Regulation of alternate peripheral pathways of glucose catabolism during aerobic and anaerobic growth of Pseudomonas aeruginosa.

Glucose may be converted to 6-phosphogluconate by alternate pathways in Pseudomonas aeruginosa. Glucose is phosphorylated to glucose-6-phosphate, which is oxidized to 6-phosphogluconate during anaerobic growth when nitrate is used as respiratory electron acceptor. Mutant cells lacking glucose-6-phosphate dehydrogenase are unable to catabolize glucose under these conditions. The mutant cells utilize glucose as effectively as do wild-type cells in the presence of oxygen; under these conditions, glucose is utilized via direct oxidation to gluconate, which is converted to 6-phosphogluconate. The membrane-associated glucose dehydrogenase activity was not formed during anaerobic growth with glucose. Gluconate, the product of the enzyme, appeared to be the inducer of the gluconate transport system, gluconokinase, and membrane-associated gluconate dehydrogenase. 6-Phosphogluconate is probably the physiological inducer of glucokinase, glucose-6-phosphate dehydrogenase, and the dehydratase and aldolase of the Entner-Doudoroff pathway. Nitrate-linked respiration is required for the anaerobic uptake of glucose and gluconate by independently regulated transport systems in cells grown under denitrifying conditions.     

Convergent peripheral pathways catalyze initial glucose catabolism in Pseudomonas putida: genomic and flux analysis

In this study, we show that glucose catabolism in Pseudomonas putida occurs through the simultaneous operation of three pathways that converge at the level of 6-phosphogluconate, which is metabolized by the Edd and Eda Entner/Doudoroff enzymes to central metabolites. When glucose enters the periplasmic space through specific OprB porins, it can either be internalized into the cytoplasm or be oxidized to gluconate. Glucose is transported to the cytoplasm in a process mediated by an ABC uptake system encoded by open reading frames PP1015 to PP1018 and is then phosphorylated by glucokinase (encoded by the glk gene) and converted by glucose-6-phosphate dehydrogenase (encoded by the zwf genes) to 6-phosphogluconate. Gluconate in the periplasm can be transported into the cytoplasm and subsequently phosphorylated by gluconokinase to 6-phosphogluconate or oxidized to 2-ketogluconate, which is transported to the cytoplasm, and subsequently phosphorylated and reduced to 6-phosphogluconate. In the wild-type strain, glucose was consumed at a rate of around 6 mmol g(-1) h(-1), which allowed a growth rate of 0.58 h(-1) and a biomass yield of 0.44 g/g carbon used. Flux analysis of (13)C-labeled glucose revealed that, in the Krebs cycle, most of the oxalacetate fraction was produced by the pyruvate shunt rather than by the direct oxidation of malate by malate dehydrogenase. Enzymatic and microarray assays revealed that the enzymes, regulators, and transport systems of the three peripheral glucose pathways were induced in response to glucose in the outer medium. We generated a series of isogenic mutants in one or more of the steps of all three pathways and found that, although all three functioned simultaneously, the glucokinase pathway and the 2-ketogluconate loop were quantitatively more important than the direct phosphorylation of gluconate. In physical terms, glucose catabolism genes were organized in a series of clusters scattered along the chromosome. Within each of the clusters, genes encoding porins, transporters, enzymes, and regulators formed operons, suggesting that genes in each cluster coevolved. The glk gene encoding glucokinase was located in an operon with the edd gene, whereas the zwf-1 gene, encoding glucose-6-phosphate dehydrogenase, formed an operon with the eda gene. Therefore, the enzymes of the glucokinase pathway and those of the Entner-Doudoroff pathway are physically linked and induced simultaneously. It can therefore be concluded that the glucokinase pathway is a sine qua non condition for P. putida to grow with glucose.     

Glucose and Gluconate Metabolism in a Mutant of Escherichia coli Lacking Gluconate-6-phosphate Dehydrase

A mutant lacking gluconate-6-phosphate dehydrase (the first enzyme of the Entner-Doudoroff pathway) was isolated after ethyl methane sulfonate mutagenesis of Escherichia coli. Other enzymes of gluconate metabolism (gluconokinase, gluconate-6-phosphate dehydrogenase, and 2-keto-3-deoxygluconate-6-phosphate aldolase) were present in the mutant. When the mutant was grown on gluconate-1-(14)C, alanine isolated from protein was unlabeled, showing that the dehydrase was absent in vivo and that the sole pathway of gluconate metabolism in the mutant was the hexose monophosphate shunt. The mutant grew on gluconate with a doubling time of 155 min, compared with the parent strain's 56 min. On glucose and fructose it grew with normal doubling times. Thus, in E. coli, the Entner-Doudoroff pathway is used for gluconate metabolism but not for glucose metabolism.     

Enzymes of glucose metabolism in Frankia sp.

Enzymes of glucose metabolism were assayed in crude cell extracts of Frankia strains HFPArI3 and HFPCcI2 as well as in isolated vesicle clusters from Alnus rubra root nodules. Activities of the Embden-Meyerhof-Parnas pathway enzymes glucokinase, phosphofructokinase, and pyruvate kinase were found in Frankia strain HFPArI3 and glucokinase and pyruvate kinase were found in Frankia strain HFPCcI2 and in the vesicle clusters. An NADP+-linked glucose 6-phosphate dehydrogenase and an NAD-linked 6-phosphogluconate dehydrogenase were found in all of the extracts, although the role of these enzymes is unclear. No NADP+-linked 6-phosphogluconate dehydrogenase was found. Both dehydrogenases were inhibited by adenosine 5-triphosphate, and the apparent Km's for glucose 6-phosphate and 6-phosphogluconate were 6.86 X 10(-4) and 7.0 X 10(-5) M, respectively. In addition to the enzymes mentioned above, an NADP+-linked malic enzyme was detected in the pure cultures but not in the vesicle clusters. In contrast, however, the vesicle clusters had activity of an NAD-linked malic enzyme. The possibility that this enzyme resulted from contamination from plant mitochondria trapped in the vesicle clusters could not be discounted. None of the extracts showed activities of the Entner-Doudoroff enzymes or the gluconate metabolism enzymes gluconate dehydrogenase or gluconokinase. Propionate- versus trehalose-grown cultures of strain HFPArI3 showed similar activities of most enzymes except malic enzyme, which was higher in the cultures grown on the organic acid. Nitrogen-fixing cultures of strain HFPArI3 showed higher specific activities of glucose 6-phosphate and 6-phosphogluconate dehydrogenases and phosphofructokinase than ammonia-grown cultures.     

Studies on gluconate metabolism in Aspergillus niger

Summary A citric acid accumulating strain of Aspergillus niger adapted to grow on gluconic acid lactone as sole carbon source was studied with regards to the enzymatic changes occurring during its adaptive growth and compared with those of the parent strain cultivated on sucrose.Glucose oxidase, glucose dehydrogenase, gluconate dehydrogenase and enzymes of Entner Doudoroff pathway could not be detected in the strain. Gluconokinase was detected in the strain and its inducible nature was established. An adaptive increase of gluconokinase, phosphogluconate dehydrogenase and ribose phosphate isomerase was observed. While the levels of the above enzymes were significantly higher, that of glucose 6-phosphate dehydrogenase were significantly low indicating the repression of the enzyme. Levels of hexokinase and fructose diphosphate aldolase remained more or less the same as that of parent strain. It was inferred from the results that gluconate metabolism occurs via the pentose phosphate pathway following initial phosphorylation.Part of this work was presented at the 8th Annual Microbiological Congress of Association of Microbiologists of India, held in New Delhi 1967.     

Characterization of enzymes involved in the central metabolism of Gluconobacter oxydans

Gluconobacter oxydans is an industrially important bacterium that lacks a complete Embden–Meyerhof pathway (glycolysis). The organism instead uses the pentose phosphate pathway to oxidize sugars and their phosphorylated intermediates. However, the lack of glycolysis limits the amount of NADH as electron donor for electron transport phosphorylation. It has been suggested that the pentose phosphate pathway contributes to NADH production. Six enzymes predicted to play central roles in intracellular glucose and gluconate flux were heterologously overproduced in Escherichia coli and characterized to investigate the intracellular flow of glucose and gluconates into the pentose phosphate pathway and to explore the contribution of the pentose phosphate pathway to NADH generation. The key pentose phosphate enzymes glucose 6-phosphate dehydrogenase (Gox0145) and 6-phosphogluconate dehydrogenase (Gox1705) had dual cofactor specificities but were physiologically NADP- and NAD-dependent, respectively. Putative glucose dehydrogenase (Gox2015) was NADP-dependent and exhibited a preference for mannose over glucose, whereas a 2-ketogluconate reductase (Gox0417) displayed dual cofactor specificity for NAD(P)H. Furthermore, a putative gluconokinase and a putative glucokinase were identified. The gluconokinase displayed high activities with gluconate and is thought to shuttle intracellular gluconate into the pentose phosphate pathway. A model for the trafficking of glucose and gluconates into the pentose phosphate pathway and its role in NADH generation is presented. The role of NADPH in chemiosmotic energy conservation is also discussed.     

Glucose and Gluconate Metabolism in an Escherichia coli Mutant Lacking Phosphoglucose Isomerase

A single gene mutant lacking phosphoglucose isomerase (pgi) was selected after ethyl methane sulfonate mutagenesis of Escherichia coli strain K-10. Enzyme assays revealed no pgi activity in the mutant, whereas levels of glucokinase, glucose-6-phosphate dehydrogenase, and gluconate-6-phosphate dehydrogenase were similar in parent and mutant. The amount of glucose released by acid hydrolysis of the mutant cells after growth on gluconate was less than 2% that released from parent cells; when grown in the presence of glucose, mutant and parent cells contained the same amount of glucose residues. The mutant grew on glucose one-third as fast as the parent; it also grew much slower than the parent on galactose, maltose, and lactose. On fructose, gluconate, and other carbon sources, growth was almost normal. In both parent and mutant, gluconokinase and gluconate-6-phosphate dehydrase were present during growth on gluconate but not during growth on glucose. Assay and degradation of alanine from protein hydrolysates after growth on glucose-1-(14)C and gluconate-1-(14)C showed that in the parent strain glucose was metabolized by the glycolytic path and the hexose monophosphate shunt. Gluconate was metabolized by the Entner-Doudoroff path and the hexose monophosphate shunt. The mutant used glucose chiefly by the shunt, but may also have used the Entner-Doudoroff path to a limited extent.     

Gluconate metabolism in germinated spores of Bacillus megaterium QM B1551: primary roles of gluconokinase and the pentose cycle

The metabolic pathway of gluconate, a major product of glucose metabolism during spore germination, was investigated in Bacillus megaterium QM B1551. Compared to the parent, mutant spores lacking gluconokinase could not metabolize gluconate, whereas the revertant simultaneously restored the enzyme activity and the ability to metabolize it, indicating that gluconokinase was solely responsible for the onset of gluconate metabolism. To identify a further metabolic route for gluconate, we determined 14C yields in acetate and CO2 formed from [14C]gluconate, and found that experimental ratios of 14CO2/[14C]acetate obtained from [2-14C]gluconate and [3,4-14C]gluconate were not compatible with the ratios predicted from the Entner-Doudoroff pathway. In contrast, when CO2 release caused by recycling (approx. 30%) was corrected, the ratios almost agreed with those from the pentose cycle. Comparison of specific radioactivities in acetate also supported the conclusion that gluconate was metabolized via the pentose cycle, subsequently metabolized via the Embden-Meyerhof pathway, and finally degraded to acetate and CO2 without a contribution by the Krebs cycle.     

Regulation of the glucolytic enzymes inPseudomonas putida

Summary The synthesis of glucose catabolizing enzymes is under inductive control inPseudomonas putida. Glucose, gluconate and 2-ketogluconate are the best nutritional inducers of these enzymes. Mutants unable to catabolize gluconate or 2-ketogluconate synthesized relatively high levels of glucose dehydrogenase and gluconate-6P dehydrase activities when grown in the presence of these substrates. This identifies both compounds as true inducers of these enzymes. KDGP aldolase is induced by its substrate, as evidenced by the inability of mutant cells unable to form KDGP to produce this enzyme at levels above the basal one. A 3-carbon compound appears to be the inducer of glyceraldehyde-3P dehydrogenase. This pattern of regulation suggests that there is a low degree of coordinate control in the synthesis of the glucolytic enzymes byP. putida. This is also supported by the lack of proportionality found in the levels of two enzymes governed by the same inducers, glucose dehydrogenase and gluconate-6P dehydrase, in cells grown on different conditions.Abbrevitions P phosphate - KDGP 2-Keto-3-deoxygluconate-6-phosphate - GDH glucose dehydrogenase - GNDH gluconate dehydrogenase - GK glucokinase - GNK gluconokinase - KGK ketogluconokinase - KGR 2-Ketogluconate-6-phosphate reductase - GPDH glucose-6-phosphate dehydrogenase - GNPD gluconate-6-phosphate dehydrase - KDGPA 2-Keto-3-deoxygluconate-6-phosphate aldolase - GAPDH glyceraldehyde-3-phosphate dehydrogenase     

Role of the ptsN gene product in catabolite repression of the Pseudomonas putida TOL toluene degradation pathway in chemostat cultures

The Pseudomonas putida KT2440 TOL upper pathway is repressed under nonlimiting conditions in cells growing in chemostat with succinate as a carbon source. We show that the ptsN gene product IIA(Ntr) participates in this repression. Crc, involved in yeast extract-dependent repression in batch cultures, did not influence expression when cells were growing in a chemostat with succinate at maximum rate.     

The role of glucose limitation in the regulation of the transport of glucose, gluconate and 2-oxogluconate, and of glucose metabolism in Pseudomonas aeruginosa.

The pathway of glucose metabolism in Pseudomonas aeruginosa was regulated by the availability of glucose and related compounds. On changing from an ammonium limitation to a glucose limitation, the organism responded by adjusting its metabolism substantially from the extracellular direct oxidative pathway to the intracellular phosphorylative route. This change was achieved by repression of the transport systems for gluconate and 2-oxogluconate and of the associated enzymes for 2-oxogluconate metabolism and gluconate kinase, while increasing the levels of glucose transport, hexokinase and glucose 6-phosphate dehydrogenase. The role of gluconate, produced by the action of glucose dehydrogenase, as a major inhibitory factor for glucose transport, and the possible significance of these regulatory mechanisms to the organism in its natural environment, are discussed.     

Effect of glucose on polyol metabolism by Rhizobium trifolii.

The effect of glucose on polyol metabolism by Rhizobium trifolii was studied. Phenomena similar to catabolite repression and catabolite inhibition were observed. The catabolism of glucose to at least glucose 6-phosphate was required for the effects to be exerted.     

The IIANtr (PtsN) protein of Pseudomonas putida mediates the C source inhibition of the sigma54-dependent Pu promoter of the TOL plasmid.

The gene cluster adjacent to the sequence of rpoN (encoding sigma factor sigma54) of Pseudomonas putida has been studied with respect to the C source regulation of the Pu promoter of the upper TOL (toluene catabolism) operon. The region includes four open reading frames (ORFs), two of which (named ptsN and ptsO genes) encode proteins similar to components of the phosphoenolpyruvate:sugar phosphotransferase system. Each of the four genes was disrupted with a nonpolar insertion, and the effects in the inhibition caused by glucose on Pu activity were inspected with a lacZ reporter system. Although cells lacking ORF102, ORF284, and ptsO did not display any evident phenotype under the conditions tested, the loss of ptsN, which encodes the IIANtr protein, made Pu unresponsive to repression by glucose. The ptsN mutant had rates of glucose/gluconate consumption identical to those of the wild type, thus ruling out indirect effects mediated by the transport of the carbohydrate. A site-directed ptsN mutant in which the conserved phospho-acceptor site His68 of IIANtr was replaced by an aspartic acid residue made Pu blind to the presence or absence of glucose, thus supporting the notion that phosphorylation of IIANtr mediates the C source inhibition of the promoter. These data substantiate the existence of a molecular pathway for co-regulation of some sigma54 promoters in which IIANtr is a key protein intermediate.