Analysis of the vitamin B6 biosynthesis pathway in the human malaria parasite Plasmodium falciparum

Pyridoxal kinase gene deletion leads to impaired growth, deranged redox metabolism and cell cycle arrest in Leishmania donovani

Pyridoxal kinase (PdxK) is a vitamin B6 salvage pathway enzyme which produces pyridoxal phosphate. We have investigated the impact of PdxK deletion in Leishmania donovani on parasite survivability, infectivity and cellular metabolism. LdPdxK mutants were generated by gene replacement strategy. All mutants showed significant reduction in growth in comparison to wild type. For PdxK mediated biochemical perturbations, only heterozygous mutants and complementation mutants were used as the growth of null mutants were compromised. Heterozygous mutant showed reduction invitro infectivity and higher cytosolic and mitochondrial ROS levels. Glutathione levels decreased significantly in heterozygous mutant indicating its involvement in cellular oxidative metabolism. Pyridoxal kinase gene deletion resulted in reduced ATP levels in parasites and arrest at G0/G1 phase of cell cycle. All these perturbations were rescued by PdxK gene complementation. This is the first report to confirm that LdPdxK plays an indispensable role in cell survival, pathogenicity, redox metabolism and cell cycle progression of L. donovani parasites. These results provide substantial evidence supporting PdxK as a therapeutic target for the development of specific antileishmanial drug candidates.

Structure and dynamics of the staphylococcal pyridoxal 5-phosphate synthase complex reveal transient interactions at the enzyme interface

Infectious diseases are a significant cause of death, and recent studies estimate that common bacterial infectious diseases were responsible for 13.6% of all global deaths in 2019. Among the most significant bacterial pathogens is Staphylococcus aureus, accounting for more than 1.1 million deaths worldwide in 2019. Vitamin biosynthesis has been proposed as a promising target for antibacterial therapy. Here, we investigated the biochemical, structural, and dynamic properties of the enzyme complex responsible for vitamin B6 (pyridoxal 5-phosphate, PLP) biosynthesis in S. aureus, which comprises enzymes SaPdx1 and SaPdx2. The crystal structure of the 24-mer complex of SaPdx1-SaPdx2 enzymes indicated that the S. aureus PLP synthase complex forms a highly dynamic assembly with transient interaction between the enzymes. Solution scattering data indicated that SaPdx2 typically binds to SaPdx1 at a substoichiometric ratio. We propose a structure-based view of the PLP synthesis mechanism initiated with the assembly of SaPLP synthase complex that proceeds in a highly dynamic interaction between Pdx1 and Pdx2. This interface interaction can be further explored as a potentially druggable site for the design of new antibiotics.

Antiplasmodial and Antimalarial Activity of 3,5-Diarylidenetetrahydro-2H-pyran-4(3H)-ones via Inhibition of Plasmodium falciparum Pyridoxal Synthase

A series of 22 different 3,5-diarylidenetetrahydro-2H-pyran-4(3H)-ones (DATPs) were synthesized, characterized, and screened for their in vitro antiplasmodial activities against chloroquine (CQ)-sensitive Pf3D7, CQ-resistant PfINDO, and artemisinin-resistant PfMRA-1240 strains of Plasmodium falciparum. DATP 19 (3,5-bis(4-hydroxy-3,5-dimethoxybenzylidene)tetrahydro-2H-pyran-4(3H)-one) was found to be the most potent (IC₅₀ 1.07 μM) against PfMRA-1240, whereas 21 (3,5-bis(3,4,5-trimethoxybenzylidene)tetrahydro-2H-pyran-4(3H)-one) showed IC₅₀ values of 1.72 and 1.44 μM against Pf3D7 and PfINDO, respectively. Resistance indices (RI) as low as 0.2 to 0.5 for 10 (3,5-bis(4-nitrobenzylidene)tetrahydro-2H-pyran-4(3H)-one) and 20 (3,5-bis(3-nitrobenzylidene)tetrahydro-2H-pyran-4(3H)-one), and <1 for most other DATPs reveals their greater potency against resistant strains than the sensitive one. The single-crystal XRD data for DATP 21 are reported. In silico support was obtained through docking studies. Killing all three strains within 4-8 h, these DATPs showed rapid kill kinetics toward the trophozoite stage. Furthermore, DATP 18 (3,5-bis(quinolin-4-ylmethylene)tetrahydro-2H-pyran-4(3H)-one) inhibited PfPdx1 enzyme activity with IC₅₀ 20.34 μM, which is about twofold lower than that (IC₅₀ 43 μM) for an already known inhibitor 4PEHz. At an oral dose of 300 mg/kg body weight, DATPs 19 and 21 were found to be nontoxic to mice, and at 100 mg/kg body weight, DATP 19 was found to suppress parasitaemia, which led to an increase in median survival time by three days relative to untreated control mice in a malaria curative study.

AIBP Regulates Metabolism of Ketone and Lipids but Not Mitochondrial Respiration

Accumulating evidence indicates that the APOA1 binding protein (AIBP)-a secreted protein-plays a profound role in lipid metabolism. Interestingly, AIBP also functions as an NAD(P)H-hydrate epimerase to catalyze the interconversion of NAD(P)H hydrate [NAD(P)HX] epimers and is renamed as NAXE. Thus, we call it NAXE hereafter. We investigated its role in NAD(P)H-involved metabolism in murine cardiomyocytes, focusing on the metabolism of hexose, lipids, and amino acids as well as mitochondrial redox function. Unbiased metabolite profiling of cardiac tissue shows that NAXE knockout markedly upregulates the ketone body 3-hydroxybutyric acid (3-HB) and increases or trends increasing lipid-associated metabolites cholesterol, α-linolenic acid and deoxycholic acid. Paralleling greater ketone levels, ChemRICH analysis of the NAXE-regulated metabolites shows reduced abundance of hexose despite similar glucose levels in control and NAXE-deficient blood. NAXE knockout reduces cardiac lactic acid but has no effect on the content of other NAD(P)H-regulated metabolites, including those associated with glucose metabolism, the pentose phosphate pathway, or Krebs cycle flux. Although NAXE is present in mitochondria, it has no apparent effect on mitochondrial oxidative phosphorylation. Instead, we detected more metabolites that can potentially improve cardiac function (3-HB, adenosine, and α-linolenic acid) in the Naxe^-/- heart; these mice also perform better in aerobic exercise. Our data reveal a new role of NAXE in cardiac ketone and lipid metabolism.

Pyridoxal Kinase of Disease-causing Human Parasites: Structural and Functional Insights to Understand its Role in Drug Discovery

Abstract:

Human parasites cause several diseased conditions with high morbidity and mortality in a large section of the population residing in various geographical areas. Nearly three billion people suffer from either one or many parasitic infections globally, with almost one million deaths annually. In spite of extensive research and advancement in the medical field, no effective vaccine is available against prominent human parasitic diseases that necessitate identification of novel targets for designing specific inhibitors. Vitamin B6 is an important ubiquitous co-enzyme that participates in several biological processes and plays an important role in scavenging ROS (reactive oxygen species) along with providing resistance to oxidative stress. Moreover, the absence of the de novo vitamin B6 biosynthetic pathway in human parasites makes this pathway indispensable for the survival of these pathogens. Pyridoxal kinase (PdxK) is a crucial enzyme for vitamin B6 salvage pathway and participates in the process of vitamers B6 phosphorylation. Since the parasites are dependent on pyridoxal kinase for their survival and infectivity to the respective hosts, it is considered a promising candidate for drug discovery. The detailed structural analysis of PdxK from disease-causing parasites has provided insights into the catalytic mechanism of this enzyme as well as significant differences from their human counterpart. Simultaneously, structure-based studies have identified small lead molecules that can be exploited for drug discovery against protozoan parasites. The present review provides structural and functional highlights of pyridoxal kinase for its implication in developing novel and potent therapeutics to combat fatal parasitic diseases.

Cloning and Characterization of Pyridoxal Kinase from Geobacillus sp. H6a

Pyridoxal kinase encoded by pdxK gene, is the important key enzyme in the salvage pathway of vitamin B6 biosynthesis. The enzyme catalyzes the phosphorylation of the 5′ alcohol groups of free form vitamin B6 into their 5′-phosphate forms that requires metal ion and ATP. Pyridoxal kinase have been reported in many organisms except in the thermophilic bacterium. Therefore, this study aimed to clone, express and characterize pyridoxal kinase of Geobacillus sp. H6a isolated from the hot spring in the North of Thailand. The GhpdxK gene (810 base pairs) was inserted into pET28a(+) plasmids at restriction site of NdeI and BamHI and transformed into E.coli BL21(DE3). The expressed pyridoxal kinase of this bacterium exhibits a homodimer, in which each subunit had a molecular mass of about 32 kDa when examined by SDS-PAGE and gel filtration. The enzyme showed maximal activity at 70°C and at pH 8.0. The expressed enzyme obtained in this study was found to be more active (>50%) in the broad pH range (6.0 – 9.0) than those previously reported. This enzyme prefers Mg2+ and also accepts other cations to the less extent. Under optimal conditions, the expressed enzyme has higher affinity toward PN (20 ± 1.35 µM), while it showed the same affinity to pyridoxal (100 ± 0.76 µM) and pyridoxamine (100 ± 1.21 µM). The Km value for ATP and 4-amino-5-hydroxymethyl-2-methylpyridine were 8.99 ± 1.76 µM and 19 ± 0.85 µM, respectively. With high activity at high temperature and active in the broad pH range, it could be considered as a potential candidate for future application particularly bioconversion of vitamin B6.

Reactive Oxygen Species as the Brainbox in Malaria Treatment

Several measures are in place to combat the worldwide spread of malaria, especially in regions of high endemicity. In part, most common antimalarials, such as quinolines and artemisinin and its derivatives, deploy an ROS-mediated approach to kill malaria parasites. Although some antimalarials may share similar targets and mechanisms of action, varying levels of reactive oxygen species (ROS) generation may account for their varying pharmacological activities. Regardless of the numerous approaches employed currently and in development to treat malaria, concerningly, there has been increasing development of resistance by Plasmodium falciparum, which can be connected to the ability of the parasites to manage the oxidative stress from ROS produced under steady or treatment states. ROS generation has remained the mainstay in enforcing the antiparasitic activity of most conventional antimalarials. However, a combination of conventional drugs with ROS-generating ability and newer drugs that exploit vital metabolic pathways, such antioxidant machinery, could be the way forward in effective malaria control.

Recombinant Expression, Purification and Characterization of Pyridoxal 5'-phosphate Synthase from Geobacillus sp. H6a, Thermophilic Bacterium Producing Extracellular Vitamin B6

Background

Pyridoxal 5' -phosphate synthase (PLPS) is present in deoxyxylose 5'-phosphate-independent of the de novo vitamin B6 biosynthesis pathway. This enzyme complex consists of PdxS and PdxT, which function as synthase and glutamine amidotranferase respectively to produce PLP.

Objectives

This study aimed to clone, express, and purify PLPS of Geobacillus sp. H6a, followed by its characterization.

Material and Methods

The PdxS and PdxT genes were amplified from Geobacillus (Gh) sp. H6a. Recombinant vectors pET28a-GhpdxS and pET28a-GhpdxT were constructed and the resulting His-tagged proteins were expressed in E. coli BL21(DE3). The soluble rGhpdxS and rGhpdxT were purified via nickel-affinity chromatography and cation-exchange chromatography. The mixture of rGhpdxS and rGhpdxT was further characterized.

Results

The molecular weights of rGhpdxS and rGhpdxT were estimated to be 35 and 23 kDa by SDS-PAGE, respectively. The native form of rGhpdxS showed hexamer and dodecamer, whereas those of rGhpdxT were a monomer upon detection with non-denaturing gel electrophoresis and gel filtration. A molar ratio of 1:1 of rGhpdxS:rGhpdxT showed the highest PLP synthesis activity (4.16 U.mg^-1) and was used for analyzing the biochemical properties. The kinetic values were obtained by using glyceraldehyde 3-phosphate, ribose 5-phosphate, and glutamine as the substrates. The rGhPLPS showed pentose phosphate isomerization without triose phosphate isomerase activity. The metal ions affected PLP synthesis activity. The optimum pH and optimum temperature of rGhPLPS were 9 and 70 °C, respectively. The rGhPLPS was active over a broad range of temperatures and pH values.

Conclusions

These results support the potential of rGhPLPS as a candidate for industrial application.

Structural Dynamics and Perspectives of Vitamin B6 Biosynthesis Enzymes in Plasmodium: Advances and Open Questions

Malaria is still today one of the most concerning diseases, with 219 million infections in 2019, most of them in Sub-Saharan Africa and Latin America, causing approx. 409,000 deaths per year. Despite the tremendous advances in malaria treatment and prevention, there is still no vaccine for this disease yet available and the increasing parasite resistance to already existing drugs is becoming an alarming issue globally. In this context, several potential targets for the development of new drug candidates have been proposed and, among those, the de novo biosynthesis pathway for the B6 vitamin was identified to be a promising candidate. The reason behind its significance is the absence of the pathway in humans and its essential presence in the metabolism of major pathogenic organisms. The pathway consists of two enzymes i.e. Pdx1 (PLP synthase domain) and Pdx2 (glutaminase domain), the last constituting a transient and dynamic complex with Pdx1 as the prime player and harboring the catalytic center. In this review, we discuss the structural biology of Pdx1 and Pdx2, together with and the understanding of the PLP biosynthesis provided by the crystallographic data. We also highlight the existing evidence of the effect of PLP synthesis inhibition on parasite proliferation. The existing data provide a flourishing environment for the structure-based design and optimization of new substrate analogs that could serve as inhibitors or even suicide inhibitors.

Systemic metabolite profiling reveals sexual dimorphism of AIBP control of metabolism in mice

Emerging studies indicate that APOA-I binding protein (AIBP) is a secreted protein and functions extracellularly to promote cellular cholesterol efflux, thereby disrupting lipid rafts on the plasma membrane. AIBP is also present in the mitochondria and acts as an epimerase, facilitating the repair of dysfunctional hydrated NAD(P)H, known as NAD(P)H(X). Importantly, AIBP deficiency contributes to lethal neurometabolic disorder, reminiscent of the Leigh syndrome in humans. Whereas cyclic NADPHX production is proposed to be the underlying cause, we hypothesize that an unbiased metabolic profiling may: 1) reveal new clues for the lethality, e.g., changes of mitochondrial metabolites., and 2) identify metabolites associated with new AIBP functions. To this end, we performed unbiased and profound metabolic studies of plasma obtained from adult AIBP knockout mice and control littermates of both genders. Our systemic metabolite profiling, encompassing 9 super pathways, identified a total of 640 compounds. Our studies demonstrate a surprising sexual dimorphism of metabolites affected by AIBP deletion, with more statistically significant changes in the AIBP knockout female vs male when compared with the corresponding controls. AIBP knockout trends to reduce cholesterol but increase the bile acid precursor 7-HOCA in female but not male. Complex lipids, phospholipids, sphingomyelin and plasmalogens were reduced, while monoacylglycerol, fatty acids and the lipid soluble vitamins E and carotene diol were elevated in AIBP knockout female but not male. NAD metabolites were not significantly different in AIBP knockout vs control mice but differed for male vs female mice. Metabolites associated with glycolysis and the Krebs cycle were unchanged by AIBP knockout. Importantly, polyamine spermidine, critical for many cellular functions including cerebral cortex synapses, was reduced in male but not female AIBP knockout. This is the first report of a systemic metabolite profile of plasma samples from AIBP knockout mice, and provides a metabolic basis for future studies of AIBP regulation of cellular metabolism and the pathophysiological presentation of AIBP deficiency in patients.

Synthesis, in vitro anti-plasmodial potency, in-silico-cum-SPR binding with inhibition of PfPyridoxal synthase and rapid parasiticidal action by 3,5-bis{(E) arylidene}-N-methyl-4-piperidones

DANMPs have been identified as new pharmacophores that have the ability to target PfPyridoxal synthase and cause rapid killing of the malaria parasite.

Solution Structures and Dynamic Assembly of the 24-Meric Plasmodial Pdx1-Pdx2 Complex

Plasmodium species are protozoan parasites causing the deadly malaria disease. They have developed effective resistance mechanisms against most antimalarial medication, causing an urgent need to identify new antimalarial drug targets. Ideally, new drugs would be generated to specifically target the parasite with minimal or no toxicity to humans, requiring these drug targets to be distinctly different from the host’s metabolic processes or even absent in the host. In this context, the essential presence of vitamin B₆ biosynthesis enzymes in Plasmodium, the pyridoxal phosphate (PLP) biosynthesis enzyme complex, and its absence in humans is recognized as a potential drug target. To characterize the PLP enzyme complex in terms of initial drug discovery investigations, we performed structural analysis of the Plasmodium vivax PLP synthase domain (Pdx1), glutaminase domain (Pdx2), and Pdx1–Pdx2 (Pdx) complex (PLP synthase complex) by utilizing complementary bioanalytical techniques, such as dynamic light scattering (DLS), X-ray solution scattering (SAXS), and electron microscopy (EM). Our investigations revealed a dodecameric Pdx1 and a monodispersed Pdx complex. Pdx2 was identified in monomeric and in different oligomeric states in solution. Interestingly, mixing oligomeric and polydisperse Pdx2 with dodecameric monodisperse Pdx1 resulted in a monodispersed Pdx complex. SAXS measurements revealed the low-resolution dodecameric structure of Pdx1, different oligomeric structures for Pdx2, and a ring-shaped dodecameric Pdx1 decorated with Pdx2, forming a heteromeric 24-meric Pdx complex.

Essential Metabolic Routes as a Way to ESKAPE From Antibiotic Resistance

Antibiotic resistance is a worldwide concern that requires a concerted action from physicians, patients, governmental agencies, and academia to prevent infections and the spread of resistance, track resistant bacteria, improve the use of current antibiotics, and develop new antibiotics. Despite the efforts spent so far, the current antibiotics in the market are restricted to only five general targets/pathways highlighting the need for basic research focusing on the discovery and evaluation of new potential targets. Here we interrogate two biosynthetic pathways as potentially druggable pathways in bacteria. The biosynthesis pathway for thiamine (vitamin B1), absent in humans, but found in many bacteria, including organisms in the group of the ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumanii, Pseudomonas aeruginosa, and Enterobacter sp.) and the biosynthesis pathway for pyridoxal 5'-phosphate and its vitamers (vitamin B6), found in S. aureus. Using current genomic data, we discuss the possibilities of inhibition of enzymes in the pathway and review the current state of the art in the scientific literature.

Structural attributes and substrate specificity of pyridoxal kinase from Leishmania donovani

The enzyme pyridoxal kinase (PdxK) catalyzes the conversion of pyridoxal to pyridoxal-5'-phosphate (PLP) using ATP as the co-factor. The product pyridoxal-5'-phosphate plays a key role in several biological processes such as transamination, decarboxylation and deamination. In the present study, full-length ORF of PdxK from Leishmania donovani (LdPdxK) was cloned and then purified using affinity chromatography. LdPdxK exists as a homo-dimer in solution and shows more activity at near to physiological pH. Biochemical analysis of LdPdxK with pyridoxal, pyridoxamine, pyridoxine and ginkgotoxin revealed its affinity preference towards different substrates. The secondary structure analysis using circular dichroism spectroscopy showed LdPdxK to be predominantly α-helical in organization which tends to decline at lower and higher pH. Simultaneously, LdPdxK was crystallized and its three-dimensional structure in complex with ADP and different substrates were determined. Crystal structure of LdPdxK delineated that it has a central core of β-sheets surrounded by α-helices with a conserved GTGD ribokinase motif. The structures of LdPdxK disclosed no major structural changes between ADP and ADP- substrate bound structures. In addition, comparative structural analysis highlighted the key differences between the active site pockets of leishmanial and human PdxK, rendering LdPdxK an attractive candidate for the designing of novel and specific inhibitors.

Vitamin and cofactor acquisition in apicomplexans: Synthesis versus salvage

The Apicomplexa phylum comprises diverse parasitic organisms that have evolved from a free-living ancestor. These obligate intracellular parasites exhibit versatile metabolic capabilities reflecting their capacity to survive and grow in different hosts and varying niches. Determined by nutrient availability, they either use their biosynthesis machineries or largely depend on their host for metabolite acquisition. Because vitamins cannot be synthesized by the mammalian host, the enzymes required for their synthesis in apicomplexan parasites represent a large repertoire of potential therapeutic targets. Here, we review recent advances in metabolic reconstruction and functional studies coupled to metabolomics that unravel the interplay between biosynthesis and salvage of vitamins and cofactors in apicomplexans. A particular emphasis is placed on Toxoplasma gondii, during both its acute and latent stages of infection.

Regulation of lipid rafts, angiogenesis and inflammation by AIBP

PURPOSE OF REVIEW

Recent studies demonstrate an important role of the secreted apolipoprotein A-I binding protein (AIBP) in regulation of cholesterol efflux and lipid rafts. The article discusses these findings in the context of angiogenesis and inflammation.

RECENT FINDINGS

Lipid rafts are cholesterol-rich and sphingomyelin-rich membrane domains in which many receptor complexes assemble upon activation. AIBP mediates selective cholesterol efflux, in part via binding to toll-like receptor-4 (TLR4) in activated macrophages and microglia, and thus reverses lipid raft increases in activated cells. Recent articles report AIBP regulation of vascular endothelial growth factor receptor-2, Notch1 and TLR4 function. In zebrafish and mouse animal models, AIBP deficiency results in accelerated angiogenesis, increased inflammation and exacerbated atherosclerosis. Spinal delivery of recombinant AIBP reduces neuraxial inflammation and reverses persistent pain state in a mouse model of chemotherapy-induced polyneuropathy. Inhalation of recombinant AIBP reduces lipopolysaccharide-induced acute lung injury in mice. These findings are discussed in the perspective of AIBP's proposed other function, as an NAD(P)H hydrate epimerase, evolving into a regulator of cholesterol trafficking and lipid rafts.

SUMMARY

Novel findings of AIBP regulatory circuitry affecting lipid rafts and related cellular processes may provide new therapeutic avenues for angiogenic and inflammatory diseases.

Overexpression of PDX-II gene in potato (Solanum tuberosum L.) leads to the enhanced accumulation of vitamin B6 in tuber tissues and tolerance to abiotic stresses

Vitamin B6 is a vital metabolite required for living organisms as a cofactor in several metabolic biochemical reactions and recognized as a potent antioxidant molecule which modulates the expression of the proteins responsible for the scavenging of cellular reactive oxygen species. It is well established that the microorganisms and plants can synthesize the B6 de novo, therefore, all the animals including humans must acquire it from the plant dietary resources. However, the bioavailability of the vitamin in the edible portions of the commonly consumed plants is insufficient to meet the daily recommended doses. Genetic engineering techniques have proven successful in increasing the vitamin B6 content in the model plants. Present study describe the development of transgenic potato (Solanum tuberosum L. cv. Kufri chipsona) overexpressing key vitamin B6 pathway gene, the PDXII (NCBI database Ref. ID- NM_125447.2) isolated from Arabidopsis thaliana under the control of CaMV 35S constitutive promoter. The stable integration and expression of transgene in the transgenic lines were confirmed by PCR, Southern blot and RT-PCR analysis. Transgenic tubers exhibited considerably improved vitamin B6 accumulation (up to 107-150%) in comparison to the untransformed controls potato. This increase in vitamin B6 was also correlated with the increased mRNA expression of PDXII gene. The prominent increase in the B6 content of transgenic potato was also associated with the capability to survive under abiotic stresses, therefore, the transgenic lines were able to withstand various abiotic stresses imposed by salinity (NaCl) or methyl viologen (MV). We thus demonstrated that overexpression of PDXII gene under the control of a constitutive promoter enhanced the accumulation of the vitamin B6 which also augmented the tolerance under various abiotic stresses in potato (Solanum tuberosum L.).

Modeling metabolism and stage-specific growth of Plasmodium falciparum HB3 during the intraerythrocytic developmental cycle

The human malaria parasite Plasmodium falciparum goes through a complex life cycle, including a roughly 48-hour-long intraerythrocytic developmental cycle (IDC) in human red blood cells. A better understanding of the metabolic processes required during the asexual blood-stage reproduction will enhance our basic knowledge of P. falciparum and help identify critical metabolic reactions and pathways associated with blood-stage malaria. We developed a metabolic network model that mechanistically links time-dependent gene expression, metabolism, and stage-specific growth, allowing us to predict the metabolic fluxes, the biomass production rates, and the timing of production of the different biomass components during the IDC. We predicted time- and stage-specific production of precursors and macromolecules for P. falciparum (strain HB3), allowing us to link specific metabolites to specific physiological functions. For example, we hypothesized that coenzyme A might be involved in late-IDC DNA replication and cell division. Moreover, the predicted ATP metabolism indicated that energy was mainly produced from glycolysis and utilized for non-metabolic processes. Finally, we used the model to classify the entire tricarboxylic acid cycle into segments, each with a distinct function, such as superoxide detoxification, glutamate/glutamine processing, and metabolism of fumarate as a byproduct of purine biosynthesis. By capturing the normal metabolic and growth progression in P. falciparum during the IDC, our model provides a starting point for further elucidation of strain-specific metabolic activity, host-parasite interactions, stress-induced metabolic responses, and metabolic responses to antimalarial drugs and drug candidates.

Purification, crystallization and preliminary X-ray diffraction analysis of pyridoxal kinase from Plasmodium falciparum (PfPdxK)

Pyridoxal kinases (PdxK) catalyze the phosphorylation of vitamin B6 precursors. Thus, these enzymes are an essential part of many metabolic processes in all organisms. The protozoan parasite Plasmodium falciparum (the main causative agent of Malaria tropica) possesses a unique de novo B6-biosynthesis pathway in addition to a interconversion pathway based on the activity of plasmodial PdxK (PfPdxK). The role of PdxK in B6 salvage has prompted previous authors to suggest PdxK as a promising target for structure-based antimalarial drug design. Here, the expression, purification, crystallization and preliminary X-ray diffraction analysis of PfPdxK are reported. PfPdxK crystals have been grown in space group P2₁, with unit-cell parameters a=52.7, b=62.0, c=93.7 Å, β=95°. A data set has been collected to 2 Å resolution and an initial molecular-replacement solution is described.

Vitamin B6-dependent enzymes in the human malaria parasite Plasmodium falciparum: a druggable target?

Malaria is a deadly infectious disease which affects millions of people each year in tropical regions. There is no effective vaccine available and the treatment is based on drugs which are currently facing an emergence of drug resistance and in this sense the search for new drug targets is indispensable. It is well established that vitamin biosynthetic pathways, such as the vitamin B6 de novo synthesis present in Plasmodium, are excellent drug targets. The active form of vitamin B6, pyridoxal 5-phosphate, is, besides its antioxidative properties, a cofactor for a variety of essential enzymes present in the malaria parasite which includes the ornithine decarboxylase (ODC, synthesis of polyamines), the aspartate aminotransferase (AspAT, involved in the protein biosynthesis), and the serine hydroxymethyltransferase (SHMT, a key enzyme within the folate metabolism).

Targeting the vitamin biosynthesis pathways for the treatment of malaria

The most severe form of malaria is Malaria tropica, caused by Plasmodium falciparum. There are more than 1 billion people that are exposed to malaria parasites leading to more than 500,000 deaths annually. Vaccines are not available and the increasing drug resistance of the parasite prioritizes the need for novel drug targets and chemotherapeutics, which should be ideally designed to target selectively the parasite. In this sense, parasite-specific pathways, such as the vitamin biosyntheses, represent perfect drug-target characteristics because they are absent in humans. In the past, the vitamin B9 (folate) metabolism has been exploited by antifolates to treat infections caused by malaria parasites. Recently, two further vitamin biosynthesis pathways - for the vitamins B6 (pyridoxine) and B1 (thiamine) - have been identified in Plasmodium and analyzed for their suitability to discover new drugs. In this review, the current status of the druggability of plasmodial vitamin biosynthesis pathways is summarized.

The vitamin B₆ biosynthesis pathway in Streptococcus pneumoniae is controlled by pyridoxal 5'-phosphate and the transcription factor PdxR and has an impact on ear infection

Vitamin B₆ is an essential cofactor for a large number of enzymes in both prokaryotes and eukaryotes. In this study, we characterized the pyridoxal 5'-phosphate (PLP) biosynthesis pathway in Streptococcus pneumoniae. Our results revealed that S. pneumoniae possesses a de novo vitamin B₆ biosynthesis pathway encoded by the pdxST genes. Purified PdxS functionally displayed as PLP synthase, whereas PdxT exhibited glutaminase activity in vitro. Deletion of pdxS, but not pdxT, resulted in a vitamin B₆ auxotrophic mutant. The defective growth of the ΔpdxS mutant in a vitamin B₆-depleted medium could be chemically restored in the presence of the B₆ vitamers at optimal concentrations. By analyzing PdxS expression levels, we demonstrated that the expression of pdxS was repressed by PLP and activated by a transcription factor, PdxR. A pneumococcal ΔpdxR mutant also exhibited as a vitamin B₆ auxotroph. In addition, we found that disruption of the vitamin B₆ biosynthesis pathway in S. pneumoniae caused a significant attenuation in a chinchilla middle ear infection model and a minor attenuation in a mouse pneumonia model, indicating that the impact of vitamin B₆ synthesis on virulence depends upon the bacterial infection niche.

Exploring inhibition of Pdx1, a component of the PLP synthase complex of the human malaria parasite Plasmodium falciparum

Malaria tropica is a devastating infectious disease caused by Plasmodium falciparum. This parasite synthesizes vitamin B6 de novo via the PLP (pyridoxal 5'-phosphate) synthase enzymatic complex consisting of PfPdx1 and PfPdx2 proteins. Biosynthesis of PLP is largely performed by PfPdx1, ammonia provided by PfPdx2 subunits is condensed together with R5P (D-ribose 5-phosphate) and G3P (DL-glyceraldehyde 3-phosphate). PfPdx1 accommodates both the R5P and G3P substrates and intricately co-ordinates the reaction mechanism, which is composed of a series of imine bond formations, leading to the production of PLP. We demonstrate that E4P (D-erythrose 4-phosphate) inhibits PfPdx1 in a dose-dependent manner. We propose that the acyclic phospho-sugar E4P, with a C1 aldehyde group similar to acyclic R5P, could interfere with R5P imine bond formations in the PfPdx1 reaction mechanism. Molecular docking and subsequent screening identified the E4P hydrazide analogue 4PEHz (4-phospho-D-erythronhydrazide), which selectively inhibited PfPdx1 with an IC50 of 43 μM. PfPdx1 contained in the heteromeric PLP synthase complex was shown to be more sensitive to 4PEHz and was inhibited with an IC50 of 16 μM. Moreover, the compound had an IC50 value of 10 μM against cultured P. falciparum intraerythrocytic parasites. To analyse further the selectivity of 4PEHz, transgenic cell lines overexpressing PfPdx1 and PfPdx2 showed that additional copies of the protein complex conferred protection against 4PEHz, indicating that the PLP synthase is directly affected by 4PEHz in vivo. These PfPdx1 inhibitors represent novel lead scaffolds which are capable of targeting PLP biosynthesis, and we propose this as a viable strategy for the development of new therapeutics against malaria.

Oxidative stress in malaria

Malaria is a significant public health problem in more than 100 countries and causes an estimated 200 million new infections every year. Despite the significant effort to eradicate this dangerous disease, lack of complete knowledge of its physiopathology compromises the success in this enterprise. In this paper we review oxidative stress mechanisms involved in the disease and discuss the potential benefits of antioxidant supplementation as an adjuvant antimalarial strategy.

Recent advances and perspectives on tropical diseases: Malaria

Malaria remains a major health problem in the world. It is a neglected disease because it occurs almost exclusively in poor developing countries, which offer negligible marketable and profitable opportunities. Malaria (together with Tuberculosis), is responsible for an unprecedented global health crisis with devastating effects in developing countries. The 2011 Word Malaria Report indicated that 106 countries showed endemic malaria. Malaria control depends mainly on drug treatment, which is increasingly difficult due to the spread of drug resistant parasites and requires expensive drug combinations. Part of the inability to combat this disease is attributed to an incomplete understanding of its pathogenesis and pathophysiology. Improving the knowledge of the underlying pathogenic mechanisms of malaria transmission and of the exclusive metabolic pathways of the parasites (protozoa of the genus Plasmodium), should promote efficient treatment of disease and help the identification of novel targets for potential therapeutic interventions. Moreover, the elucidation of determinants involved in the spread of malaria will provide important information for efficient planning of strategies for targeted control.

The antioxidative effect of de novo generated vitamin B6 in Plasmodium falciparum validated by protein interference

The malaria parasite Plasmodium falciparum is able to synthesize de novo PLP (pyridoxal 5'-phosphate), the active form of vitamin B6. In the present study, we have shown that the de novo synthesized PLP is used by the parasite to detoxify 1O2 (singlet molecular oxygen), a highly destructive reactive oxygen species arising from haemoglobin digestion. The formation of 1O2 and the response of the parasite were monitored by live-cell fluorescence microscopy, by transcription analysis and by determination of PLP levels in the parasite. Pull-down experiments of transgenic parasites overexpressing the vitamin B6-biosynthetic enzymes PfPdx1 and PfPdx2 clearly demonstrated an interaction of the two proteins in vivo which results in an elevated PLP level from 12.5 μM in wild-type parasites to 36.6 μM in the PfPdx1/PfPdx2-overexpressing cells and thus to a higher tolerance towards 1O2. In contrast, by applying the dominant-negative effect on the cellular level using inactive mutants of PfPdx1 and PfPdx2, P. falciparum becomes susceptible to 1O2. Our results demonstrate clearly the crucial role of vitamin B6 biosynthesis in the detoxification of 1O2 in P. falciparum. Besides the known role of PLP as a cofactor of many essential enzymes, this second important task of the vitamin B6 de novo synthesis as antioxidant emphasizes the high potential of this pathway as a target of new anti-malarial drugs.

Assembly of the eukaryotic PLP-synthase complex from Plasmodium and activation of the Pdx1 enzyme

Biosynthesis of vitamins is fundamental to malaria parasites. Plasmodia synthesize the active form of vitamin B(6) (pyridoxal 5'-phosphate, PLP) using a PLP synthase complex. The EM analysis shown here reveals a random association pattern of up to 12 Pdx2 glutaminase subunits to the dodecameric Pdx1 core complex. Interestingly, Plasmodium falciparum PLP synthase organizes in fibers. The crystal structure shows differences in complex formation to bacterial orthologs as interface variations. Alternative positioning of an α helix distinguishes an open conformation from a closed state when the enzyme binds substrate. The pentose substrate is covalently attached through its C1 and forms a Schiff base with Lys84. Ammonia transfer between Pdx2 glutaminase and Pdx1 active sites is regulated by a transient tunnel. The mutagenesis analysis allows defining the requirement for conservation of critical methionines, whereas there is also plasticity in ammonia tunnel construction as seen from comparison across different species.

Clinical proteomics of the neglected human malarial parasite Plasmodium vivax

Recent reports highlight the severity and the morbidity of disease caused by the long neglected malaria parasite Plasmodium vivax. Due to inherent difficulties in the laboratory-propagation of P. vivax, the biology of this parasite has not been adequately explored. While the proteome of P. falciparum, the causative agent of cerebral malaria, has been extensively explored from several sources, there is limited information on the proteome of P. vivax. We have, for the first time, examined the proteome of P. vivax isolated directly from patients without adaptation to laboratory conditions. We have identified 153 proteins from clinical P. vivax, majority of which do not show homology to any previously known gene products. We also report 29 new proteins that were found to be expressed in P. vivax for the first time. In addition, several proteins previously implicated as anti-malarial targets, were also found in our analysis. Most importantly, we found several unique proteins expressed by P. vivax.This study is an important step in providing insight into physiology of the parasite under clinical settings.

Pyridoxal phosphate: biosynthesis and catabolism

Vitamin B(6) is an essential cofactor that participates in a large number of biochemical reactions. Pyridoxal phosphate is biosynthesized de novo by two different pathways (the DXP dependent pathway and the R5P pathway) and can also be salvaged from the environment. It is one of the few cofactors whose catabolic pathway has been comprehensively characterized. It is also known to function as a singlet oxygen scavenger and has protective effects against oxidative stress in fungi. Enzymes utilizing vitamin B(6) are important targets for therapeutic agents. This review provides a concise overview of the mechanistic enzymology of vitamin B(6) biosynthesis and catabolism. This article is part of a Special Issue entitled: Pyridoxal Phosphate Enzymology.

Vitamin B6 biosynthesis is essential for survival and virulence of Mycobacterium tuberculosis

With 500000 cases of multidrug-resistant tuberculosis there is an urgent need for attractive targets to enable the discovery of novel antimycobacterials. The biosynthesis of essential cofactors is of particular interest as these pathways are absent in man and their inhibition is expected to affect the metabolism of Mycobacterium tuberculosis at multiple sites. Our data demonstrate that the pathogen synthesizes pyridoxal 5-phosphate (PLP), the bioactive form of vitamin B6, by a heteromeric PLP synthase composed of Pdx1 (Rv2606c) and Pdx2 (Rv2604c). Disruption of the pdx1 gene generated a strictly B6 auxotrophic M. tuberculosis mutant, Δpdx1. Removal of the cofactor during exponential growth or stationary phase demonstrated the essentiality of vitamin B6 biosynthesis for growth and survival of the pathogen in culture. In a tuberculosis dormancy model based on gradual oxygen depletion, de novo biosynthesis of PLP was required for regrowth of the bacillus after direct oxygen exposure. The Δpdx1 mutant showed a severe growth defect in immunocompetent mice: bacilli applied intranasally failed to persist in host tissues and were quickly cleared. We conclude that vitamin B6 biosynthesis is required for survival of M. tuberculosis in vivo and thus might represent a candidate pathway for the development of new antitubercular agents.

Defining the structural requirements for ribose 5-phosphate-binding and intersubunit cross-talk of the malarial pyridoxal 5-phosphate synthase

Most organisms synthesise the B(6) vitamer pyridoxal 5-phosphate (PLP) via the glutamine amidotransferase PLP synthase, a large enzyme complex of 12 Pdx1 synthase subunits with up to 12 Pdx2 glutaminase subunits attached. Deletion analysis revealed that the C-terminus has four distinct functionalities: assembly of the Pdx1 monomers, binding of the pentose substrate (ribose 5-phosphate), formation of the reaction intermediate I(320), and finally PLP synthesis. Deletions of distinct C-terminal regions distinguish between these individual functions. PLP formation is the only function that is conferred to the enzyme by the C-terminus acting in trans, explaining the cooperative nature of the complex.

Vitamin B6: a molecule for human health?

Vitamin B₆ is an intriguing molecule that is involved in a wide range of metabolic, physiological and developmental processes. Based on its water solubility and high reactivity when phosphorylated, it is a suitable co-factor for many biochemical processes. Furthermore the vitamin is a potent antioxidant, rivaling carotenoids or tocopherols in its ability to quench reactive oxygen species. It is therefore not surprising that the vitamin is essential and unquestionably important for the cellular metabolism and well-being of all living organisms. The review briefly summarizes the biosynthetic pathways of vitamin B₆ in pro- and eukaryotes and its diverse roles in enzymatic reactions. Finally, because in recent years the vitamin has often been considered beneficial for human health, the review will also sum up and critically reflect on current knowledge how human health can profit from vitamin B₆.

Secretion of an acid phosphatase provides a possible mechanism to acquire host nutrients by Plasmodium falciparum

As an intracellular proliferating parasite, Plasmodium falciparum exploits the human host to acquire nutrients. However, nutrients such as nucleotides and cofactors are mostly phosphorylated in the host cell cytosol and thus have to be dephosphorylated in order to be taken up by the parasite. Here we report the functional characterization of a unique secreted phosphatase in P. falciparum, which is expressed throughout the developmental stages in the red blood cell. We show that this enzyme, formerly described as anchoring glideosome-associated protein 50 (GAP50), reveals a broad substrate profile with preference for di- and triphosphates at pH 5-7. Bioinformatic studies of the protein sequence identified an N-terminal signal anchor (SA) as well as a C-terminal transmembrane domain. By means of live microscopy of parasites transfected with GFP-fusions of this secreted acid phosphatase (PfSAP), we demonstrate that PfSAP enters the secretory pathway en route to the parasite periphery - mediated by SA - and is subsequently engulfed into the food vacuole. We corroborate this with independent data where acid phosphatase activity is visualized in close proximity to hemozoin. The biochemical as well as the trafficking results support the proposed role of PfSAP in the acquisition of host nutrients by dephosphorylation.

Vitamin B metabolism in Plasmodium falciparum as a source of drug targets

The malaria parasite Plasmodium falciparum depends primarily on nutrient sources from its human host. Most compounds, such as glucose, purines, amino acids, as well as cofactors and vitamins, are abundantly available in the host cell, and can be readily salvaged by the parasite. However, in some cases the parasite can also synthesize cofactors de novo in reactions that appear to be essential. Importantly, the three biosynthetic pathways that produce vitamins B(1), B(6) and B(9) are absent from the host, but are well established in P. falciparum. This review summarizes and updates the current knowledge of vitamin B de novo synthesis and salvage in P. falciparum and focuses on their potential as targets for drug intervention.

GintPDX1 encodes a protein involved in vitamin B6 biosynthesis that is up-regulated by oxidative stress in the arbuscular mycorrhizal fungus Glomus intraradices

Vitamin B6 is an essential metabolite that has recently been implicated in defense against cellular oxidative stress. In fungi, the de novo biosynthetic pathway of vitamin B6 involves two genes, PDX1 and PDX2. Here, we report a component of the PDX1/PDX2 vitamin B6 biosynthetic pathway in an arbuscular mycorrhizal (AM) fungus. Using rapid amplification of cDNA ends, we isolated the full-length cDNA of a PDX-like gene, GintPDX1, from Glomus intraradices. GintPDX1 expression was analysed by real-time reverse transcription-polymerase chain reaction (RT-PCR). GintPDX1 activity and function were investigated by heterologous complementation of the yeast strainDeltasnz1, which is deficient in vitamin B6 biosynthesis. Sequence data revealed that GintPDX1 is highly homologous to other identified PDX1 proteins. GintPDX1 restores prototrophy to the vitamin B6 auxotrophic yeast mutant and reverts its superoxide sensitivity. GintPDX1 is expressed throughout the fungal life cycle, with the highest transcription levels found in the intraradical fungal structures. GintPDX1 expression was induced in response to hydrogen peroxide, paraquat and copper. The results demonstrate that AM fungi possess at least one component of the machinery necessary for vitamin B6 biosynthesis. Transcriptional regulation of GintPDX1 suggests a role for vitamin B6 as an antioxidant and modulator of reactive oxygen species in G. intraradices.

Enhancement of vitamin B(6) levels in seeds through metabolic engineering

As a versatile cofactor for many enzymes catalyzing important biochemical reactions, vitamin B(6) is required for all cellular organisms. In contrast to bacteria, fungi and plants, which have the ability to synthesize vitamin B(6)de novo, animals have to take up the vitamin from their diet. Plants are the major source of vitamin B(6) for animals. The recent identification of vitamin B(6) biosynthetic enzymes PDX1 and PDX2 in plants makes it possible to regulate the biosynthesis of this important vitamin. In this study, we generated Arabidopsis plants overexpressing the PDX1 and/or PDX2 gene and used a liquid chromatography/mass spectrometry/mass spectrometry method to determine the levels of different forms of vitamin B(6) in these transgenic plants. It was found that expression of the PDX genes under control of the CaMV 35S promoter caused only a limited increase in pyridoxine contents in dry seeds but not in shoots or roots. When using the Arabidopsis seed-specific 12S promoter to drive the expression of the PDX genes, the levels of vitamin B(6) increased more than twofold in transgenic plants. Our work demonstrates that it is feasible to enhance vitamin B(6) content in seeds by metabolic engineering.

Mobility of the conserved glycine 155 is required for formation of the active plasmodial Pdx1 dodecamer

BACKGROUND

Vitamin B6 synthesis requires a functional Pdx1 assembly that is dodecameric in vivo. We have previously shown that mutation of a catalytic lysine in the plasmodial Pdx1 protein results in a protein that is both inactive and hexameric in vitro.

METHODS

Static and dynamic light scattering, circular dichroism, co-purification and enzyme assays are used to investigate the role of a glycine conserved in all Pdx1 family members.

RESULTS

Static light scattering indicates that a glycine to alanine mutant is present as a hexamer in vitro. Subsequent circular dichroism experiments demonstrate that a significant change in secondary structure content is induced by this mutation. However, this mutant is still competent to bind and support Pdx2 activity.

CONCLUSIONS

As the mutated glycine occupies an unrestricted region of the Ramachandran plot the additional stereo-chemical restrictions imposed on alanine residues strongly support our hypothesis that significant structural rearrangement of Pdx1 is required during the transition from hexamer to dodecamer.

GENERAL SIGNIFICANCE

The presented results demonstrate that reduction in the mobility of this region in Pdx1 proteins is required for formation of the in vivo dodecamer, negatively affecting the activity of Pdx1, opening the possibility of allosteric Pdx1 inhibitors.

Poisoning pyridoxal 5-phosphate-dependent enzymes: a new strategy to target the malaria parasite Plasmodium falciparum

The human malaria parasite Plasmodium falciparum is able to synthesize de novo pyridoxal 5-phosphate (PLP), a crucial cofactor, during erythrocytic schizogony. However, the parasite possesses additionally a pyridoxine/pyridoxal kinase (PdxK) to activate B6 vitamers salvaged from the host. We describe a strategy whereby synthetic pyridoxyl-amino acid adducts are channelled into the parasite. Trapped upon phosphorylation by the plasmodial PdxK, these compounds block PLP-dependent enzymes and thus impair the growth of P. falciparum. The novel compound PT3, a cyclic pyridoxyl-tryptophan methyl ester, inhibited the proliferation of Plasmodium very efficiently (IC(50)-value of 14 microM) without harming human cells. The non-cyclic pyridoxyl-tryptophan methyl ester PT5 and the pyridoxyl-histidine methyl ester PHME were at least one order of magnitude less effective or completely ineffective in the case of the latter. Modeling in silico indicates that the phosphorylated forms of PT3 and PT5 fit well into the PLP-binding site of plasmodial ornithine decarboxylase (PfODC), the key enzyme of polyamine synthesis, consistent with the ability to abolish ODC activity in vitro. Furthermore, the antiplasmodial effect of PT3 is directly linked to the capability of Plasmodium to trap this pyridoxyl analog, as shown by an increased sensitivity of parasites overexpressing PfPdxK in their cytosol, as visualized by GFP fluorescence.

Analysis of a horizontally transferred pathway involved in vitamin B6 biosynthesis from the soybean cyst nematode Heterodera glycines

Heterodera glycines is an obligate plant parasite capable of biochemically and developmentally altering its host's cells in order to create a specialized feeding cell. Although the exact mechanism of feeding cell morphogenesis remains a mystery, the nematode's ability to manipulate the plant is thought to be due in part to horizontal gene transfers (HGTs). A bioinformatic screen of the nematode genome has revealed homologues of the genes SNZ and SNO, which comprise a metabolic pathway for the de novo biosynthesis of pyridoxal 5'-phosphate, the active form of vitamin B(6) (VB(6)). Analysis of the 2 genes, HgSNZ and HgSNO, show that they contain nematode-like introns, generate polyadenylated mRNAs, and map to the soybean cyst nematode genetic linkage map, indicating that they are part of the nematode genome. However, gene synteny, protein homology, and phylogenetic evidence suggest prokaryotic origin. This would represent the first case of the HGT of a complete pathway into a nematode or terrestrial animal. VB(6) acts as a cofactor in over 140 different enzymes, and recent studies point toward an important role as a potent quencher of reactive oxygen species. With H. glycines' penchant for acquiring parasitism genes through HGT along with the absence of this pathway in other land-based animals suggests a specific need for VB(6) which may involve the parasite-host interaction.

Heterologous expression of plasmodial proteins for structural studies and functional annotation

Malaria remains the world's most devastating tropical infectious disease with as many as 40% of the world population living in risk areas. The widespread resistance of Plasmodium parasites to the cost-effective chloroquine and antifolates has forced the introduction of more costly drug combinations, such as Coartem^®. In the absence of a vaccine in the foreseeable future, one strategy to address the growing malaria problem is to identify and characterize new and durable antimalarial drug targets, the majority of which are parasite proteins. Biochemical and structure-activity analysis of these proteins is ultimately essential in the characterization of such targets but requires large amounts of functional protein. Even though heterologous protein production has now become a relatively routine endeavour for most proteins of diverse origins, the functional expression of soluble plasmodial proteins is highly problematic and slows the progress of antimalarial drug target discovery. Here the status quo of heterologous production of plasmodial proteins is presented, constraints are highlighted and alternative strategies and hosts for functional expression and annotation of plasmodial proteins are reviewed.

The assembly of the plasmodial PLP synthase complex follows a defined course

Background

Plants, fungi, bacteria and the apicomplexan parasite Plasmodium falciparum are able to synthesize vitamin B6 de novo, whereas mammals depend upon the uptake of this essential nutrient from their diet. The active form of vitamin B6 is pyridoxal 5-phosphate (PLP). For its synthesis two enzymes, Pdx1 and Pdx2, act together, forming a multimeric complex consisting of 12 Pdx1 and 12 Pdx2 protomers.

Methodology/Principal Findings

Here we report amino acid residues responsible for stabilization of the structural and enzymatic integrity of the plasmodial PLP synthase, identified by using distinct mutational analysis and biochemical approaches. Residues R85, H88 and E91 (RHE) are located at the Pdx1:Pdx1 interface and play an important role in Pdx1 complex assembly. Mutation of these residues to alanine impedes both Pdx1 activity and Pdx2 binding. Furthermore, changing D26, K83 and K151 (DKK), amino acids from the active site of Pdx1, to alanine obstructs not only enzyme activity but also formation of the complex. In contrast to the monomeric appearance of the RHE mutant, alteration of the DKK residues results in a hexameric assembly, and does not affect Pdx2 binding or its activity. While the modelled position of K151 is distal to the Pdx1:Pdx1 interface, it affects the assembly of hexameric Pdx1 into a functional dodecamer, which is crucial for PLP synthesis.

Conclusions/Significance

Taken together, our data suggest that the assembly of a functional Pdx1:Pdx2 complex follows a defined pathway and that inhibition of this assembly results in an inactive holoenzyme.