in peritoneal and in urinary tract infection, with virulence restored by complementation. CFT073 degS shows growth similar to that of the wild type at 37 degrees C but is impaired at 43 degrees C or in 3% ethanol LB broth at 37 degrees C. Compared to the wild type, the mutant shows similar serum survival, motility, hemolysis, erythrocyte agglutination, and tolerance to oxidative stress. It also has the same lipopolysaccharide appearance on a silver-stained gel. The basis for the virulence attenuation is unclear, but because DegS is needed for sigma(E) activity, our findings implicate sigma(E) and its regulon in E. coli extraintestinal pathogenesis.

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.

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

The first committed step in the biosynthesis of menaquinone (vitamin K2) is the conversion of chorismate to isochorismate, which is mediated by an isochorismate synthase encoded by the menF gene. This isochorismate synthase (MenF) is distinct from the entC-encoded isochorismate synthase (EntC) involved in enterobactin biosynthesis. MenF has been overexpressed under the influence of the T7 promoter and purified to homogeneity. The purified protein was found to have a molecular mass of 98 kDa as determined by gel filtration column chromatography on Sephacryl S-200. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a molecular mass of 48 kDa. Thus, the enzyme is a homodimer. The purified enzyme showed a pH optimum of 7.5 to 8.0 and a temperature optimum of 37 degrees C. The enzyme carries out the irreversible conversion of chorismate to isochorismate in the presence of Mg2+. The enzyme was found to have a Km of 195 +/- 23 microM and a k(cat) of 80 min(-1). In the presence of 30 mM beta-mercaptoethanol (BME), the k(cat) increased to 176 min(-1). The reducing agents BME and dithiothreitol stimulated the enzymatic activity more than twofold. Treatment of the enzyme with the cysteine-specific modifying reagent N-ethylmaleimide (NEM) resulted in the complete loss of activity. Preincubation of the enzyme with the substrate, chorismate, before NEM treatment resulted in complete protection of the enzyme from inactivation.

Anaerobic biosynthesis of enterobactin Escherichia coli: regulation of entC gene expression and evidence against its involvement in menaquinone (vitamin K2) biosynthesis

In Escherichia coli, isochorismate is a common precursor for the biosynthesis of the siderophore enterobactin and menaquinone (vitamin K2). Isochorismate is formed by the shikimate pathway from chorismate by the enzyme isochorismate synthase encoded by the entC gene. Since enterobactin is involved in the aerobic assimilation of iron, and menaquinone is involved in anaerobic electron transport, we investigated the regulation of entC by iron and oxygen. An operon fusion between entC with its associated regulatory region and lacZ+ was constructed and introduced into the chromosome in a single copy. Expression of entC-lacZ was found to be regulated by the concentration of iron both aerobically and anaerobically. An established entC::kan mutant deficient in enterobactin biosynthesis was found to grow normally and synthesize wild-type levels of menaquinone under anaerobic conditions in iron-sufficient media. These results led to the demonstration of an alternate isochorismate synthase specifically involved in menaquinone synthesis encoded by the menF gene. Consistent with these findings, the entC+ strains were found to synthesize enterobactin anaerobically under iron-deficient conditions while the ent mutants failed to do so.

An isochorismate hydroxymutase isogene in Escherichia coli

The pivotal step in enterobactin and menaquinone biosynthesis is the conversion of chorismate to isochorismate. Circumstantial evidence pointed to Escherichia coli isochorismate hydroxymutase isogenes being responsible for this conversion. While the gene involved in enterobactin synthesis (entC) was known, the corresponding gene for menaquinone biosynthesis (menF) was not but has now been identified and sequenced. The amino acid sequence of MenF is 23.5% identical and 57.8% similar to that of EntC.

Menaquinone (vitamin K2) biosynthesis: cloning, nucleotide sequence, and expression of the menC gene from Escherichia coli

The benzenoid aromatic compound o-succinylbenzoic acid is formed by dehydration of the prearomatic compound 2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylic acid by the enzyme o-succinylbenzoate synthase, encoded by the menC gene. A 1.3-kb PstI-PvuII fragment was found to complement the menC mutation. The complete nucleotide sequence of this fragment revealed a single open reading frame of 954 bp capable of encoding a 35-kDa protein. A consensus sequence for a ribosomal binding site but no promoter consensus sequences were found. However, the first base of the initiating codon of this open reading frame overlaps the upstream menB gene termination codon, suggesting an operon-like organization for these genes. Consistent with this suggestion, the menB promoter can initiate transcription of the menC gene.

Nucleotide sequence and genetic organization of the ferric enterobactin transport system: homology to other periplasmic binding protein-dependent systems in Escherichia coli

The nucleotide sequence of the Escherichia coli fep genomic region has been determined. Three new loci were identified. One of these, P43, encodes a membrane protein that is not essential for ferric enterobactin transport. Two others, fepD and fepG, were found to be essential for transport and their translational products showed extensive homology to other integral membrane proteins involved in TonB-dependent transport processes. The FepC amino acid sequence suggested a peripheral membrane location and revealed conserved ATP-binding domains. Together these data indicate that ferric enterobactin is transported through a typical periplasmic binding protein-dependent system. In addition, the transcriptional organization of these genes was examined and primer extension analysis identified a single iron-regulated bidirectional promoter between the P43 gene and the fepDGC operon.

Overexpression, purification, and characterization of isochorismate synthase (EntC), the first enzyme involved in the biosynthesis of enterobactin from chorismate

Isochorismate synthase (EC 5.4.99.6), the entC gene product of Escherichia coli, catalyzes the conversion of chorismate to isochorismate, the first step in the biosynthesis of the powerful iron-chelating agent enterobactin. A sequence-specific deletion method has been used to construct an EntC overproducer, which allows for the purification and characterization of the E. coli isochorismate synthase for the first time. The N-terminal sequence and the subunit molecular weight (43,000) of the polypeptide derived from SDS-polyacrylamide gel electrophoresis agree with those deduced from DNA sequence data. The enzyme is an active monomer with a native molecular weight of 42,000. It was shown that EntC alone is fully capable of catalyzing the interconversion of chorismate and isochorismate in both directions and the associated activity is not affected by EntA of the same biosynthetic pathway as has recently been speculated [Elkins, M. F., & Earhart, C. F. (1988) FEMS Microbiol. Lett. 56, 35; Liu, J., Duncan, K., & Walsh, C.T. (1989) J. Bacteriol. 171, 791; Ozenberger, B. A., Brickman, T.J., & McIntosh, M. A. (1989) J. Bacteriol. 171, 775]. The kinetic constants were determined with Km = 14 microM and kcat = 173 min-1 for chorismate in the forward direction and Km = 5 microM and kcat = 108 min-1 for isochorismate in the backward direction. The equilibrium constant for the reaction derived from the kinetic data is 0.56 with the equilibrium lying toward the side of chorismate, corresponding to a free energy difference of 0.36 kcal/mol between chorismate and isochorismate.(ABSTRACT TRUNCATED AT 250 WORDS)