Binding of purine nucleotides to two regulatory sites results in synergistic feedback inhibition of glutamine 5-phosphoribosylpyrophosphate amidotransferase

The roles of molecular chaperones in regulating cell metabolism

Fluctuations in nutrient and biomass availability, often as a result of disease, impart metabolic challenges that must be overcome in order to sustain cell survival and promote proliferation. Cells adapt to these environmental changes and stresses by adjusting their metabolic networks through a series of regulatory mechanisms. Our understanding of these rewiring events has largely been focused on those genetic transformations that alter protein expression and the biochemical mechanisms that change protein behavior, such as post-translational modifications and metabolite-based allosteric modulators. Mounting evidence suggests that a class of proteome surveillance proteins called molecular chaperones also can influence metabolic processes. Here, we summarize several ways the Hsp90 and Hsp70 chaperone families act on human metabolic enzymes and their supramolecular assemblies to change enzymatic activities and metabolite flux. We further highlight how these chaperones can assist in the translocation and degradation of metabolic enzymes. Collectively, these studies provide a new view for how metabolic processes are regulated to meet cellular demand and inspire new avenues for therapeutic intervention.

Diverse strategies adopted by nature for regulating purine biosynthesis via fine-tuning of purine metabolic enzymes

Purine nucleotides, generated by de novo synthesis and salvage pathways, are essential for metabolism and act as building blocks of genetic material. To avoid an imbalance in the nucleotide pool, nature has devised several strategies to regulate/tune the catalytic performance of key purine metabolic enzymes. Here, we discuss some recent examples, such as stress-regulating alarmones that bind to select pathway enzymes, huge ensembles like dynamic metabolons and self-assembled filaments that highlight the layered fine-control prevalent in the purine metabolic pathway to fulfill requisite purine demands. Examples of enzymes that turn-on only under allosteric control, are regulated via long-distance communication that facilitates transient conduits have additionally been explored.

Genetic analysis using vitamin B6 antagonist 4-deoxypyridoxine uncovers a connection between pyridoxal 5'-phosphate and coenzyme A metabolism in Salmonella enterica

Pyridoxal 5'-phosphate (PLP) is an essential cofactor for organisms in all three domains of life. Despite the central role of PLP, many aspects of vitamin B₆ metabolism, including its integration with other biological pathways, are not fully understood. In this study, we examined the metabolic perturbations caused by the vitamin B₆ antagonist 4-deoxypyridoxine (dPN) in a ptsJ mutant of Salmonella enterica serovar Typhimurium LT2. Our data suggest that PdxK (PL/PN/PM kinase, EC 2.7.1.35) phosphorylates dPN to 4-deoxypyridoxine 5'-phosphate (dPNP), which in turn can compromise the de novo biosynthesis of PLP. The data are consistent with the hypothesis that accumulated dPNP inhibits GlyA (serine hydroxymethyltransferase, EC 2.1.2.1) and/or GcvP (glycine decarboxylase, EC 1.4.4.2), two PLP-dependent enzymes involved in the generation of one-carbon units. Our data suggest this inhibition leads to reduced flux to coenzyme A precursors and subsequently lower synthesis of CoA and thiamine. This study uncovers a link between vitamin B₆ metabolism and the biosynthesis of CoA and thiamine, highlighting the integration of biochemical pathways in microbes. IMPORTANCE PLP is a ubiquitous cofactor required by enzymes in diverse metabolic networks. The data herein expand our understanding of the toxic effects of dPN, a vitamin B₆ antagonist often used to mimic vitamin B₆ deficiency and to study PLP-dependent enzyme kinetics. In addition to de novo PLP biosynthesis, we define a metabolic connection between vitamin B₆ metabolism and synthesis of thiamine and CoA. This work provides a foundation for the use of dPN to study vitamin B₆ metabolism in other organisms.

NOTCH1-driven UBR7 stimulates nucleotide biosynthesis to promote T cell acute lymphoblastic leukemia

Ubiquitin protein ligase E3 component N-recognin 7 (UBR7) is the most divergent member of UBR box-containing E3 ubiquitin ligases/recognins that mediate the proteasomal degradation of its substrates through the N-end rule. Here, we used a proteomic approach and found phosphoribosyl pyrophosphate synthetases (PRPSs), the essential enzymes for nucleotide biosynthesis, as strong interacting partners of UBR7. UBR7 stabilizes PRPS catalytic subunits by mediating the polyubiquitination-directed degradation of PRPS-associated protein (PRPSAP), the negative regulator of PRPS. Loss of UBR7 leads to nucleotide biosynthesis defects. We define UBR7 as a transcriptional target of NOTCH1 and show that UBR7 is overexpressed in NOTCH1-driven T cell acute lymphoblastic leukemia (T-ALL). Impaired nucleotide biosynthesis caused by UBR7 depletion was concomitant with the attenuated cell proliferation and oncogenic potential of T-ALL. Collectively, these results establish UBR7 as a critical regulator of nucleotide metabolism through the regulation of the PRPS enzyme complex and uncover a metabolic vulnerability in NOTCH1-driven T-ALL.

Identification of Z nucleotides as an ancient signal for two-component system activation in bacteria

Two-component systems (TCSs) in bacteria are molecular circuits that allow the perception of and response to diverse stimuli. These signaling circuits rely on phosphoryl-group transfers between transmitter and receiver domains of sensor kinase and response regulator proteins, and regulate several cellular processes in response to internal or external cues. Phosphorylation, and thereby activation, of response regulators has been demonstrated to occur by their cognate histidine kinases but also by low molecular weight phosphodonors such as acetyl phosphate and carbamoyl phosphate. Here, we present data indicating that the intermediates of the de novo syntheses of purines and histidine, 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranosyl 5'-monophosphate (ZMP) and/or 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranosyl 5'-triphosphate (ZTP), activate the response regulator UvrY, by promoting its autophosphorylation at the conserved aspartate at position 54. Moreover, these Z nucleotides are shown to also activate the nonrelated response regulators ArcA, CpxR, RcsB, and PhoQ. We propose that ZMP and/or ZTP act as alarmones for a wide range of response regulators in vivo, providing a novel mechanism by which they could impact gene expression in response to metabolic cues.

RNase I Modulates Escherichia coli Motility, Metabolism, and Resistance

Bacteria are constantly adapting to their environment by sensing extracellular factors that trigger production of intracellular signaling molecules, known as second messengers. Recently, 2',3'-cyclic nucleotide monophosphates (2',3'-cNMPs) were identified in Escherichia coli and have emerged as possible novel signaling molecules. 2',3'-cNMPs are produced through endonucleolytic cleavage of short RNAs by the T2 endoribonuclease, RNase I; however, the physiological roles of RNase I remain unclear. Our transcriptomic analysis suggests that RNase I is involved in modulating numerous cellular processes, including nucleotide metabolism, motility, acid sensitivity, metal homeostasis, and outer membrane morphology. Through a combination of deletion strain and inhibitor studies, we demonstrate that RNase I plays a previously unknown role in E. coli stress resistance by affecting pathways that are part of the defense mechanisms employed by bacteria when introduced to external threats, including antibiotics. Thus, this work provides insight into the emerging roles of RNase I in bacterial signaling and physiology and highlights the potential of RNase I as a target for antibacterial adjuvants.

Highly Efficient Biosynthesis of Hypoxanthine in Escherichia coli and Transcriptome-Based Analysis of the Purine Metabolism

Nucleosides and purine analogues have multiple functions in cell physiology, food additives, and pharmaceuticals, and some are produced on a large scale using different microorganisms. However, biosynthesis of purines is still lacking. In the present study, we engineered the de novo purine biosynthesis pathway, branched pathways, and a global regulator to ensure highly efficient hypoxanthine production by Escherichia coli. The engineered strain Q2973 produced 1243 mg/L hypoxanthine in fed-batch fermentation, accompanied by an extremely low accumulation of byproducts such as acetate and xanthine. We also performed global gene expression analysis to illustrate the mechanism for improving hypoxanthine production. This study demonstrated the feasibility of large-scale hypoxanthine production byan engineered E. coli strain, and provides a reference for subsequent studies on purine analogues and nucleosides.

Mapping Post-Translational Modifications of de Novo Purine Biosynthetic Enzymes: Implications for Pathway Regulation

Purines represent a class of essential metabolites produced by the cell to maintain cellular homeostasis and facilitate cell proliferation. In times of high purine demand, the de novo purine biosynthetic pathway is activated; however, the mechanisms that facilitate this process are largely unknown. One plausible mechanism is through intracellular signaling, which results in enzymes within the pathway becoming post-translationally modified to enhance their individual enzyme activities and the overall pathway metabolic flux. Here, we employ a proteomic strategy to investigate the extent to which de novo purine biosynthetic pathway enzymes are post-translationally modified in 293T cells. We identified 7 post-translational modifications on 135 residues across the 6 human pathway enzymes. We further asked whether there were differences in the post-translational modification state of each pathway enzyme isolated from cells cultured in the presence or absence of purines. Of the 174 assigned modifications, 67% of them were only detected in one experimental growth condition in which a significant number of serine and threonine phosphorylations were noted. A survey of the most-probable kinases responsible for these phosphorylation events uncovered a likely AKT phosphorylation site at residue Thr397 of PPAT, which was only detected in cells under purine-supplemented growth conditions. These data suggest that this modification might alter enzyme activity or modulate its interaction(s) with downstream pathway enzymes. Together, these findings propose a role for post-translational modifications in pathway regulation and activation to meet intracellular purine demand.

Emerging Role of Purine Metabolizing Enzymes in Brain Function and Tumors

The growing evidence of the involvement of purine compounds in signaling, of nucleotide imbalance in tumorigenesis, the discovery of purinosome and its regulation, cast new light on purine metabolism, indicating that well known biochemical pathways may still surprise. Adenosine deaminase is important not only to preserve functionality of immune system but also to ensure a correct development and function of central nervous system, probably because its activity regulates the extracellular concentration of adenosine and therefore its function in brain. A lot of work has been done on extracellular 5′-nucleotidase and its involvement in the purinergic signaling, but also intracellular nucleotidases, which regulate the purine nucleotide homeostasis, play unexpected roles, not only in tumorigenesis but also in brain function. Hypoxanthine guanine phosphoribosyl transferase (HPRT) appears to have a role in the purinosome formation and, therefore, in the regulation of purine synthesis rate during cell cycle with implications in brain development and tumors. The final product of purine catabolism, uric acid, also plays a recently highlighted novel role. In this review, we discuss the molecular mechanisms underlying the pathological manifestations of purine dysmetabolisms, focusing on the newly described/hypothesized roles of cytosolic 5′-nucleotidase II, adenosine kinase, adenosine deaminase, HPRT, and xanthine oxidase.

Perturbation of the metabolic network in Salmonella enterica reveals cross-talk between coenzyme A and thiamine pathways

Microorganisms respond to a variety of metabolic perturbations by repurposing or recruiting pathways to reroute metabolic flux and overcome the perturbation. Elimination of the 2-dehydropantoate 2-reductase, PanE, both reduces total coenzyme A (CoA) levels and causes a conditional HMP-P auxotrophy in Salmonella enterica. CoA or acetyl-CoA has no demonstrable effect on the HMP-P synthase, ThiC, in vitro. Suppressors aimed at probing the connection between the biosynthesis of thiamine and CoA contained mutations in the gene encoding the ilvC transcriptional regulator, ilvY. These mutations may help inform the structure and mechanism of action for the effector-binding domain, as they represent the first sequenced substitutions in the effector-binding domain of IlvY that cause constitutive expression of ilvC. Since IlvC moonlights as a 2-dehydropantoate 2-reductase, the resultant increase in ilvC transcription increased synthesis of CoA. This study failed to identify mutations overcoming the need for CoA for thiamine synthesis in S. enterica panE mutants, suggesting that a more integrated approach may be necessary to uncover the mechanism connecting CoA and ThiC activity in vivo.

A New View into the Regulation of Purine Metabolism: The Purinosome

Other than serving as building blocks for DNA and RNA, purine metabolites provide a cell with the necessary energy and cofactors to promote cell survival and proliferation. A renewed interest in how purine metabolism may fuel cancer progression has uncovered a new perspective into how a cell regulates purine need. Under cellular conditions of high purine demand, the de novo purine biosynthetic enzymes cluster near mitochondria and microtubules to form dynamic multienzyme complexes referred to as 'purinosomes'. In this review, we highlight the purinosome as a novel level of metabolic organization of enzymes in cells, its consequences for regulation of purine metabolism, and the extent that purine metabolism is being targeted for the treatment of cancers.

Nucleotides, Nucleosides, and Nucleobases

We review literature on the metabolism of ribo- and deoxyribonucleotides, nucleosides, and nucleobases in Escherichia coli and Salmonella,including biosynthesis, degradation, interconversion, and transport. Emphasis is placed on enzymology and regulation of the pathways, at both the level of gene expression and the control of enzyme activity. The paper begins with an overview of the reactions that form and break the N-glycosyl bond, which binds the nucleobase to the ribosyl moiety in nucleotides and nucleosides, and the enzymes involved in the interconversion of the different phosphorylated states of the nucleotides. Next, the de novo pathways for purine and pyrimidine nucleotide biosynthesis are discussed in detail.Finally, the conversion of nucleosides and nucleobases to nucleotides, i.e.,the salvage reactions, are described. The formation of deoxyribonucleotides is discussed, with emphasis on ribonucleotidereductase and pathways involved in fomation of dUMP. At the end, we discuss transport systems for nucleosides and nucleobases and also pathways for breakdown of the nucleobases.

High-Frequency Variation of Purine Biosynthesis Genes Is a Mechanism of Success in Campylobacter jejuni

Phenotypic variation is prevalent in the zoonotic pathogen Campylobacter jejuni, the leading agent of enterocolitis in the developed world. Heterogeneity enhances the survival and adaptive malleability of bacterial populations because variable phenotypes may allow some cells to be protected against future stress. Exposure to hyperosmotic stress previously revealed prevalent differences in growth between C. jejuni strain 81-176 colonies due to resistant or sensitive phenotypes, and these isolated colonies continued to produce progeny with differential phenotypes. In this study, whole-genome sequencing of isolated colonies identified allelic variants of two purine biosynthesis genes, purF and apt, encoding phosphoribosyltransferases that utilize a shared substrate. Genetic analyses determined that purF was essential for fitness, while apt was critical. Traditional and high-depth amplicon-sequencing analyses confirmed extensive intrapopulation genetic variation of purF and apt that resulted in viable strains bearing alleles with in-frame insertion duplications, deletions, or missense polymorphisms. Different purF and apt alleles were associated with various stress survival capabilities under several niche-relevant conditions and contributed to differential intracellular survival in an epithelial cell infection model. Amplicon sequencing revealed that intracellular survival selected for stress-fit purF and apt alleles, as did exposure to oxygen and hyperosmotic stress. Putative protein recognition direct repeat sequences were identified in purF and apt, and a DNA-protein affinity screen captured a predicted exonuclease that promoted the global spontaneous mutation rate. This work illustrates the adaptive properties of high-frequency genetic variation in two housekeeping genes, which influences C. jejuni survival under stress and promotes its success as a pathogen.

IMPORTANCE

C. jejuni is an important cause of bacterial diarrheal illness. Bacterial populations have many strategies for stress survival, but phenotypic variation due to genetic diversity has a powerful advantage: no matter how swift the change in environment, a fraction of the population already expresses the survival trait. Nonclonality is thus increasingly viewed as a mechanism of population success. Our previous work identified prominent resistant/sensitive colonial variation in C. jejuni bacteria in response to hyperosmotic stress; in the work presented here, we attribute that to high-frequency genetic variation in two purine biosynthesis genes, purF and apt. We demonstrated selective pressure for nonlethal mutant alleles of both genes, showed that single-cell variants had the capacity to give rise to diverse purF and apt populations, and determined that stress exposure selected for desirable alleles. Thus, a novel C. jejuni adaptive strategy was identified, which was, unusually, reliant on prevalent genetic variation in two housekeeping genes.

Deregulation of purine pathway in Bacillus subtilis and its use in riboflavin biosynthesis

Background

Purine nucleotides are essential metabolites for living organisms because they are involved in many important processes, such as nucleic acid synthesis, energy supply, and biosynthesis of several amino acids and riboflavin. Owing to the pivotal roles of purines in cell physiology, the pool of intracellular purine nucleotides must be maintained under strict control, and hence the de novo purine biosynthetic pathway is tightly regulated by transcription repression and inhibition mechanism. Deregulation of purine pathway is essential for this pathway engineering in Bacillus subtilis.

Results

Deregulation of purine pathway was attempted to improve purine nucleotides supply, based on a riboflavin producer B. subtilis strain with modification of its rib operon. To eliminate transcription repression, the pur operon repressor PurR and the 5’-UTR of pur operon containing a guanine-sensing riboswitch were disrupted. Quantitative RT-PCR analysis revealed that the relative transcription levels of purine genes were up-regulated about 380 times. Furthermore, site-directed mutagenesis was successfully introduced into PRPP amidotransferase (encoded by purF) to remove feedback inhibition by homologous alignment and analysis. Overexpression of the novel mutant PurF (D293V, K316Q and S400W) significantly increased PRPP amidotransferase activity and triggered a strong refractory effect on purine nucleotides mediated inhibition. Intracellular metabolite target analysis indicated that the purine nucleotides supply in engineered strains was facilitated by a stepwise gene-targeted deregulation. With these genetic manipulations, we managed to enhance the metabolic flow through purine pathway and consequently increased riboflavin production 3-fold (826.52 mg/L) in the purF-VQW mutant strain.

Conclusions

A sequential optimization strategy was applied to deregulate the rib operon and purine pathway of B. subtilis to create genetic diversities and to improve riboflavin production. Based on the deregulation of purine pathway at transcription and metabolic levels, an extended application is recommended for the yield of products, like inosine, guanosine, adenosine and folate which are directly stemming from purine pathway in B. subtilis.

Discovery of novel allosteric effectors based on the predicted allosteric sites for Escherichia coli D-3-phosphoglycerate dehydrogenase

D-3-phosphoglycerate dehydrogenase (PGDH) from Escherichia coli catalyzes the first critical step in serine biosynthesis, and can be allosterically inhibited by serine. In a previous study, we developed a computational method for allosteric site prediction using a coarse-grained two-state Gō Model and perturbation. Two potential allosteric sites were predicted for E. coli PGDH, one close to the active site and the nucleotide binding site (Site I) and the other near the regulatory domain (Site II). In the present study, we discovered allosteric inhibitors and activators based on site I, using a high-throughput virtual screen, and followed by using surface plasmon resonance (SPR) to eliminate false positives. Compounds 1 and 2 demonstrated a low-concentration activation and high-concentration inhibition phenomenon, with IC₅₀ values of 34.8 and 58.0 µM in enzymatic bioassays, respectively, comparable to that of the endogenous allosteric effector, L-serine. For its activation activity, compound 2 exhibited an AC₅₀ value of 34.7 nM. The novel allosteric site discovered in PGDH was L-serine- and substrate-independent. Enzyme kinetics studies showed that these compounds influenced K_m, k_cat, and k_cat/K_m. We have also performed structure-activity relationship studies to discover high potency allosteric effectors. Compound 2-2, an analog of compound 2, showed the best in vitro activity with an IC₅₀ of 22.3 µM. Compounds targeting this site can be used as new chemical probes to study metabolic regulation in E. coli. Our study not only identified a novel allosteric site and effectors for PGDH, but also provided a general strategy for designing new regulators for metabolic enzymes.

Stroke neuroprotection: targeting mitochondria

Stroke is the fourth leading cause of death and the leading cause of long-term disability in the United States. Blood flow deficit results in an expanding infarct core with a time-sensitive peri-infarct penumbra that is considered salvageable and is the primary target for treatment strategies. The only current FDA-approved drug for treating ischemic stroke is recombinant tissue plasminogen activator (rt-PA). However, this treatment is limited to within 4.5 h of stroke onset in a small subset of patients. The goal of this review is to focus on mitochondrial-dependent therapeutic agents that could provide neuroprotection following stroke. Dysfunctional mitochondria are linked to neurodegeneration in many disease processes including stroke. The mechanisms reviewed include: (1) increasing ATP production by purinergic receptor stimulation, (2) decreasing the production of ROS by superoxide dismutase, or (3) increasing antioxidant defenses by methylene blue, and their benefits in providing neuroprotection following a stroke.

Metabolic engineering of the purine biosynthetic pathway in Corynebacterium glutamicum results in increased intracellular pool sizes of IMP and hypoxanthine

Background

Purine nucleotides exhibit various functions in cellular metabolism. Besides serving as building blocks for nucleic acid synthesis, they participate in signaling pathways and energy metabolism. Further, IMP and GMP represent industrially relevant biotechnological products used as flavor enhancing additives in food industry. Therefore, this work aimed towards the accumulation of IMP applying targeted genetic engineering of Corynebacterium glutamicum.

Results

Blocking of the degrading reactions towards AMP and GMP lead to a 45-fold increased intracellular IMP pool of 22 μmol g_CDW^-1. Deletion of the pgi gene encoding glucose 6-phosphate isomerase in combination with the deactivated AMP and GMP generating reactions, however, resulted in significantly decreased IMP pools (13 μmol g_CDW^-1). Targeted metabolite profiling of the purine biosynthetic pathway further revealed a metabolite shift towards the formation of the corresponding nucleobase hypoxanthine (102 μmol g_CDW^-1) derived from IMP degradation.

Conclusions

The purine biosynthetic pathway is strongly interconnected with various parts of the central metabolism and therefore tightly controlled. However, deleting degrading reactions from IMP to AMP and GMP significantly increased intracellular IMP levels. Due to the complexity of this pathway further degradation from IMP to the corresponding nucleobase drastically increased suggesting additional targets for future strain optimization.

Analysis of ThiC variants in the context of the metabolic network of Salmonella enterica

In bacteria, the 4-amino-hydroxymethyl-2-methylpyrimidine (HMP) moiety of thiamine is synthesized from 5-aminoimidazole ribotide (AIR), a branch point metabolite of purine and thiamine biosynthesis. ThiC is a member of the radical S-adenosylmethionine (AdoMet) superfamily and catalyzes the complex chemical rearrangement of AIR to HMP-P. As reconstituted in vitro, the ThiC reaction requires AdoMet, AIR, and reductant. This study analyzed variants of ThiC in vivo and in vitro to probe the metabolic network surrounding AIR in Salmonella enterica. Several variants of ThiC that required metabolic perturbations to function in vivo were biochemically characterized in vitro. Results presented herein indicate that the subtleties of the metabolic network have not been captured in the current reconstitution of the ThiC reaction.

Accumulation of gene-targeted Bacillus subtilis mutations that enhance fermentative inosine production

In order to test a possible approach to enhance fermentative inosine production by Bacillus subtilis, seven gene-targeted mutations were introduced in the laboratory standard strain168 in a stepwise fashion. The mutations were employed in order to prevent inosine 5'-monophosphate (IMP) from being consumed for AMP and GMP synthesis, to minimize inosine degradation, and to expand the intracellular IMP pool. First, the genes for adenylosuccinate synthase (purA) and IMP dehydrogenase (guaB) were inactivated. Second, two genes for purine nucleoside phosphorylase, punA and deoD, were inactivated. Third, to enhance purine nucleotide biosynthesis, the pur operon repressor PurR and the 5'-UTR of the operon, containing the guanine riboswitch, were disrupted. Finally, the -10 sequence of the pur promoter was optimized to elevate its transcription level. The resulting mutant was capable of producing 6 g/L inosine from 30 g/L glucose in culture broth without the detectable by-production of hypoxanthine. This indicates the validity of this approach for the breeding of the next generation of B. subtilis strains for industrial nucleoside production.

Effect of amplification of desensitized purF and prs on inosine accumulation in Escherichia coli

The effect of a phosphoribosylpyrophosphate (PRPP) synthetase gene (prs) that was desensitized to feedback inhibition by ADP on inosine accumulation was investigated using an inosine-producing mutant of Escherichia coli. At the same time, various types of plasmid having a PRPP amidotransferase gene (purF) that was desensitized to feedback inhibition by AMP and GMP were also investigated to improve inosine productivity using a compatible plasmid containing prs with a plasmid containing purF. The recombinant E. coli I-9 harboring a low-copy-number plasmid having the desensitized-purF (pMWKQ) accumulated 3.6 g/l inosine from 40 g/l glucose in a 2-d culture. Furthermore, desensitized-prs amplification, in addition to purF, resulted in the accumulation of 6.2 g/l inosine. Additionally, through these experiments, a spontaneous mutant with an enhanced inosine-producing ability compared with the parent strain I-9 was obtained. The spontaneous mutant I-9m harboring only pMWKQ and I-9m harboring both pMWKQ and pSTVDA (a plasmid having the desensitized-prs) accumulated 6.7 g/l and 7.5 g/l inosine, respectively, from 40 g/l glucose in a 3-d culture.

Metabolic engineering of the purine pathway for riboflavin production in Ashbya gossypii

Purine nucleotides are essential precursors for living organisms because they are involved in many important processes, such as nucleic acid synthesis, energy supply, and the biosynthesis of several amino acids and vitamins such as riboflavin. GTP is the immediate precursor for riboflavin biosynthesis, and its formation through the purine pathway is subject to several regulatory mechanisms in different steps. Extracellular purines repress the transcription of most genes required for de novo ATP and GTP synthesis. Additionally, three enzymes of the pathway, phosphoribosyl pyrophosphate (PRPP) amidotransferase, adenylosuccinate synthetase, and IMP dehydrogenase, are subject to feedback inhibition by their end products. Here we report the characterization and manipulation of the committed step in the purine pathway of the riboflavin overproducer Ashbya gossypii. We report that phosphoribosylamine biosynthesis in A. gossypii is negatively regulated at the transcriptional level by extracellular adenine. Furthermore, we show that ATP and GTP exert a strong inhibitory effect on the PRPP amidotransferase from A. gossypii. We constitutively overexpressed the AgADE4 gene encoding PRPP amidotransferase in A. gossypii, thereby abolishing the adenine-mediated transcriptional repression. In addition, we replaced the corresponding residues (aspartic acid310, lysine333, and alanine417) that have been described to be important for PRPP amidotransferase feedback inhibition in other organisms by site-directed mutagenesis. With these manipulations, we managed to enhance metabolic flow through the purine pathway and to increase the production of riboflavin in the triple mutant strain 10-fold (228 mg/liter).

Feedback inhibition of amidophosphoribosyltransferase regulates the rate of cell growth via purine nucleotide, DNA, and protein syntheses

To clarify the contributions of amidophosphoribosyltransferase (ATase) and its feedback regulation to the rates of purine de novo synthesis, DNA synthesis, protein synthesis, and cell growth, mutated human ATase (mhATase) resistant to feedback inhibition by purine ribonucleotides was engineered by site-directed mutagenesis and expressed in CHO ade (-)A cells (an ATase-deficient cell line of Chinese hamster ovary fibroblasts) and in transgenic mice (mhATase-Tg mice). In Chinese hamster ovary transfectants with mhATase, the following parameters were examined: ATase activity and its subunit structure, the metabolic rates of de novo and salvage pathways, DNA and protein synthesis rates, and the rate of cell growth. In mhATase-Tg mice, ATase activity in the liver and spleen, the metabolic rate of the de novo pathway in the liver, serum uric acid concentration, urinary excretion of purine derivatives, and T lymphocyte proliferation by phytohemagglutinin were examined. We concluded the following. 1) ATase and its feedback inhibition regulate not only the rate of purine de novo synthesis but also DNA and protein synthesis rates and the rate of cell growth in cultured fibroblasts. 2) Suppression of the de novo pathway by the salvage pathway is mainly due to the feedback inhibition of ATase by purine ribonucleotides produced via the salvage pathway, whereas the suppression of the salvage pathway by the de novo pathway is due to consumption of 5-phosphoribosyl 1-pyrophosphate by the de novo pathway. 3) The feedback inhibition of ATase is more important for the regulation of the de novo pathway than that of 5-phosphoribosyl 1-pyrophosphate synthetase. 4) ATase superactivity leads to hyperuricemia and an increased bromodeoxyuridine incorporation in T lymphocytes stimulated by phytohemagglutinin.

Investigation of various genotype characteristics for inosine accumulation in Escherichia coli W3110

For the derivation of an inosine-overproducing strain from the wild type microorganism, it is known that the addition of an adenine requirement, removal of purine nucleoside hydrolyzing activity, removal of the feedback inhibition, and repression of key enzymes in the purine nucleotides biosynthetic pathway are essential. Thus, the disruption of purA (adenine requirement), deoD (removal of purine nucleosides phosphorylase activity), purR (derepression of the regulation of purine nucleotides biosynthetic pathway), and the insensitivity of the feedback inhibition of phosphoribosylpyrophosphate (PRPP) amidotransferase by adenosine 5'-monophosphate (AMP) and guanosine 5'-monophosphate (GMP) were done in the Escherichia coli strain W3110, and then the inosine productivity was estimated. In the case of using a plasmid harboring the PRPP amidotransferase gene (purF) that encoded a desensitized PRPP amidotransferase, purF disrupted mutants were used as the host strains. It was found that the innovation of the four genotypes brought about a small amount of inosine accumulation. Furthermore, an adenine auxotrophic mutant of E. coli showed inappropriate adenine use because its growth could not respond efficiently to the concentration of adenine added. As the presence of adenosine deaminase is well known in E. coli and it is thought to be involved in adenine use, a mutant disrupted adenosine deaminase gene (add) was constructed and tested. The mutant, which is deficient in purF, purA, deoD, purR, and add genes, and harboring the desensitized purF as a plasmid, accumulated about 1 g of inosine per liter. Although we investigated the effects of purR disruption and purF gene improvement, unexpectedly an increase in the inosine productivity could not be found with this mutant.

Glutamine PRPP amidotransferase: snapshots of an enzyme in action

Recent studies of glutamine PRPP amidotransferase have provided a new understanding of the function and mechanism of this rather complicated enzyme that may be a paradigm for other complex enzymes. New insights have been gained into the mechanisms of catalysis in the active sites of the two half-reactions, catalytic coupling, allosteric control by feedback inhibitors and the channeling of reaction and metabolic intermediates.

Enzymes utilizing glutamine as an amide donor

Amide nitrogen from glutamine is a major source of nitrogen atoms incorporated biosynthetically into other amino acids, purine and pyrimidine bases, amino-sugars, and coenzymes. A family comprised of at least sixteen amidotransferases are known to catalyze amide nitrogen transfer from glutamine to their acceptor substrates. Recent fine structural advances, largely as a result of X-ray crystallography, now provide structure-based mechanisms that help to explain fundamental aspects of the catalytic and regulatory interactions of several of these aminotransferases. This chapter provides an overview of this recent progress made on the characterization of amidotransferase structure and mechanism.

Crystal structure of glutamine phosphoribosylpyrophosphate amidotransferase from Escherichia coli

Crystal structures of glutamine phosphoribosylpyrophosphate (PRPP) amidotransferase from Escherichia coli have been determined to 2.0-A resolution in the absence of ligands, and to 2.5-A resolution with the feedback inhibitor AMP bound to the PRPP catalytic site. Glutamine PRPP amidotransferase (GPATase) employs separate catalytic domains to abstract nitrogen from the amide of glutamine and to transfer nitrogen to the acceptor substrate PRPP. The unliganded and AMP-bound structures, which are essentially identical, are interpreted as the inhibited form of the enzyme because the two active sites are disconnected and the PRPP active site is solvent exposed. The structures were compared with a previously reported 3.0-A structure of the homologous Bacillus subtilis enzyme (Smith JL et al., 1994, Science 264:1427-1433). The comparison indicates a pattern of conservation of peptide structures involved with catalysis and variability in enzyme regulatory functions. Control of glutaminase activity, communication between the active sites, and regulation by feedback inhibitors are addressed differently by E. coli and B. subtilis GPATases. The E. coli enzyme is a prototype for the metal-free GPATases, whereas the B. subtilis enzyme represents the metal-containing enzymes. The structure of the E. coli enzyme suggests that a common ancestor of the two enzyme subfamilies may have included an Fe-S cluster.

Structure and function of the glutamine phosphoribosylpyrophosphate amidotransferase glutamine site and communication with the phosphoribosylpyrophosphate site

Glutamine phosphoribosylpyrophosphate (PRPP) amidotransferase from Escherichia coli exhibits a basal PRPP-independent glutaminase activity having a kcat/Km that is 0.3% of fully active enzyme. Binding of PRPP activates the enzyme by a structural change that lowers the Km for glutamine 100-fold and couples glutamine hydrolysis to synthesis of 5-phosphoribosylamine. By analysis of the x-ray structure of the glutamine site containing bound 6-diazo-5-oxonorleucine, a glutamine affinity analog, and by site-directed mutagenesis we have identified residues important for glutamine binding, catalysis, and coupling with PRPP. Tyr74 is a key residue in the coupling between the sites for glutamine in the NH2-terminal domain and PRPP in the COOH-terminal domain. Arg73 and Asp127 have roles in glutamine binding. The x-ray structure indicates that there are no amino acid side chains sufficiently close to Cys1 to participate as a proton acceptor in formation of the thiolate needed for nucleophilic attack on the carboxamide of glutamine, nor as a general acid for amide nitrogen transfer. Based on the x-ray model of the glutamine site and analysis of a mutant enzyme we propose that the free NH2 terminus of Cys1 functions as the proton acceptor and donor. The results indicate that the side chain of Asn101 and the backbone nitrogen of Gly102 function to stabilize a tetrahedral oxyanion resulting from attack of Cys1 on the glutamine carboxamide. Cys1, Arg73, Asn101, Gly102, and Asp127 are conserved in the NH2-terminal domain of a subfamily of amidotransferases that includes asparagine synthetase, glucosamine 6-phosphate synthase, and glutamate synthase, implying a common function in the four enzymes. Tyr74, on the other hand, is conserved only in glutamine PRPP amidotransferase sequences consistent with a specific role in interdomain coupling. The catalytic framework of key glutamine site residues supports the assignment of glutamine PRPP amidotransferase to a recently described Ntn (NH2-terminal nucleophile) hydrolase family of enzymes.

A stable carbocyclic analog of 5-phosphoribosyl-1-pyrophosphate to probe the mechanism of catalysis and regulation of glutamine phosphoribosylpyrophosphate amidotransferase

Glutamine phosphoribosylpyrophosphate (PRPP) amidotransferase catalysis and regulation were studied using a new stable carbocyclic analog of PRPP, 1-alpha-pyrophosphoryl-2-alpha, 3-alpha-dihydroxy-4-beta-cyclopentane-methanol-5-phosphate (cPRPP). Although cPRPP competes with PRPP for binding to the catalytic C site of the Escherichia coli enzyme, two lines of evidence demonstrate that cPRPP, unlike PRPP, does not promote an active enzyme conformation. First, cPRPP was not able to "activate" Cys1 for reaction with glutamine or a glutamine affinity analog. The ring oxygen of PRPP may thus be necessary for the conformation change that activates Cys1 for catalysis. Second, binding of cPRPP to the C site blocks binding of AMP and GMP, nucleotide end product inhibitors, to this site. However, the binding of nucleotide to the allosteric site was essentially unaffected by cPRPP in the C site. Since it is expected that nucleotide inhibitors would bind with low affinity to the active enzyme conformation, the nucleotide binding data support the conclusion that cPRPP does not activate the enzyme.