Action of the thiamine antagonist bacimethrin on thiamine biosynthesis

Whole cell affinity for 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP) in the marine bacterium Candidatus Pelagibacter st. HTCC7211 explains marine dissolved HMP concentrations

Vitamin B1 is a universally required coenzyme in carbon metabolism. However, most marine microorganisms lack the complete biosynthetic pathway for this compound and must acquire thiamin, or precursor molecules, from the dissolved pool. The most common version of Vitamin B1 auxotrophy is for thiamin's pyrimidine precursor moiety, 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP). Frequent HMP auxotrophy in plankton and vanishingly low dissolved concentrations (approximately 0.1-50 pM) suggest that high-affinity HMP uptake systems are responsible for maintaining low ambient HMP concentrations. We used tritium-labelled HMP to investigate HMP uptake mechanisms and kinetics in cell cultures of Candidatus Pelagibacter st. HTCC7211, a representative of the globally distributed and highly abundant SAR11 clade. A single protein, the sodium solute symporter ThiV, which is conserved across SAR11 genomes, is the likely candidate for HMP transport. Experimental evidence indicated transport specificity for HMP and mechanistically complex, high-affinity HMP uptake kinetics. Km values ranged from 9.5 pM to 1.2 nM and were dramatically lower when cells were supplied with a carbon source. These results suggest that HMP uptake in HTCC7211 is subject to complex regulation and point to a strategy for high-affinity uptake of this essential growth factor that can explain natural HMP levels in seawater.

Phosphomethylpyrimidine Synthase (ThiC): Trapping of Five Intermediates Provides Mechanistic Insights on a Complex Radical Cascade Reaction in Thiamin Biosynthesis

Phosphomethylpyrimidine synthase (ThiC) catalyzes the conversion of AIR to the thiamin pyrimidine HMP-P. This reaction is the most complex enzyme-catalyzed radical cascade identified to date, and the detailed mechanism has remained elusive. In this paper, we describe the trapping of five new intermediates that provide snapshots of the ThiC reaction coordinate and enable the formulation of a revised mechanism for the ThiC-catalyzed reaction.

Sulfite-Catalyzed Nucleophilic Substitution Reactions with Thiamin and Analogous Pyrimidine Donors Proceed via an SNAE Mechanism

When treated with SO₃^2-, thiamin undergoes a substitution reaction to release a thiazole leaving group and the corresponding sulfonate. Although this reaction could proceed via a simple S_N2-like mechanism, a multistep addition-elimination (S_NAE) mechanism involving the addition of SO₃^2- to C6' of the 4-aminopyrimidine of thiamin has also been proposed. Although this reaction has potential utility in the synthesis of substituted pyrimidines and provides a direct analogue to reactions catalyzed by thiaminases, a detailed mechanistic picture of the SO₃^2--catalyzed cleavage of thiamin has remained elusive. Here, DFT calculations have been used to probe the relative energetics and the factors that shape the potential energy surfaces that define the possible mechanisms of substitution. These calculations provide clear support for the S_NAE mechanism over an S_N2-like process and illustrate that the unique ability of SO₃^2- to activate thiamin toward nucleophilic displacement is due to the combined nucleophilicity and relatively poor leaving group ability of SO₃^2-. Both of these factors favor the forward partitioning of the sulfite adduct toward the cleavage products whereas adducts formed with other nucleophiles overwhelmingly revert to reactants. Calculations performed with a range of substrates with various electrophilicities and nucleofugalities consistently suggest that the S_NAE pathway is significantly lower in energy than the direct substitution, illustrating that this SO₃^2--catalyzed multistep process is likely to be broadly applicable both in solution and in catalysis by thiaminases.

Pharmacological perturbation of thiamine metabolism sensitizes Pseudomonas aeruginosa to multiple antibacterial agents

New therapeutic concepts are critically needed for carbapenem-resistant Pseudomonas aeruginosa, an opportunistic pathogen particularly recalcitrant to antibiotics. The screening of around 230,000 small molecules yielded a very low hit rate of 0.002% after triaging for known antibiotics. The only novel hit that stood out was the antimetabolite oxythiamine. Oxythiamine is a known transketolase inhibitor in eukaryotic cells, but its antibacterial potency has not been reported. Metabolic and transcriptomic analyses indicated that oxythiamine is intracellularly converted to oxythiamine pyrophosphate and subsequently inhibits several vitamin-B1-dependent enzymes, sensitizing the bacteria to several antibiotic and non-antibiotic drugs such as tetracyclines, 5-fluorouracil, and auranofin. The positive interaction between 5-fluorouracil and oxythiamine was confirmed in a murine ocular infection model, indicating relevance during infection. Together, this study revealed a system-level significance of thiamine metabolism perturbation that sensitizes P. aeruginosa to multiple small molecules, a property that could inform on the development of a rational drug combination.

Molecular properties prediction, synthesis, and antimicrobial activity of bis(azolyl)sulfonamidoacetamides

A library of bis(azolyl)sulfonamidoacetamides was prepared by the reaction of azolylsulfonylamines with azolylchloroacetamides in the presence of pyridine/4-(dimethylamino)pyridine (DMAP) under ultrasonication. The reaction proceeded well with DMAP, resulting in a higher yield of the products. The antimicrobial activity of the compounds indicated that N-{5-[N-(2-{[4-(4-chloro-1H-pyrrol-2-yl)-1H-imidazol-2-yl)amino}-2-oxoethyl)sulfamoyl]-4-phenylthiazol-2-yl}benzamide (22a), N-{5-[N-(2-{[4-(4-chloro-1H-pyrrol-2-yl)-1H-imidazol-2-yl]amino}-2-oxoethyl)sulfamoyl]-4-(4-chlorophenyl)thiazol-2-yl}benzamide (22c), and N-{5-[N-(2-{[4-(4-chloro-1H-pyrrol-2-yl)-1H-imidazol-2-yl]amino}-2-oxoethyl)sulfamoyl]-4-(4-chloro-phenyl)-1H-imidazol-2-yl}benzamide (24c) exhibited a low minimal inhibitory concentration (MIC) against Bacillus subtilis, equal to the standard drug, chloramphenicol. Compounds 22c and 24c also showed low MICs against Aspergillus niger, equal to the standard drug, ketoconazole. The molecular properties of the synthesized molecules were studied to identify druglikeness properties of the target compounds. On the basis of molecular properties prediction, 19a, 19b, 20b, 20c, 21a-c, 22b, 22c, and 23a-c can be treated as drug candidates.

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.

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.

Utilization of nitriles as the nitrogen source: practical and economical construction of 4-aminopyrimidine and β-enaminonitrile skeletons

A highly practical and economical method for the synthesis of 4-aminopyrimidines and β-enaminonitriles from mixed organonitriles is reported.

Exploiting the Sensitivity of Nutrient Transporter Deletion Strains in Discovery of Natural Product Antimetabolites

Actinomycete secondary metabolites are a renowned source of antibacterial chemical scaffolds. Herein, we present a target-specific approach that increases the detection of antimetabolites from natural sources by screening actinomycete-derived extracts against nutrient transporter deletion strains. On the basis of the growth rescue patterns of a collection of 22 Escherichia coli (E. coli) auxotrophic deletion strains representative of the major nutrient biosynthetic pathways, we demonstrate that antimetabolite detection from actinomycete-derived extracts prepared using traditional extraction platforms is masked by nutrient supplementation. In particular, we find poor sensitivity for the detection of antimetabolites targeting vitamin biosynthesis. To circumvent this and as a proof of principle, we exploit the differential activity of actinomycete extracts against E. coli ΔyigM, a biotin transporter deletion strain versus wildtype E. coli. We achieve more than a 100-fold increase in antimetabolite sensitivity using this method and demonstrate a successful bioassay-guided purification of the known biotin antimetabolite, amiclenomycin. Our findings provide a unique solution to uncover the full potential of naturally derived antibiotics.

Genomic insights into the thiamin metabolism of Paenibacillus thiaminolyticus NRRL B-4156 and P. apiarius NRRL B-23460

Paenibacillus thiaminolyticus is the model organism for studying thiaminase I, an enigmatic extracellular enzyme. Originally isolated from the feces of clinical patients suffering from thiamin deficiency, P. thiaminolyticus has been implicated in thiamin deficiencies in humans and other animals due to its ability to produce this thiamin-degrading enzyme. Its close relative, P. apiarius, also produces thiaminase I and was originally isolated from dead honeybee larvae, though it has not been reported to be a honeybee pathogen. We generated draft genomes of the type strains of both species, P. thiaminolyticus NRRL B-4156 and P. apiarius NRRL B-23460, to deeply explore potential routes of thiamin metabolism. We discovered that the thiaminase I gene is located in a highly conserved operon with thiamin biosynthesis and salvage genes, as well as genes involved in the biosynthesis of the antibiotic bacimethrin. Based on metabolic pathway predictions, P. apiarius NRRL B-23460 has the genomic capacity to synthesize thiamin de novo using a pathway that is rarely seen in bacteria, but P. thiaminolyticus NRRL B-4156 is a thiamin auxotroph. Both genomes encode importers for thiamin and the pyrimidine moiety of thiamin, as well as enzymes to synthesize thiamin from pyrimidine and thiazole.

A strictly monofunctional bacterial hydroxymethylpyrimidine phosphate kinase precludes damaging errors in thiamin biosynthesis

The canonical kinase (ThiD) that converts the thiamin biosynthesis intermediate hydroxymethylpyrimidine (HMP) monophosphate into the diphosphate can also very efficiently convert free HMP into the monophosphate in prokaryotes, plants, and fungi. This HMP kinase activity enables salvage of HMP, but it is not substrate-specific and so allows toxic HMP analogs and damage products to infiltrate the thiamin biosynthesis pathway. Comparative analysis of bacterial genomes uncovered a gene, thiD2, that is often fused to the thiamin synthesis gene thiE and could potentially encode a replacement for ThiD. Standalone ThiD2 proteins and ThiD2 fusion domains are small (∼130 residues) and do not belong to any previously known protein family. Genetic and biochemical analyses showed that representative standalone and fused ThiD2 proteins catalyze phosphorylation of HMP monophosphate, but not of HMP or its toxic analogs and damage products such as bacimethrin and 5-(hydroxymethyl)-2-methylpyrimidin-4-ol. As strictly monofunctional HMP monophosphate kinases, ThiD2 proteins eliminate a potentially fatal vulnerability of canonical ThiD, at the cost of the ability to reclaim HMP formed by thiamin turnover.

Pre- and post-weaning diet alters the faecal metagenome in the cat with differences in vitamin and carbohydrate metabolism gene abundances

Dietary format, and its role in pet nutrition, is of interest to pet food manufacturers and pet owners alike. The aim of the present study was to investigate the effects of pre- and post-weaning diets (kibbled or canned) on the composition and function of faecal microbiota in the domestic cat by shotgun metagenomic sequencing and gene taxonomic and functional assignment using MG-RAST. Post-weaning diet had a dramatic effect on community composition; 147 of the 195 bacterial species identified had significantly different mean relative abundances between kittens fed kibbled and canned diets. The kittens fed kibbled diets had relatively higher abundances of Lactobacillus (>100-fold), Bifidobacterium (>100-fold), and Collinsella (>9-fold) than kittens fed canned diets. There were relatively few differences in the predicted microbiome functions associated with the pre-weaning diet. Post-weaning diet affected the abundance of functional gene groups. Genes involved in vitamin biosynthesis, metabolism, and transport, were significantly enriched in the metagenomes of kittens fed the canned diet. The impact of post-weaning diet on the metagenome in terms of vitamin biosynthesis functions suggests that modulation of the microbiome function through diet may be an important avenue for improving the nutrition of companion animals.

Investigation of C-5 alkynyl (alkynyloxy or hydroxymethyl) and/or N-3 propynyl substituted pyrimidine nucleoside analogs as a new class of antimicrobial agents

The resurgence of mycobacterial infections and the emergence of drug-resistant strains urgently require a new class of agents that are distinct than current therapies. A group of 5-ethynyl (6-10), 5-(2-propynyloxy) (16, 18, 20, 22, 24), 5-(2-propynyloxy)-3-N-(2-propynyl) (17, 19, 21, 23, 25) and 5-hydroxymethyl-3-N-(2-propynyl) (30-33) derivatives of pyrimidine nucleosides were synthesized and evaluated against mycobacteria [Mycobacterium tuberculosis (Mtb), Mycobacterium bovis (BCG) and Mycobacterium avium], gram-positive bacteria (Staphylococcus aureus and Enterococcus faecalis) and gram-negative bacteria (Escherichia coli, Salmonella typhimurium and Pseudomonas aeruginosa) alone and in combination with existing drugs in in vitro assays. Although several compounds exhibited marked inhibitory activity at a higher concentration against Mtb, M. bovis, S. aureus and E. faecalis, they displayed unexpected synergistic and additive interactions at their lower concentrations with antitubercular drugs isoniazid and rifampicin, and antibacterial drug gentamicin. The active analogues were also found to inhibit intracellular Mtb in a human monocytic cell line infected with H37Ra. Oral administration of 5-hydroxymethyl-3-N-(2-propynyl)-3'-azido-2',3'-dideoxyuridine (32) and 5-hydroxymethyl-3-N-(2-propynyl)-2',3'-dideoxyuridine (33) at a dose of 100mg/kg for two weeks showed promising in vivo effects in mice infected with Mtb (H37Ra). No in vitro cytotoxicity of the test compounds was observed up to the highest concentration tested (CC50>300μg/mL).

Competence of Thiamin Diphosphate-Dependent Enzymes with 2'-Methoxythiamin Diphosphate Derived from Bacimethrin, a Naturally Occurring Thiamin Anti-vitamin

Bacimethrin (4-amino-5-hydroxymethyl-2-methoxypyrimidine), a natural product isolated from some bacteria, has been implicated as an inhibitor of bacterial and yeast growth, as well as in inhibition of thiamin biosynthesis. Given that thiamin biosynthetic enzymes could convert bacimethrin to 2'-methoxythiamin diphosphate (MeOThDP), it is important to evaluate the effect of this coenzyme analogue on thiamin diphosphate (ThDP)-dependent enzymes. The potential functions of MeOThDP were explored on five ThDP-dependent enzymes: the human and Escherichia coli pyruvate dehydrogenase complexes (PDHc-h and PDHc-ec, respectively), the E. coli 1-deoxy-D-xylulose 5-phosphate synthase (DXPS), and the human and E. coli 2-oxoglutarate dehydrogenase complexes (OGDHc-h and OGDHc-ec, respectively). Using several mechanistic tools (fluorescence, circular dichroism, kinetics, and mass spectrometry), it was demonstrated that MeOThDP binds in the active centers of ThDP-dependent enzymes, however, with a binding mode different from that of ThDP. While modest activities resulted from addition of MeOThDP to E. coli PDHc (6-11%) and DXPS (9-14%), suggesting that MeOThDP-derived covalent intermediates are converted to the corresponding products (albeit with rates slower than that with ThDP), remarkably strong activity (up to 75%) resulted upon addition of the coenzyme analogue to PDHc-h. With PDHc-ec and PDHc-h, the coenzyme analogue could support all reactions, including communication between components in the complex. No functional substitution of MeOThDP for ThDP was in evidence with either OGDH-h or OGDH-ec, shown to be due to tight binding of ThDP.

Biosynthesis of a thiamin antivitamin in Clostridium botulinum

Bacimethrin-derived 2′-methoxythiamin pyrophosphate inhibits microbial growth by disrupting metabolic pathways dependent on thiamin-utilizing enzymes. This study describes the discovery of the bacimethrin biosynthetic gene cluster of Clostridium botulinum A ATCC 19397 and in vitro reconstitution of bacimethrin biosynthesis from cytidine 5′-monophosphate.

Discovery of novel 5-(ethyl or hydroxymethyl) analogs of 2'-'up' fluoro (or hydroxyl) pyrimidine nucleosides as a new class of Mycobacterium tuberculosis, Mycobacterium bovis and Mycobacterium avium inhibitors

Discovery of novel antimycobacterial compounds that work on distinctive targets and by diverse mechanisms of action is urgently required for the treatment of mycobacterial infections due to the emerging global health threat of tuberculosis. We have identified a new class of 5-ethyl or hydroxy (or methoxy) methyl-substituted pyrimidine nucleosides as potent inhibitors of Mycobacterium bovis, Mycobacterium tuberculosis (H37Ra, H37Rv) and Mycobacterium avium. A series of 2'-'up' fluoro (or hydroxy) nucleosides (1, 2, 4-6, 9, 10, 13, 16, 18, 21, 24) was synthesized and evaluated for antimycobacterial activity. Among 2'-fluorinated compounds, 1-(3-bromo-2,3-dideoxy-2-fluoro-β-d-arabinofuranosyl)-5-ethyluracil (13) exhibited promising activity against M. bovis and Mtb alone, and showed synergism when combined with isoniazid. The most active compound emerging from these studies, 1-(β-d-arabinofuranosyl)-4-thio-5-hydroxymethyluracil (21) inhibited Mtb (H37Ra) (MIC(50)=0.5 μg/mL) and M. bovis (MIC(50)=0.5 μg/mL) at low concentrations, and was ten times more potent against Mtb (H37Ra) than cycloserine (MIC(50)=5.0 μg/mL), a second line drug. It also showed an additive effect when combined with isoniazid. Compound 21 retained sensitivity against a rifampicin-resistant (H37Rv) strain of Mtb (MIC(50)=1 μg/mL) at concentrations similar to that for a rifampicin-sensitive (H37Rv) strain, suggesting that it has no cross-resistance to a first-line anti-TB drug. In addition, the replication of M. avium was also inhibited by 21 (MIC(50)=10 μg/mL). No cellular toxicity of 13 or 21 was observed up to the highest concentration tested (CC(50)>100 μg/mL). These observations offer promise for a new drug treatment regimen to augment and complement the current chemotherapy of TB.

Thiamin (vitamin B1) biosynthesis and regulation: a rich source of antimicrobial drug targets?

Drug resistance of pathogens has necessitated the identification of novel targets for antibiotics. Thiamin (vitamin B1) is an essential cofactor for all organisms in its active form thiamin diphosphate (ThDP). Therefore, its metabolic pathways might be one largely untapped source of antibiotics targets. This review describes bacterial thiamin biosynthetic, salvage, and transport pathways. Essential thiamin synthetic enzymes such as Dxs and ThiE are proposed as promising drug targets. The regulation mechanism of thiamin biosynthesis by ThDP riboswitch is also discussed. As drug targets of existing antimicrobial compound pyrithiamin, the ThDP riboswitch might serves as alternative targets for more antibiotics.

ThiC is an [Fe-S] cluster protein that requires AdoMet to generate the 4-amino-5-hydroxymethyl-2-methylpyrimidine moiety in thiamin synthesis

Thiamin pyrophosphate is a required cofactor in all organisms. The biosynthesis of thiamin requires the independently synthesized 4-amino-5-hydroxymethyl-2-methylpyrimidine pyrophosphate (HMP-PP) and 5-hydroxyethyl-4-methylthiazole phosphate (THZ-P) moieties. In bacteria, the pyrimidine moiety is derived from 5-aminoimidazole ribotide (AIR), and ThiC is the only gene product known to be required for this conversion in vivo. We report here the purification and characterization of the ThiC protein from Salmonella enterica. The data showed this protein generated HMP when AIR, S-adenosylmethionine (AdoMet), and an appropriate reducing agent were present. It is further shown that ThiC carries an oxygen labile [Fe-S] cluster essential for this activity.

Vitamin B1 and B6 in the malaria parasite: requisite or dispensable?

Vitamins are essential compounds mainly involved in acting as enzyme co-factors or in response to oxidative stress. In the last two years it became apparent that apicomplexan parasites are able to generate B vitamers such as vitamin B1 and B6 de novo. The biosynthesis pathways responsible for vitamin generation are considered as drug targets, since both provide a high degree of selectivity due to their absence in the human host. This report updates the current knowledge about vitamin B1 and B6 biosynthesis in malaria and other apicomplexan parasites. Owing to the urgent need for novel antimalarials, the significance of the biosynthesis and salvage of these vitamins is critically discussed in terms of parasite survival and their exploitation for drug development.

Vitamin and cofactor biosynthesis pathways in Plasmodium and other apicomplexan parasites

Vitamins are essential components of the human diet. By contrast, the malaria parasite Plasmodium falciparum and related apicomplexan parasites synthesize certain vitamins de novo, either completely or in parts. The various biosynthesis pathways are specific to different apicomplexan parasites and emphasize the distinct requirements of these parasites for nutrients and growth factors. The absence of vitamin biosynthesis in humans implies that inhibition of the parasite pathways might be a way to interfere specifically with parasite development. However, the roles of biosynthesis and uptake of vitamins in the regulation of vitamin homeostasis in parasites needs to be established first. In this article, the procurement of vitamins B(1), B(5) and B(6) by Plasmodium and other apicomplexan parasites is discussed.

Thiamine is synthesized by a salvage pathway in Rhizobium leguminosarum bv. viciae strain 3841

In the absence of added thiamine, Rhizobium leguminosarum bv. viciae strain 3841 does not grow in liquid medium and forms only "pin" colonies on agar plates, which contrasts with the good growth of Sinorhizobium meliloti 1021, Mesorhizobium loti 303099, and Rhizobium etli CFN42. These last three organisms have thiCOGE genes, which are essential for de novo thiamine synthesis. While R. leguminosarum bv. viciae 3841 lacks thiCOGE, it does have thiMED. Mutation of thiM prevented formation of pin colonies on agar plates lacking added thiamine, suggesting thiamine intermediates are normally present. The putative functions of ThiM, ThiE, and ThiD are 4-methyl-5-(beta-hydroxyethyl) thiazole (THZ) kinase, thiamine phosphate pyrophosphorylase, and 4-amino-5-hydroxymethyl-2-methyl pyrimidine (HMP) kinase, respectively. This suggests that a salvage pathway operates in R. leguminosarum, and addition of HMP and THZ enabled growth at the same rate as that enabled by thiamine in strain 3841 but elicited no growth in the thiM mutant (RU2459). There is a putative thi box sequence immediately upstream of the thiM, and a gfp-mut3.1 fusion to it revealed the presence of a promoter that is strongly repressed by thiamine. Using fluorescent microscopy and quantitative reverse transcription-PCR, it was shown that thiM is expressed in the rhizosphere of vetch and pea plants, indicating limitation for thiamine. Pea plants infected by RU2459 were not impaired in nodulation or nitrogen fixation. However, colonization of the pea rhizosphere by the thiM mutant was impaired relative to that of the wild type. Overall, the results show that a thiamine salvage pathway operates to enable growth of Rhizobium leguminosarum in the rhizosphere, allowing its survival when thiamine is limiting.

Vitamin B1 de novo synthesis in the human malaria parasite Plasmodium falciparum depends on external provision of 4-amino-5-hydroxymethyl-2-methylpyrimidine

Vitamin B1 (thiamine) is an essential cofactor for several key enzymes of carbohydrate metabolism. Mammals have to salvage this crucial nutrient from their diet to complement their deficiency of de novo synthesis. In contrast, bacteria, fungi, plants and, as reported here, Plasmodium falciparum, possess a vitamin B1 biosynthesis pathway. The plasmodial pathway identified consists of the three vitamin B1 biosynthetic enzymes 5-(2-hydroxy-ethyl)-4-methylthiazole (THZ) kinase (ThiM), 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP)/HMP-P kinase (ThiD) and thiamine phosphate synthase (ThiE). Recombinant PfThiM and PfThiD proteins were biochemically characterised, revealing K(m)app values of 68 microM for THZ and 12 microM for HMP. Furthermore, the ability of PfThiE for generating vitamin B1 was analysed by a complementation assay with thiE-negative E. coli mutants. All three enzymes are expressed throughout the developmental blood stages, as shown by Northern blotting, which indicates the presence of the vitamin B1 biosynthesis enzymes. However, cultivation of the parasite in minimal medium showed a dependency on the provision of HMP or thiamine. These results demonstrate that the human malaria parasite P. falciparum possesses active vitamin B1 biosynthesis, which depends on external provision of thiamine precursors.

A Genetic Screen for the Identification of Thiamin Metabolic Genes

A genetic screen was developed for the identification of genes related to thiamin biosynthesis and degradation. Genes conferring resistance to bacimethrin or 4-amino-2-trifluoromethyl-5-hydroxymethylpyrimidine were selected from Escherichia coli and Bacillus subtilis genomic libraries. Hits from the selection included the known thiamin biosynthetic genes thiC, thiE, and dxs as well as five genes of previously unknown function (E. coli yjjX, yajO, ymfB, and cof and B. subtilis yveN). The gene products YmfB and Cof catalyze the hydrolysis of 4-amino-2-methyl-5-hydroxymethylpyrimidine pyrophosphate to 4-amino-2-methyl-5-hydroxymethylpyrimidine phosphate. YmfB also converts thiamin pyrophosphate into thiamin phosphate.

Crystal structure of 4-amino-5-hydroxymethyl-2-methylpyrimidine phosphate kinase from Salmonella typhimurium at 2.3 A resolution

The crystal structures of Salmonella typhimurium 4-amino-5-hydroxymethyl-2-methylpyrimidine phosphate kinase (HMPP kinase) and its complex with substrate HMP have been determined. HMPP kinase catalyzes two separate ATP-dependent phosphorylation reactions and is an essential enzyme in the thiamin biosynthetic pathway. HMPP kinase is a homodimer with one active site per monomer and is structurally homologous to members of the ribokinase family. A comparison of the structure of HMPP kinase with other members of the ribokinase family suggests an evolutionary progression. Modeling studies suggest that HMPP kinase catalyzes both of its phosphorylation reactions using in-line displacement mechanisms. We propose that the active site accommodates the two separate reactions by providing two different binding modes for the phosphate group of HMP phosphate.

The mechanism of action of bacimethrin, a naturally occurring thiamin antimetabolite

The mechanism of bacimethrin (2) toxicity has been determined. This compound is converted to 2'-methoxy-thiamin pyrophosphate (10) by the thiamin biosynthetic enzymes. Of the seven thiamin pyrophosphate utilizing enzymes in Escherichia coli, 2'-methoxy-thiamin pyrophosphate inhibits alpha-ketoglutarate dehydrogenase, transketolase, and deoxy-D-xylulose-5-phosphate synthase. Bacimethrin does not cause repression of the genes coding for the thiamin biosynthetic enzymes.