Glucose 6-phosphate dehydrogenase 6-phosphogluconolactonase: characterization of the Plasmodium vivax enzyme and inhibitor studies

Glucose 6-phosphate dehydrogenase variants increase NADPH pools for yeast isoprenoid production

Isoprenoid biosynthesis has a significant requirement for the co-factor NADPH. Thus, increasing NADPH levels for enhancing isoprenoid yields in synthetic biology is critical. Previous efforts have focused on diverting flux into the pentose phosphate pathway or overproducing enzymes that generate NADPH. In this study, we instead focused on increasing the efficiency of enzymes that generate NADPH. We first established a robust genetic screen that allowed us to screen improved variants. The pentose phosphate pathway enzyme, glucose 6-phosphate dehydrogenase (G6PD), was chosen for further improvement. Different gene fusions of G6PD with the downstream enzyme in the pentose phosphate pathway, 6-phosphogluconolactonase (6PGL), were created. The linker-less G6PD-6PGL fusion displayed the highest activity, and although it had slightly lower activity than the WT enzyme, the affinity for G6P was higher and showed higher yields of the diterpenoid sclareol in vivo. A second gene fusion approach was to fuse G6PD to truncated HMG-CoA reductase, the rate-limiting step and also the major NADPH consumer in the pathway. Both domains were functional, and the fusion also yielded higher sclareol levels. We simultaneously carried out a rational mutagenesis approach with G6PD, which led to the identification of two mutants of G6PD, N403D and S238QI239F, that showed 15-25% higher activity in vitro. The diterpene sclareol yields were also increased in the strains overexpressing these mutants relative to WT G6PD, and these will be very beneficial in synthetic biology applications.

Structural and functional characterization of 6-phosphogluconate dehydrogenase in Plasmodium falciparum (3D7) and identification of its potent inhibitors

The malarial parasite Plasmodium falciparum predominantly causes severe malaria and deaths worldwide. Moreover, resistance developed by P. falciparum to frontline drugs in recent years has markedly increased malaria-related deaths in South Asian Countries. Ribulose 5-phosphate and NADPH synthesized by Pentose Phosphate Pathway (PPP) act as a direct precursor for nucleotide synthesis and P. falciparum survival during oxidative challenges in the intra-erythrocytic growth phase . In the present study, we have elucidated the structure and functional characteristics of 6-phosphogluconate dehydrogenase (6PGD) in P. falciparum and have identified potent hits against 6PGD by pharmacophore-based virtual screening with ZINC and ChemBridge databases. Molecular docking and Molecular dynamics simulation, binding free energies (MMGBSA & MMPBSA), and Density Functional Theory (DFT) calculations were integratively employed to validate and prioritize the most potential hits. The 6PGD structure was found to have an open and closed conformation during MD simulation. The apo form of 6PGD was found to be in closed conformation, while a open conformation attributed to facilitating binding of cofactor. It was also inferred from the conformational analysis that the small domain of 6PGD has a high influence in altering the conformation that may aid in open/closed conformation of 6PGD. The top three hits identified using pharmacophore hypotheses were ChemBridge_11084819, ChemBridge_80178394, and ChemBridge_17912340. Though all three hits scored a high glide score, MMGBSA, and favorable ADMET properties, ChemBridge_11084819 and ChemBrdige_17912340 showed higher stability and binding free energy. Moreover, these hits also featured stable H-bond interactions with the active loop of 6PGD with binding free energy comparable to substrate-bound complex. Therefore, the ChemBridge_11084819 and ChemBridge_17912340 moieties demonstrate to have high therapeutic potential against 6PGD in P. falciparum.Communicated by Ramaswamy H. Sarma.

Fused Enzyme Glucose-6-Phosphate Dehydrogenase::6-Phosphogluconolactonase (G6PD::6PGL) as a Potential Drug Target in Giardia lamblia, Trichomonas vaginalis, and Plasmodium falciparum

Several microaerophilic parasites such as Giardia lamblia, Trichomonas vaginalis, and Plasmodium falciparum are major disease-causing organisms and are responsible for spreading infections worldwide. Despite significant progress made in understanding the metabolism and molecular biology of microaerophilic parasites, chemotherapeutic treatment to control it has seen limited progress. A current proposed strategy for drug discovery against parasitic diseases is the identification of essential key enzymes of metabolic pathways associated with the parasite's survival. In these organisms, glucose-6-phosphate dehydrogenase::6-phosphogluconolactonase (G6PD:: 6PGL), the first enzyme of the pentose phosphate pathway (PPP), is essential for its metabolism. Since G6PD:: 6PGL provides substrates for nucleotides synthesis and NADPH as a source of reducing equivalents, it could be considered an anti-parasite drug target. This review analyzes the anaerobic energy metabolism of G. lamblia, T. vaginalis, and P. falciparum, with a focus on glucose metabolism through the pentose phosphate pathway and the significance of the fused G6PD:: 6PGL enzyme as a therapeutic target in the search for new drugs.

Structural Analysis of Plasmodium falciparum Hexokinase Provides Novel Information about Catalysis Due to a Plasmodium-Specific Insertion

The protozoan parasite Plasmodium falciparum is the causative pathogen of the most severe form of malaria, for which novel strategies for treatment are urgently required. The primary energy supply for intraerythrocytic stages of Plasmodium is the production of ATP via glycolysis. Due to the parasite's strong dependence on this pathway and the significant structural differences of its glycolytic enzymes compared to its human counterpart, glycolysis is considered a potential drug target. In this study, we provide the first three-dimensional protein structure of P. falciparum hexokinase (PfHK) containing novel information about the mechanisms of PfHK. We identified for the first time a Plasmodium-specific insertion that lines the active site. Moreover, we propose that this insertion plays a role in ATP binding. Residues of the insertion further seem to affect the tetrameric interface and therefore suggest a special way of communication among the different monomers. In addition, we confirmed that PfHK is targeted and affected by oxidative posttranslational modifications (oxPTMs). Both S-glutathionylation and S-nitrosation revealed an inhibitory effect on the enzymatic activity of PfHK.

An Undefined Interaction between Polyamines and Heat Shock Proteins Leads to Cellular Protection in Plasmodium falciparum and Proliferating Cells in Various Organisms

Environmental stimuli can distress the internal reaction of cells and their normal function. To react promptly to sudden environmental changes, a cascade of heat shock proteins (Hsps) functions to protect and act as housekeepers inside the cells. In parallel to the heat shock response, the metabolic polyamine (PA) status changes. Here, we discuss possible ways of putative interactions between Hsps and polyamines in a wide lineage of eukaryotic model organisms with a particular focus on parasitic protozoa such as Plasmodium falciparum (P. falciparum). The supposed interaction between polyamines and Hsps may protect the parasite from the sudden change in temperature during transmission from the female Anopheles mosquito to a human host. Recent experiments performed with the spermidine mimetic inhibitor 15-deoxyspergualine in Plasmodium in vitro cultures show that the drug binds to the C-terminal EEVD motif of Hsp70. This leads to inhibition of protein biosynthesis caused by prevention of eIF5A2 phosphorylation and eukaryotic initiation factor 5A (eIF5A) modification. These observations provide further evidence that PAs are involved in the regulation of protein biosynthesis of Hsps to achieve a protective effect for the parasite during transmission.

Crystal structure of Leishmania donovani glucose 6-phosphate dehydrogenase reveals a unique N-terminal domain

Since unicellular parasites highly depend on NADPH as a source for reducing equivalents, the pentose phosphate pathway, especially the first and rate-limiting NADPH-producing enzyme glucose 6-phosphate dehydrogenase (G6PD), is considered an excellent antitrypanosomatid drug target. Here we present the crystal structure of Leishmania donovani G6PD (LdG6PD) elucidating the unique N-terminal domain of Kinetoplastida G6PDs. Our investigations on the function of the N-domain suggest its involvement in the formation of a tetramer that is completely different from related Trypanosoma G6PDs. Structural and functional investigations further provide interesting insights into the binding mode of LdG6PD, following an ordered mechanism, which is confirmed by a G6P-induced domain shift and rotation of the helical N-domain. Taken together, these insights into LdG6PD contribute to the understanding of G6PDs' molecular mechanisms and provide an excellent basis for further drug discovery approaches.

Chemistry of polyhalogenated nitrobutadienes, 17: Efficient synthesis of persubstituted chloroquinolinyl-1H-pyrazoles and evaluation of their antimalarial, anti-SARS-CoV-2, antibacterial, and cytotoxic activities

A series of 26 novel 1-(7-chloroquinolin-4-yl)-4-nitro-1H-pyrazoles bearing a dichloromethyl and an amino or thio moiety at C3 and C5 has been prepared in yields up to 72% from the reaction of 1,1-bisazolyl-, 1-azolyl-1-amino-, and 1-thioperchloro-2-nitrobuta-1,3-dienes with 7-chloro-4-hydrazinylquinoline. A new way for the formation of a pyrazole cycle from 3-methyl-2-(2,3,3-trichloro-1-nitroallylidene)oxazolidine (6) is also described. In addition, the antimalarial activity of the synthesized compounds has been evaluated in vitro against the protozoan malaria parasite Plasmodium falciparum. Notably, the 7-chloro-4-(5-(dichloromethyl)-4-nitro-3-(1H-1,2,4-triazol-1-yl)-1H-pyrazol-1-yl)quinoline (3b) and 7-chloro-4-(3-((4-chlorophenyl)thio)-5-(dichloromethyl)-4-nitro-1H-pyrazol-1-yl)quinoline (9e) inhibited the growth of the chloroquine-sensitive Plasmodium falciparum strain 3D7 with EC50 values of 0.2 ± 0.1 µM (85 ng/mL, 200 nM) and 0.2 ± 0.04 µM (100 ng/mL, 200 nM), respectively. Two compounds (3b and 10d) have also been tested for anti-SARS-CoV-2, antibacterial, and cytotoxic activity.

Boosting the Discovery of Small Molecule Inhibitors of Glucose-6-Phosphate Dehydrogenase for the Treatment of Cancer, Infectious Diseases, and Inflammation

We present an overview of small molecule glucose-6-phosphate dehydrogenase (G6PD) inhibitors that have potential for use in the treatment of cancer, infectious diseases, and inflammation. Both steroidal and nonsteroidal inhibitors have been identified with steroidal inhibitors lacking target selectivity. The main scaffolds encountered in nonsteroidal inhibitors are quinazolinones and benzothiazinones/benzothiazepinones. Three molecules show promise for development as antiparasitic (25 and 29) and anti-inflammatory (32) agents. Regarding modality of inhibition (MOI), steroidal inhibitors have been shown to be uncompetitive and reversible. Nonsteroidal small molecules have exhibited all types of MOI. Strategies to boost the discovery of small molecule G6PD inhibitors include exploration of structure-activity relationships (SARs) for established inhibitors, employment of high-throughput screening (HTS), and fragment-based drug discovery (FBDD) for the identification of new hits. We discuss the challenges and gaps associated with drug discovery efforts of G6PD inhibitors from in silico, in vitro, and in cellulo to in vivo studies.

An Optimized Dihydrodibenzothiazepine Lead Compound (SBI-0797750) as a Potent and Selective Inhibitor of Plasmodium falciparum and P. vivax Glucose 6-Phosphate Dehydrogenase 6-Phosphogluconolactonase

In Plasmodium, the first two and rate-limiting enzymes of the pentose phosphate pathway, glucose 6-phosphate dehydrogenase (G6PD) and the 6-phosphogluconolactonase, are bifunctionally fused to a unique enzyme named GluPho, differing structurally and mechanistically from the respective human orthologs. Consistent with the enzyme's essentiality for malaria parasite proliferation and propagation, human G6PD deficiency has immense impact on protection against severe malaria, making PfGluPho an attractive antimalarial drug target. Herein we report on the optimized lead compound N-(((2R,4S)-1-cyclobutyl-4-hydroxypyrrolidin-2-yl)methyl)-6-fluoro-4-methyl-11-oxo-10,11-dihydrodibenzo[b,f][1,4]thiazepine-8-carboxamide (SBI-0797750), a potent and fully selective PfGluPho inhibitor with robust nanomolar activity against recombinant PfGluPho, PvG6PD, and P. falciparum blood-stage parasites. Mode-of-action studies have confirmed that SBI-0797750 disturbs the cytosolic glutathione-dependent redox potential, as well as the cytosolic and mitochondrial H₂O₂ homeostasis of P. falciparum blood stages, at low nanomolar concentrations. Moreover, SBI-0797750 does not harm red blood cell (RBC) integrity and phagocytosis and thus does not promote anemia. SBI-0797750 is therefore a very promising antimalarial lead compound.

Biophysical and Structural Characterization of Ribulose-5-phosphate Epimerase from Leishmania donovani

Pentose phosphate pathway (PPP) plays a crucial role in the maintenance of NADPH/NADP⁺ homeostasis and provides protection against oxidative stress through detoxification of the reactive oxygen species. Ribulose-5-phosphate epimerase (RPE) participates in catalysis of the interconversion of ribulose-5-phosphate (Ru5P) to xylulose-5-phosphate (Xu5P) during PPP, however the structural attributes of this enzyme are still underexplored in many human pathogens including leishmanial parasites. The present study focuses upon cloning, purification and characterization of RPE of Leishmania donovani (LdRPE) using various biophysical and structural approaches. Sequence analysis has shown the presence of trypanosomatid-specific insertions at the N-terminus that are absent in humans and other eukaryotes. Gel filtration chromatography indicated recombinant LdRPE to exist as a dimer in the solution. Circular dichroism studies revealed a higher alpha helical content at physiological pH and temperature that comparatively varies with changing these parameters. Additionally, intrinsic fluorescence and quenching studies of LdRPE have depicted that tryptophan residues are mainly buried in the hydrophobic regions, and the recombinant enzyme is moderately tolerant to urea. Moreover, homology modeling was employed to generate the three-dimensional structure of LdRPE followed by molecular docking with the substrate, product, and substrate analogues. The modeled structure of LdRPE unravelled the presence of conserved active site residues as well as a single binding pocket for the substrate and product, while an in silico study suggested binding of substrate analogues into a similar pocket with more affinity than the substrate. Additionally, molecular dynamics simulation analysis has deciphered complexes of LdRPE with most of the ligands exhibiting more stability than its apo form and lesser fluctuations in active site residues in the presence of ligands. Altogether, our study presents structural insights into leishmanial RPE that could provide the basis for its implication to develop potent antileishmanials.

Glucose-6-Phosphate Dehydrogenase::6-Phosphogluconolactonase from the Parasite Giardia lamblia. A Molecular and Biochemical Perspective of a Fused Enzyme

Giardia lamblia is a single-celled eukaryotic parasite with a small genome and is considered an early divergent eukaryote. The pentose phosphate pathway (PPP) plays an essential role in the oxidative stress defense of the parasite and the production of ribose-5-phosphate. In this parasite, the glucose-6-phosphate dehydrogenase (G6PD) is fused with the 6-phosphogluconolactonase (6PGL) enzyme, generating the enzyme named G6PD::6PGL that catalyzes the first two steps of the PPP. Here, we report that the G6PD::6PGL is a bifunctional enzyme with two catalytically active sites. We performed the kinetic characterization of both domains in the fused G6PD::6PGL enzyme, as well as the individual cloned G6PD. The results suggest that the catalytic activity of G6PD and 6PGL domains in the G6PD::6PGL enzyme are more efficient than the individual proteins. Additionally, using enzymatic and mass spectrometry assays, we found that the final metabolites of the catalytic reaction of the G6PD::6PGL are 6-phosphoglucono-δ-lactone and 6-phosphogluconate. Finally, we propose the reaction mechanism in which the G6PD domain performs the catalysis, releasing 6-phosphoglucono-δ-lactone to the reaction medium. Then, this metabolite binds to the 6PGL domain catalyzing the hydrolysis reaction and generating 6-phosphogluconate. The structural difference between the G. lamblia fused enzyme G6PD::6PGL with the human G6PD indicate that the G6PD::6PGL is a potential drug target for the rational synthesis of novels anti-Giardia drugs.

Discovery and characterization of a novel glucose-6-phosphate dehydrogenase (G6PD) inhibitor via high-throughput screening

Altered glucose-6-phosphate dehydrogenase (G6PD) status is influential in many cellular pathophysiological processes and diseases, making G6PD a potential target for cancer therapy. However, the available G6PD inhibitors are very limited and restricted. Here we developed a reducing equivalent nicotinamide adenine dinucleotide phosphate (NADPH) absorption photometry assay based on enzyme kinetics to characterize G6PD activity. In this way, we performed a high-throughput screening (HTS) to an in house library. And then we identified compound named Wedelolactone inhibiting G6PD strongly in a non-competitive, reversible way. In addition, we did the surface Plasmon Resonance (SPR) assay and indicated the K_D between Wedelolactone and G6PD protein was 3.64 μM. Furthermore, our basic colony formation assay showed the inhibitory effect of Wedelolactone on the proliferation of ovarian cancer cells (IC₅₀ ~ 10 µM). Thus, we provided a high-throughput screening assay to quickly and efficiently discover G6PD inhibitors, and identified Wedelolactone as a G6PD inhibitor, implying that Wedelolactone suppresses ovarian cancer partly through targeting G6PD.

Effectiveness and mechanism study of glutamine on alleviating hypermetabolism in burned rats

OBJECTIVES

This study aimed to explore the effects of glutamine on hypermetabolic reactions in burned rats and its underlying mechanism.

METHODS

Fifty-five Sprague-Dawley rats were randomly divided into three groups, namely, the control (C), burned (B), and burned + glutamine (B + G) groups. Rats in the glutamine treatment group were supplemented with 1 g glutamine per kg body weight. Changes in body weight and resting energy expenditure in all groups were observed daily. Blood glucose and glucose tolerance level were measured on days 1, 3, 7, 10 and 14 after burn injury. On days 7 and 14 after injury, the rats were sacrificed, and the weight and protein content of the skeletal muscle were measured. Moreover, the level of glutamine, inflammatory mediator, nicotinamide adenine dinucleotide phosphate (NADPH), glutathione, and the activity of glutamine metabolic enzymes were measured.

RESULTS

The hypermetabolic reaction after burn injury was significantly inhibited by glutamine administration, and the range of variations in the resting energy expenditure and body weight indicators was narrowed remarkably (P < 0.05 or 0.01), whereas the weight and protein content of the skeletal muscle returned to normal (P < 0.05 or 0.01). Glutamine could increase glutaminase activity in various tissues, promote the utilization of glutamine, and appropriately reduce the degree of organ damage and inflammatory response (P < 0.05 or 0.01). Furthermore, glutamine could promote the synthesis of the reducing substances NADPH and glutathione (P < 0.05 or 0.01).

CONCLUSIONS

Glutamine administration effectively reduces hypermetabolic reactions by promoting NADPH synthesis, inhibiting oxidative stress, and improving glutamine utilization after burn injury.

Characterizing the Fused TvG6PD::6PGL Protein from the Protozoan Trichomonas vaginalis, and Effects of the NADP+ Molecule on Enzyme Stability

This report describes a functional and structural analysis of fused glucose-6-phosphate dehydrogenase dehydrogenase-phosphogluconolactonase protein from the protozoan Trichomonas vaginalis (T. vaginalis). The glucose-6-phosphate dehydrogenase (g6pd) gene from T. vaginalis was isolated by PCR and the sequence of the product showed that is fused with 6pgl gene. The fused Tvg6pd::6pgl gene was cloned and overexpressed in a heterologous system. The recombinant protein was purified by affinity chromatography, and the oligomeric state of the TvG6PD::6PGL protein was found as tetramer, with an optimal pH of 8.0. The kinetic parameters for the G6PD domain were determined using glucose-6-phosphate (G6P) and nicotinamide adenine dinucleotide phosphate (NADP⁺) as substrates. Biochemical assays as the effects of temperature, susceptibility to trypsin digestion, and analysis of hydrochloride of guanidine on protein stability in the presence or absence of NADP⁺ were performed. These results revealed that the protein becomes more stable in the presence of the NADP⁺. In addition, we determined the dissociation constant for the binding (K_d) of NADP⁺ in the protein and suggests the possible structural site in the fused TvG6PD::6PGL protein. Finally, computational modeling studies were performed to obtain an approximation of the structure of TvG6PD::6PGL. The generated model showed differences with the GlG6PD::6PGL protein (even more so with human G6PD) despite both being fused.