Leishmania donovani encodes a functional enzyme involved in vitamin c biosynthesis: Arabino-1,4-lactone oxidase

L-gulono-γ-lactone Oxidase, the Key Enzyme for L-Ascorbic Acid Biosynthesis

L-ascorbic acid (AsA, vitamin C) plays a vital role in preventing various diseases, particularly scurvy. AsA is known for its antioxidant properties, which help protect against reactive oxygen species generated from metabolic activities; however, at high doses, it may exhibit pro-oxidative effects. The final step in AsA biosynthesis is catalyzed by L-gulono-γ-lactone oxidase (GULO). This enzyme is present in many organisms, but some animals, including humans, guinea pigs, bats, and other primates, are unable to synthesize AsA due to the absence of a functional GULO gene. The GULO enzyme belongs to the family of aldonolactone oxidoreductases (AlORs) and contains two conserved domains, an N-terminal FAD-binding region and a C-terminal HWXK motif capable of binding the flavin cofactor. In this review, we explore AsA production, the biosynthetic pathways of AsA, and the localization of GULO-like enzymes in both animal and plant cells. Additionally, we compare the amino acid sequences of AlORs across different species and summarize the findings related to their enzymatic activity. Interestingly, a recombinant C-terminal rat GULO (the cytoplasmic domain of the rat GULO expressed in Escherichia coli) demonstrated enzymatic activity. This suggests that the binding of the flavin cofactor to the HWXK motif at the C-terminus is sufficient for the formation of the enzyme's active site. Another enzyme, GULLO7 from Arabidopsis thaliana, also lacks the N-terminal FAD-binding domain and is strongly expressed in mature pollen, although its activity has not been specifically measured.

Recombinant C-Terminal Catalytic Domain of Rat L-Gulono Lactone Oxidase Produced in Bacterial Cells Is Enzymatically Active

The L-gulonolactone oxidase enzyme (GULO) catalyzes the last step of L-ascorbic acid (vitamin C) biosynthesis. This enzymatic activity is lost in primates. The full-length rat GULO has been previously produced in plants and demonstrated to be active. In this study, we compared the activity of two variants of GULO produced in Escheriachia coli cells, full-length rat GULO (fGULO) and its C-terminal catalytic domain (cGULO). The expression and purification of the recombinant proteins were optimized, and their biological activity was confirmed by two methods, the GULO activity assay in the protein extracts and the 'in-gel' staining for GULO activity. Both variants of recombinant GULO were biologically active in both assays. However, cGULO is more promising than fGULO for ascorbic acid production because it is more efficiently produced by bacteria. Furthermore, the optimal activities of the fGULO and cGULO recombinant proteins were observed at pH 7 and 6.5, and at temperatures of 40 and 30 °C, respectively. Kinetic studies revealed that at low substrate concentrations, Km values for fGULO and cGULO were 53.5 ± 5 and 42 ± 6.3 µM, respectively.

A review on potential therapeutic targets for the treatment of leishmaniasis

Leishmania, a protozoan parasite, is responsible for the occurrence of leishmaniasis, a disease that is prevalent in tropical regions. Visceral Leishmaniasis (VL), also known as kala-azar in Asian countries, is one of the most significant forms of VL, along with Cutaneous Leishmaniasis (CL) and Mucocutaneous Leishmaniasis (ML). Management of this condition typically entails the use of chemotherapy as the sole therapeutic option. The current treatments for leishmaniasis present several drawbacks, including a multitude of side effects, prolonged treatment duration, disparate efficacy across different regions, and the emergence of resistance. To address this urgent need, it is imperative to identify alternative treatments that are both safer and more effective. The identification of appropriate pharmacological targets in conjunction with biological pathways constitutes the initial stage of drug discovery. In this review, we have addressed the key metabolic pathways that represent potential pharmacological targets as well as prominent treatment options for leishmaniasis.

Molecular-level strategic goals and repressors in Leishmaniasis - Integrated data to accelerate target-based heterocyclic scaffolds

Leishmaniasis is a complex of neglected tropical diseases caused by various species of leishmanial parasites that primarily affect the world's poorest people. A limited number of standard medications are available for this disease that has been used for several decades, these drugs have many drawbacks such as resistance, higher cost, and patient compliance, making it difficult to reach the poor. The search for novel chemical entities to treat leishmaniasis has led to target-based scaffold research. Among several identified potential molecular targets, enzymes involved in the purine salvage pathway include polyamine biosynthetic process, such as arginase, ornithine decarboxylase, S-adenosylmethionine decarboxylase, spermidine synthase, trypanothione reductase as well as enzymes in the DNA cell cycle, such as DNA topoisomerases I and II plays vital role in the life cycle survival of leishmanial parasite. This review mainly focuses on various heterocyclic scaffolds, and their specific inhibitory targets against leishmaniasis, particularly those from the polyamine biosynthesis pathway and DNA topoisomerases with estimated activity studies of various heterocyclic analogs in terms of their IC₅₀ or EC₅₀ value, reported molecular docking analysis from available published literatures.

Next-Generation Leishmanization: Revisiting Molecular Targets for Selecting Genetically Engineered Live-Attenuated Leishmania

Despite decades of research devoted to finding a vaccine against leishmaniasis, we are still lacking a safe and effective vaccine for humans. Given this scenario, the search for a new prophylaxis alternative for controlling leishmaniasis should be a global priority. Inspired by leishmanization-a first generation vaccine strategy where live L. major parasites are inoculated in the skin to protect against reinfection-live-attenuated Leishmania vaccine candidates are promising alternatives due to their robust elicited protective immune response. In addition, they do not cause disease and could provide long-term protection upon challenge with a virulent strain. The discovery of a precise and easy way to perform CRISPR/Cas-based gene editing allowed the selection of safer null mutant live-attenuated Leishmania parasites obtained by gene disruption. Here, we revisited molecular targets associated with the selection of live-attenuated vaccinal strains, discussing their function, their limiting factors and the ideal candidate for the next generation of genetically engineered live-attenuated Leishmania vaccines to control leishmaniasis.

Advances in Novel Animal Vitamin C Biosynthesis Pathways and the Role of Prokaryote-Based Inferences to Understand Their Origin

Vitamin C (VC) is an essential nutrient required for the optimal function and development of many organisms. VC has been studied for many decades, and still today, the characterization of its functions is a dynamic scientific field, mainly because of its commercial and therapeutic applications. In this review, we discuss, in a comparative way, the increasing evidence for alternative VC synthesis pathways in insects and nematodes, and the potential of myo-inositol as a possible substrate for this metabolic process in metazoans. Methodological approaches that may be useful for the future characterization of the VC synthesis pathways of Caenorhabditis elegans and Drosophila melanogaster are here discussed. We also summarize the current distribution of the eukaryote aldonolactone oxidoreductases gene lineages, while highlighting the added value of studies on prokaryote species that are likely able to synthesize VC for both the characterization of novel VC synthesis pathways and inferences on the complex evolutionary history of such pathways. Such work may help improve the industrial production of VC.

The vast repertoire of carbohydrate oxidases: An overview

Carbohydrates are widely abundant molecules present in a variety of forms. For their biosynthesis and modification, nature has evolved a plethora of carbohydrate-acting enzymes. Many of these enzymes are of particular interest for biotechnological applications, where they can be used as biocatalysts or biosensors. Among the enzymes catalysing conversions of carbohydrates are the carbohydrate oxidases. These oxidative enzymes belong to different structural families and use different cofactors to perform the oxidation reaction of CH-OH bonds in carbohydrates. The variety of carbohydrate oxidases available in nature reflects their specificity towards different sugars and selectivity of the oxidation site. Thanks to their properties, carbohydrate oxidases have received a lot of attention in basic and applied research, such that nowadays their role in biotechnological processes is of paramount importance. In this review we provide an overview of the available knowledge concerning the known carbohydrate oxidases. The oxidases are first classified according to their structural features. After a description on their mechanism of action, substrate acceptance and characterisation, we report on the engineering of the different carbohydrate oxidases to enhance their employment in biocatalysis and biotechnology. In the last part of the review we highlight some practical applications for which such enzymes have been exploited.

Suramin could block the activity of Arabinono-1, 4-lactone oxidase enzyme from Leishmania donovani: structure-based screening and molecular dynamics analyses

BACKGROUND

Leishmania donovani, a parasitic protozoan causing visceral leishmaniasis, can lead to a dangerous and often fatal disease in humans. Current treatment for leishmaniasis may have severe side effects, low efficacy and high cost, hence an immediate need for new efficient drugs is essential. Arabinono-1, 4-lactone oxidase enzyme from Leishmania donovani (LdALO), which catalyzes the last step of the ascorbate biosynthesis pathway, has been considered as a potential target for antileishmanial drugs design.

METHODS

The current study was performed with an in silico approach to predict novel inhibitory molecules against the LdALO enzyme. Various modeling and refinement processes were employed to obtain a reliable 3D structure.

RESULTS

The best LdALO model with the highest qualitative model energy analysis score was predicted by the Robetta server and subsequently refined by 3D refine and ModLoop servers. The high quality of the final LdALO model was confirmed using model assessment software. Based on docking analysis results, we predicted 10 inhibitory molecules of a US Food and Drug Administration-approved library, with appropriate criteria regarding energy binding and interaction with the main functionally active sites of LdALO, indicating that they could be significant targets for further drug design investigations against L. donovani.

CONCLUSION

Suramin is used to treat the first stage of African sleeping sickness and its mechanism of action is unknown. Our results showed that suramin was the best-predicted inhibitor compound for LdALO enzyme activity.

Quantitative secretome analysis unravels new secreted proteins in Amphotericin B resistant Leishmania donovani

Leishmaniasis is second most neglected disease after malaria and seems to be a worldwide concern because of increased drug resistance and non-availability of approved vaccine. The underlying molecular mechanism of drug resistance (Amp B) in Leishmania parasites still remains elusive. Herein, the present study investigated differentially expressed secreted proteins of Amphotericin B sensitive (S) and resistant (R) isolate of Leishmania donovani by using label free quantitative LC-MS/MS approach. A total of 406 differentially expressed secreted proteins were found between sensitive (S) and resistant (R) isolate. Among 406 proteins, 32 were significantly up regulated (>2.0 fold) while 22 were down regulated (<0.5 fold) in resistant isolate of L. donovani. Further, differentially expressed proteins were classified into 11 various biological processes. Interestingly, identified up regulated proteins in resistant parasites were dominated in carbohydrate metabolism, stress response, transporters and proteolysis. Western blot and enzymatic activity of identified proteins validate our proteomic findings. Finally, our study demonstrated some new secreted proteins associated with Amp B resistance which provides a basis for further investigations to understand the role of proteins in L. donovani. BIOLOGICAL SIGNIFICANCE: Although great advances have been achieved in the diagnosis and treatment of leishmaniasis, still drug resistance is major hurdle in control of disease. Present study will enhance the deeper understanding of altered metabolic pathways involved in Amp B resistance mechanism and provide possible new proteins which can be potential candidate either for exploring as new drug target or vaccine. Protein-protein interactions highlighted the up-regulated metabolic pathways in resistant parasites which further unravel the adaptive mechanism of parasites.

Characterization of Two Arabidopsis L-Gulono-1,4-lactone Oxidases, AtGulLO3 and AtGulLO5, Involved in Ascorbate Biosynthesis

L-Ascorbic acid (AsA, vitamin C) is an essential antioxidant for plants and animals. There are four known ascorbate biosynthetic pathways in plants: the L-galactose, L-gulose, D-galacturonate, and myo-inositol routes. These pathways converge into two AsA precursors: L-galactono-1,4-lactone and L-gulono-1,4-lactone (L-GulL). This work focuses on the study of L-gulono-1,4-lactone oxidase (GulLO), the enzyme that works at the intersect of the gulose and inositol pathways. Previous studies have shown that feeding L-gulono-1,4-lactone to multiple plants leads to increased AsA. There are also reports showing GulLO activity in plants. We describe the first detailed characterization of a plant enzyme specific to oxidize L-GulL to AsA. We successfully purified a recombinant Arabidopsis GulLO enzyme (called AtGulLO5) in a transient expression system. The biochemical properties of this enzyme are similar to the ones of bacterial isozymes in terms of substrate specificity, subcellular localization, use of flavin adenine dinucleotide (FAD) as electron acceptor, and specific activity. AtGulLO5 is an exclusive dehydrogenase with an absolute specificity for L-GulL as substrate thus differing from the existing plant L-galactono-1,4-lactone dehydrogenases and mammalian GulLOs. Feeding L-GulL to N. benthamiana leaves expressing AtGulLO5 constructs led to increased foliar AsA content, but it was not different from that of controls, most likely due to the observed low catalytic efficiency of AtGulLO5. Similar results were also obtained with another member of the AtGulLO family (AtGulLO3) that appears to have a rapid protein turnover. We propose that AsA synthesis through L-GulL in plants is regulated at the post-transcriptional level by limiting GulLO enzyme availability.

Recent progress on the characterization of aldonolactone oxidoreductases

L-Ascorbic acid (ascorbate, AsA, vitamin C) is essential for animal and plant health. Despite our dependence on fruits and vegetables to fulfill our requirement for this vitamin, the metabolic network leading to its formation in plants is just being fully elucidated. There is evidence supporting the operation of at least four biosynthetic pathways leading to AsA formation in plants. These routes use D-mannose/L-galactose, L-gulose, D-galacturonate, and myo-inositol as the main precursors. This review focuses on aldonolactone oxidoreductases, a subgroup of the vanillyl alcohol oxidase (VAO; EC 1.1.3.38) superfamily, enzymes that catalyze the terminal step in AsA biosynthesis in bacteria, protozoa, animals, and plants. In this report, we review the properties of well characterized aldonolactone oxidoreductases to date. A shared feature in these proteins is the presence of a flavin cofactor as well as a thiol group. The flavin cofactor in many cases is bound to the N terminus of the enzymes or to a recently discovered HWXK motif in the C terminus. The binding between the flavin moiety and the protein can be either covalent or non-covalent. Substrate specificity and subcellular localization differ among the isozymes of each kingdom. All oxidases among these enzymes possess dehydrogenase activity, however, exclusive dehydrogenases are also found. We also discuss recent evidence indicating that plants have both L-gulono-1,4-lactone oxidases and L-galactono-1,4-lactone dehydrogenases involved in AsA biosynthesis.

Intracellular Survival of Leishmania major Depends on Uptake and Degradation of Extracellular Matrix Glycosaminoglycans by Macrophages

Leishmania parasites replicate within the phagolysosome compartment of mammalian macrophages. Although Leishmania depend on sugars as a major carbon source during infections, the nutrient composition of the phagolysosome remains poorly described. To determine the origin of the sugar carbon source in macrophage phagolysosomes, we have generated a N-acetylglucosamine acetyltransferase (GNAT) deficient Leishmania major mutant (∆gnat) that is auxotrophic for the amino sugar, N-acetylglucosamine (GlcNAc). This mutant was unable to grow or survive in ex vivo infected macrophages even when macrophages were cultivated in presence of exogenous GlcNAc. In contrast, the L. major ∆gnat mutant induced normal skin lesions in mice, suggesting that these parasites have access to GlcNAc in tissue macrophages. Intracellular growth of the mutant in ex vivo infected macrophages was restored by supplementation of the macrophage medium with hyaluronan, a GlcNAc-rich extracellular matrix glycosaminoglycan. Hyaluronan is present and constitutively turned-over in Leishmania-induced skin lesions and is efficiently internalized into Leishmania containing phagolysosomes. These findings suggest that the constitutive internalization and degradation of host glycosaminoglycans by macrophages provides Leishmania with essential carbon sources, creating a uniquely favorable niche for these parasites.

Association of liposome-encapsulated trivalent antimonial with ascorbic acid: an effective and safe strategy in the treatment of experimental visceral leishmaniasis

BACKGROUND

Visceral leishmaniasis (VL) is a chronic debilitating disease endemic in tropical and subtropical areas, caused by protozoan parasites of the genus Leishmania. Annually, it is estimated the occurrence of 0.2 to 0.4 million new cases of the disease worldwide. Considering the lack of an effective vaccine the afflicted population must rely on both, an accurate diagnosis and successful treatment to combat the disease. Here we propose to evaluate the efficacy of trivalent antimonial encapsulated in conventional liposomes, in association with ascorbic acid, by monitoring its toxicity and efficacy in BALB/c mice infected with Leishmania infantum.

METHODOLOGY/PRINCIPAL FINDINGS

Infected mice were subjected to single-dose treatments consisting in the administration of either free or liposome-encapsulated trivalent antimony (SbIII), in association or not with ascorbic acid. Parasite burden was assessed in the liver, spleen and bone marrow using the serial limiting dilution technique. After treatment, tissue alterations were examined by histopathology of liver, heart and kidney and confirmed by serum levels of classic biomarkers. The phenotypic profile of splenocytes was also investigated by flow cytometry. Treatment with liposome-encapsulated SbIII significantly reduced the parasite burden in the liver, spleen and bone marrow. Co-administration of ascorbic acid, with either free SbIII or its liposomal form, did not interfere with its leishmanicidal activity and promoted reduced toxicity particularly to the kidney and liver tissues.

CONCLUSIONS/SIGNIFICANCE

Among the evaluated posological regimens treatment of L. infantum-infected mice with liposomal SbIII, in association with ascorbic acid, represented the best alternative as judged by its high leishmanicidal activity and absence of detectable toxic effects. Of particular importance, reduction of parasite burden in the bone marrow attested to the ability of SbIII-carrying liposomes to efficiently reach this body compartment.

Aldonolactone oxidoreductases

Vitamin C is a widely used vitamin. Here we review the occurrence and properties of aldonolactone oxidoreductases, an important group of flavoenzymes responsible for the ultimate production of vitamin C and its analogs in animals, plants, and single-cell organisms.

Ascorbate peroxidase acts as a novel determiner of redox homeostasis in Leishmania

SIGNIFICANCE

Reactive oxygen species (ROS) are produced as natural byproducts of metabolism and respiration. While physiological levels of ROS are required for vital cellular functions (e.g., development and proliferation), a living organism is faced with constant challenges due to accumulation or overproduction of ROS throughout its life. The life cycle of Leishmania parasite has led it to confront the highly oxidizing environment in the macrophage phagosomes, necessitating ROS homeostasis and signaling as key strategies for successful survival and pathogenicity.

RECENT ADVANCES

Ascorbate peroxidase from Leishmania major (LmAPX) is the only heme peroxidase identified so far in Leishmania. Structural analysis and functional characterization of LmAPX have yielded interesting and novel insight on this enzyme. The protein has been found to be a hybrid of cytochrome c peroxidase and ascorbate peroxidase. This enzyme is colocalized with cytochrome c in the inner mitochondrial membrane facing the intermembrane space and shows higher activity toward cytochrome c oxidation.

CRITICAL ISSUES

Overexpression of LmAPX in L. major cells confers tolerance to oxidative stress-mediated cardiolipin oxidation and consequently protects cells from extensive protein damage. LmAPX-/- mutants show higher intracellular hydrogen peroxide (H₂O₂), which might signal for cellular transformation from noninfective procyclic to infective metacyclic form and ultimately apoptosis.

FUTURE DIRECTIONS

Manipulation of LmAPX expression has significantly added to the present understanding of the parasite's defense network against oxidative damage caused by H₂O₂. The future investigations will address more exactly the signaling pathways involved in redox homeostasis.

Conserved or lost: molecular evolution of the key gene GULO in vertebrate vitamin C biosynthesis

L-gulono-gamma-lactone oxidase (GULO) catalyzes the final step in vertebrate vitamin C biosynthesis. Vitamin C-incapable vertebrates lack the GULO gene. Gene structure and phylogenetic analyses showed that vertebrate GULO genes are 64-95% identical at the amino acid level and consist of 11 conserved exons. GULO pseudogenes have multiple indel mutations and premature stop codons in higher primates, guinea pigs, and some bats. No GULO-like sequences were identified in teleost fishes. During animal GULO evolution, exon F was subdivided into F1 and F2. Additional GULO retropseudogenes were identified in dogs, cats, and giant pandas. GULO-flanking genome regions acquired frequent segment translocations and inversions during vertebrate evolution. Purifying selection was detected across vertebrate GULO genes (d(N)/d(S) = 0.069), except for some positively selected sites identified in sharks and frogs. These positive sites demonstrated little functional significance when mapped onto the three-dimensional GULO protein structure. Vertebrate GULO genes are conserved except for those that are lost.