X-ray crystallography-based structural elucidation of enzyme-bound intermediates along the 1-deoxy-d-xylulose 5-phosphate synthase reaction coordinate

Bisubstrate Analog Inhibitors of DXP Synthase Show Species Specificity

1-Deoxy-d-xylulose 5-phosphate synthase (DXPS) is a unique thiamin diphosphate (ThDP)-dependent enzyme that catalyzes the formation of DXP, a branchpoint metabolite required for the biosynthesis of vitamins and isoprenoids in bacterial pathogens. DXPS has relaxed substrate specificity and utilizes a gated mechanism, equipping DXPS to sense and respond to diverse substrates. We speculate that pathogens utilize this distinct gated mechanism in different ways to support metabolic adaptation during infection. DXPS is susceptible to time-dependent inhibition by bisubstrate analogs. We suggest that potential differences in the ligand-gated mechanism that may accompany alternative activities of DXPS homologues may enable the development of species-specific bisubstrate analog inhibitors. Here, we evaluate known bisubstrate analog inhibitors of Escherichia coli DXPS (EcDXPS) against DXPS from Pseudomonas aeruginosa (PaDXPS), a Gram-negative pathogen with a remarkable capacity to adapt to diverse environments. Our results indicate that these inhibitors are significantly less potent against PaDXPS compared to EcDXPS. Acceptor site residues that stabilize the phosphonolactyl-ThDP adduct (PLThDP) of bisubstrate analog d-PheTrAP on EcDXPS are not as critical for stabilization of this PLThDP adduct on PaDXPS. Substitution of EcR99 or the analogous PaR106 reduces the potency of both d-PheTrAP and the simpler BAP scaffold, suggesting a common role of these arginine residues in stabilizing PLThDP adducts. However, although EcR99 is required for potent, time-dependent inhibition of EcDXPS by d-PheTrAP, PaR106 does not appear to govern slow-onset inhibition. This work demonstrates that species-specific targeting of DXPS by bisubstrate analogs is possible and highlights mechanistic differences that should be considered in the design of homologue-specific inhibitors, toward narrow-spectrum approaches targeting DXPS.

Aldehyde-based Activation of C2α-lactylthiamin Diphosphate Decarboxylation on Bacterial 1-deoxy-d-xylulose 5-phosphate Synthase

1-Deoxy-d-xylulose 5-phosphate synthase (DXPS) catalyzes the thiamin diphosphate (ThDP)-dependent formation of DXP from pyruvate (donor substrate) and d-glyceraldehyde 3-phosphate (d-GAP, acceptor substrate) in bacterial central metabolism. DXPS uses a ligand-gated mechanism in which binding of a small molecule "trigger" activates the first enzyme-bound intermediate, C2α-lactylThDP (LThDP), to form the reactive carbanion via LThDP decarboxylation. d-GAP is the natural acceptor substrate for DXPS and also serves a role as a trigger to induce LThDP decarboxylation in the gated step. Additionally, we have shown that O2 and d-glyceraldehyde (d-GA) can induce LThDP decarboxylation. We hypothesize this ligand-gated mechanism poises DXPS to sense and respond to cellular cues in metabolic remodeling during bacterial adaptation. Here we sought to characterize features of small molecule inducers of LThDP decarboxylation. Using a combination of CD, NMR and biochemical methods, we demonstrate that the α-hydroxy aldehyde moiety of d-GAP is sufficient to induce LThDP decarboxylation en route to DXP formation. A variety of aliphatic aldehydes also induce LThDP decarboxylation. The study highlights the capacity of DXPS to respond to different molecular cues, lending support to potential multifunctionality of DXPS and its metabolic regulation by this mechanism.

Identification and Characterization of Thiamine Diphosphate-Dependent Lyases with an Unusual CDG Motif

The thiamine diphosphate (ThDP)-binding motif, characterized by the canonical GDG(X)24-27N sequence, is highly conserved among ThDP-dependent enzymes. We investigated a ThDP-dependent lyase (JanthE from Janthinobacterium sp. HH01) with an unusual cysteine (C458) replacing the first glycine of this motif. JanthE exhibits a high substrate promiscuity and accepts long aliphatic α-keto acids as donors. Sterically hindered aromatic aldehydes or non-activated ketones are acceptor substrates, giving access to a variety of secondary and tertiary alcohols as carboligation products. The crystal structure solved at a resolution of 1.9 Å reveals that C458 is not primarily involved in cofactor binding as previously thought for the canonical glycine. Instead, it coordinates methionine 406, thus ensuring the integrity of the active site and the enzyme activity. In addition, we have determined the long-sought genuine tetrahedral intermediates formed with pyruvate and 2-oxobutyrate in the pre-decarboxylation states and deciphered the atomic details for their stabilization in the active site. Collectively, we unravel an unexpected role for the first residue of the ThDP-binding motif and unlock a family of lyases that can perform valuable carboligation reactions.

Cryo-EM structure of 1-deoxy-D-xylulose 5-phosphate synthase DXPS from Plasmodium falciparum reveals a distinct N-terminal domain

Plasmodium falciparum is the main causative agent of malaria, a deadly disease that mainly affects children under five years old. Artemisinin-based combination therapies have been pivotal in controlling the disease, but resistance has arisen in various regions, increasing the risk of treatment failure. The non-mevalonate pathway is essential for the isoprenoid synthesis in Plasmodium and provides several under-explored targets to be used in the discovery of new antimalarials. 1-deoxy-D-xylulose-5-phosphate synthase (DXPS) is the first and rate-limiting enzyme of the pathway. Despite its importance, there are no structures available for any Plasmodium spp., due to the complex sequence which contains large regions of high disorder, making crystallisation a difficult task. In this manuscript, we use cryo-electron microscopy to solve the P. falciparum DXPS structure at a final resolution of 2.42 Å. Overall, the structure resembles other DXPS enzymes but includes a distinct N-terminal domain exclusive to the Plasmodium genus. Mutational studies show that destabilization of the cap domain interface negatively impacts protein stability and activity. Additionally, a density for the co-factor thiamine diphosphate is found in the active site. Our work highlights the potential of cryo-EM to obtain structures of P. falciparum proteins that are unfeasible by means of crystallography.

Evaluation of ketoclomazone and its analogues as inhibitors of 1-deoxy-d-xylulose 5-phosphate synthases and other thiamine diphosphate (ThDP)-dependent enzymes

Most pathogenic bacteria, apicomplexan parasites and plants rely on the methylerythritol phosphate (MEP) pathway to obtain precursors of isoprenoids. 1-Deoxy-d-xylulose 5-phosphate synthase (DXPS), a thiamine diphosphate (ThDP)-dependent enzyme, catalyses the first and rate-limiting step of the MEP pathway. Due to its absence in humans, DXPS is considered as an attractive target for the development of anti-infectious agents and herbicides. Ketoclomazone is one of the earliest reported inhibitors of DXPS and antibacterial and herbicidal activities have been documented. This study investigated the activity of ketoclomazone on DXPS from various species, as well as the broader ThDP-dependent enzyme family. To gain further insights into the inhibition, we have prepared analogues of ketoclomazone and evaluated their activity in biochemical and computational studies. Our findings support the potential of ketoclomazone as a selective antibacterial agent.

An activity-based probe for antimicrobial target DXP synthase, a thiamin diphosphate-dependent enzyme

This work reports an alkyl acetylphosphonate (alkylAP) activity-based probe (ABP) for 1-deoxy-d-xylulose 5-phosphate synthase DXPS, a promising antimicrobial target. This essential thiamin diphosphate (ThDP)-dependent enzyme operates at a branchpoint in bacterial central metabolism and is believed to play key roles in pathogen adaptation during infection. How different bacterial pathogens harness DXPS activity to adapt and survive within host environments remains incompletely understood, and tools for probing DXPS function in different contexts of infection are lacking. Here, we have developed alkylAP-based ABP 1, designed to react with the ThDP cofactor on active DXPS to form a stable C2α-phosphonolactylThDP adduct which subsequently crosslinks to the DXPS active site upon photoactivation. ABP 1 displays low micromolar potency against DXPS and dose-dependent labeling of DXPS that is blocked by alkylAP-based inhibitors. The probe displays selectivity for DXPS over ThDP-dependent enzymes and is capable of detecting active DXPS in a complex proteome. These studies represent an important advance toward development of tools to probe DXPS function in different contexts of bacterial infection, and for drug discovery efforts on this target.

Potent Inhibition of E. coli DXP Synthase by a gem-Diaryl Bisubstrate Analog

New antimicrobial strategies are needed to address pathogen resistance to currently used antibiotics. Bacterial central metabolism is a promising target space for the development of agents that selectively target bacterial pathogens. 1-Deoxy-d-xylulose 5-phosphate synthase (DXPS) converts pyruvate and d-glyceraldehyde 3-phosphate (d-GAP) to DXP, which is required for synthesis of essential vitamins and isoprenoids in bacterial pathogens. Thus, DXPS is a promising antimicrobial target. Toward this goal, our lab has demonstrated selective inhibition of Escherichia coli DXPS by alkyl acetylphosphonate (alkylAP)-based bisubstrate analogs that exploit the requirement for ternary complex formation in the DXPS mechanism. Here, we present the first DXPS structure with a bisubstrate analog bound in the active site. Insights gained from this cocrystal structure guided structure-activity relationship studies of the bisubstrate scaffold. A low nanomolar inhibitor (compound 8) bearing a gem-dibenzyl glycine moiety conjugated to the acetylphosphonate pyruvate mimic via a triazole-based linker emerged from this study. Compound 8 was found to exhibit slow, tight-binding inhibition, with contacts to E. coli DXPS residues R99 and R478 demonstrated to be important for this behavior. This work has discovered the most potent DXPS inhibitor to date and highlights a new role of R99 that can be exploited in future inhibitor designs toward the development of a novel class of antimicrobial agents.

Disruption of an Active Site Network Leads to Activation of C2α-Lactylthiamin Diphosphate on the Antibacterial Target 1-Deoxy-d-xylulose-5-phosphate Synthase

The bacterial metabolic enzyme 1-deoxy-d-xylulose-5-phosphate synthase (DXPS) catalyzes the thiamin diphosphate (ThDP)-dependent formation of DXP from pyruvate and d-glyceraldehyde-3-phosphate (d-GAP). DXP is an essential bacteria-specific metabolite that feeds into the biosynthesis of isoprenoids, pyridoxal phosphate (PLP), and ThDP. DXPS catalyzes the activation of pyruvate to give the C2α-lactylThDP (LThDP) adduct that is long-lived on DXPS in a closed state in the absence of the cosubstrate. Binding of d-GAP shifts the DXPS-LThDP complex to an open state which coincides with LThDP decarboxylation. This gated mechanism distinguishes DXPS in ThDP enzymology. How LThDP persists on DXPS in the absence of cosubstrate, while other pyruvate decarboxylases readily activate LThDP for decarboxylation, is a long-standing question in the field. We propose that an active site network functions to prevent LThDP activation on DXPS until the cosubstrate binds. Binding of d-GAP coincides with a conformational shift and disrupts the network causing changes in the active site that promote LThDP activation. Here, we show that the substitution of putative network residues, as well as nearby residues believed to contribute to network charge distribution, predictably affects LThDP reactivity. Substitutions predicted to disrupt the network have the effect to activate LThDP for decarboxylation, resulting in CO2 and acetate production. In contrast, a substitution predicted to strengthen the network fails to activate LThDP and has the effect to shift DXPS toward the closed state. Network-disrupting substitutions near the carboxylate of LThDP also have a pronounced effect to shift DXPS to an open state. These results offer initial insights to explain the long-lived LThDP intermediate and its activation through disruption of an active site network, which is unique to DXPS. These findings have important implications for DXPS function in bacteria and its development as an antibacterial target.

Recent Advances on Key Enzymes of Microbial Origin in the Lycopene Biosynthesis Pathway

Lycopene is a common carotenoid found mainly in ripe red fruits and vegetables that is widely used in the food industry due to its characteristic color and health benefits. Microbial synthesis of lycopene is gradually replacing the traditional methods of plant extraction and chemical synthesis as a more economical and productive manufacturing strategy. The biosynthesis of lycopene is a typical multienzyme cascade reaction, and it is important to understand the characteristics of each key enzyme involved and how they are regulated. In this paper, the catalytic characteristics of the key enzymes involved in the lycopene biosynthesis pathway and related studies are first discussed in detail. Then, the strategies applied to the key enzymes of lycopene synthesis, including fusion proteins, enzyme screening, combinatorial engineering, CRISPR/Cas9-based gene editing, DNA assembly, and scaffolding technologies are purposefully illustrated and compared in terms of both traditional and emerging multienzyme regulatory strategies. Finally, future developments and regulatory options for multienzyme synthesis of lycopene and similar secondary metabolites are also discussed.

Design of thiamine analogues for inhibition of thiamine diphosphate (ThDP)-dependent enzymes: Systematic investigation through Scaffold-Hopping and C2-Functionalisation

Thiamine diphosphate (ThDP), the bioactive form of vitamin B1, is an essential coenzyme needed for processes of cellular metabolism in all organisms. ThDP-dependent enzymes all require ThDP as a coenzyme for catalytic activity, although individual enzymes vary significantly in substrate preferences and biochemical reactions. A popular way to study the role of these enzymes through chemical inhibition is to use thiamine/ThDP analogues, which typically feature a neutral aromatic ring in place of the positively charged thiazolium ring of ThDP. While ThDP analogues have aided work in understanding the structural and mechanistic aspects of the enzyme family, at least two key questions regarding the ligand design strategy remain unresolved: 1) which is the best aromatic ring? and 2) how can we achieve selectivity towards a given ThDP-dependent enzyme? In this work, we synthesise derivatives of these analogues covering all central aromatic rings used in the past decade and make a head-to-head comparison of all the compounds as inhibitors of several ThDP-dependent enzymes. Thus, we establish the relationship between the nature of the central ring and the inhibitory profile of these ThDP-competitive enzyme inhibitors. We also demonstrate that introducing a C2-substituent onto the central ring to explore the unique substrate-binding pocket can further improve both potency and selectivity.

1-deoxy-D-xylulose-5-phosphate synthase from Pseudomonas aeruginosa and Klebsiella pneumoniae reveals conformational changes upon cofactor binding

The ESKAPE bacteria are the six highly virulent and antibiotic-resistant pathogens that require the most urgent attention for the development of novel antibiotics. Detailed knowledge of target proteins specific to bacteria is essential to develop novel treatment options. The methylerythritol-phosphate (MEP) pathway, which is absent in humans, represents a potentially valuable target for the development of novel antibiotics. Within the MEP pathway, the enzyme 1-deoxy-D-xylulose-5-phosphate synthase (DXPS) catalyzes a crucial, rate-limiting first step and a branch point in the biosynthesis of the vitamins B1 and B6. We report the high-resolution crystal structures of DXPS from the important ESKAPE pathogens Pseudomonas aeruginosa and Klebsiella pneumoniae in both the co-factor-bound and the apo forms. We demonstrate that the absence of the cofactor thiamine diphosphate results in conformational changes that lead to disordered loops close to the active site that might be important for the design of potent DXPS inhibitors. Collectively, our results provide important structural details that aid in the assessment of DXPS as a potential target in the ongoing efforts to combat antibiotic resistance.

MEP pathway products allosterically promote monomerization of deoxy-D-xylulose-5-phosphate synthase to feedback-regulate their supply

Isoprenoids are a very large and diverse family of metabolites required by all living organisms. All isoprenoids derive from the double-bond isomers isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), which are produced by the methylerythritol 4-phosphate (MEP) pathway in bacteria and plant plastids. It has been reported that IPP and DMAPP feedback-regulate the activity of deoxyxylulose 5-phosphate synthase (DXS), a dimeric enzyme that catalyzes the main flux-controlling step of the MEP pathway. Here we provide experimental insights into the underlying mechanism. Isothermal titration calorimetry and dynamic light scattering approaches showed that IPP and DMAPP can allosterically bind to DXS in vitro, causing a size shift. In silico ligand binding site analysis and docking calculations identified a potential allosteric site in the contact region between the two monomers of the active DXS dimer. Modulation of IPP and DMAPP contents in vivo followed by immunoblot analyses confirmed that high IPP/DMAPP levels resulted in monomerization and eventual aggregation of the enzyme in bacterial and plant cells. Loss of the enzymatically active dimeric conformation allows a fast and reversible reduction of DXS activity in response to a sudden increase or decrease in IPP/DMAPP supply, whereas aggregation and subsequent removal of monomers that would otherwise be available for dimerization appears to be a more drastic response in the case of persistent IPP/DMAPP overabundance (e.g., by a blockage in their conversion to downstream isoprenoids). Our results represent an important step toward understanding the regulation of the MEP pathway and rational design of biotechnological endeavors aimed at increasing isoprenoid contents in microbial and plant systems.

Antibacterial Target DXP Synthase Catalyzes the Cleavage of d-Xylulose 5-Phosphate: a Study of Ketose Phosphate Binding and Ketol Transfer Reaction

The bacterial enzyme 1-deoxy-d-xylulose 5-phosphate synthase (DXPS) catalyzes the formation of DXP from pyruvate and d-glyceraldehyde 3-phosphate (d-GAP) in a thiamin diphosphate (ThDP)-dependent manner. In addition to its role in isoprenoid biosynthesis, DXP is required for ThDP and pyridoxal phosphate biosynthesis. Due to its function as a branch-point enzyme and its demonstrated substrate and catalytic promiscuity, we hypothesize that DXPS could be key for bacterial adaptation in the dynamic metabolic landscape during infection. Prior work in the Freel Meyers laboratory has illustrated that DXPS displays relaxed specificity toward donor and acceptor substrates and varies acceptor specificity according to the donor used. We have reported that DXPS forms dihydroxyethyl (DHE)ThDP from ketoacid or aldehyde donor substrates via decarboxylation and deprotonation, respectively. Here, we tested other DHE donors and found that DXPS cleaves d-xylulose 5-phosphate (X5P) at C2-C3, producing DHEThDP through a third mechanism involving d-GAP elimination. We interrogated DXPS-catalyzed reactions using X5P as a donor substrate and illustrated (1) production of a semi-stable enzyme-bound intermediate and (2) O2, H+, and d-erythrose 4-phosphate act as acceptor substrates, highlighting a new transketolase-like activity of DXPS. Furthermore, we examined X5P binding to DXPS and suggest that the d-GAP binding pocket plays a crucial role in X5P binding and turnover. Overall, this study reveals a ketose-cleavage reaction catalyzed by DXPS, highlighting the remarkable flexibility for donor substrate usage by DXPS compared to other C-C bond-forming enzymes.

Frontiers in the enzymology of thiamin diphosphate-dependent enzymes

Enzymes that use thiamin diphosphate (ThDP), the biologically active derivative of vitamin B1, as a cofactor play important roles in cellular metabolism in all domains of life. The analysis of ThDP enzymes in the past decades have provided a general framework for our understanding of enzyme catalysis of this protein family. In this review, we will discuss recent advances in the field that include the observation of "unusual" reactions and reaction intermediates that highlight the chemical versatility of the thiamin cofactor. Further topics cover the structural basis of cooperativity of ThDP enzymes, novel insights into the mechanism and structure of selected enzymes, and the discovery of "superassemblies" as reported, for example, acetohydroxy acid synthase. Finally, we summarize recent findings in the structural organisation and mode of action of 2-keto acid dehydrogenase multienzyme complexes and discuss future directions of this exciting research field.

First crystal structures of 1-deoxy-D-xylulose 5-phosphate synthase (DXPS) from Mycobacterium tuberculosis indicate a distinct mechanism of intermediate stabilization

The development of drug resistance by Mycobacterium tuberculosis and other pathogenic bacteria emphasizes the need for new antibiotics. Unlike animals, most bacteria synthesize isoprenoid precursors through the MEP pathway. 1-Deoxy-d-xylulose 5-phosphate synthase (DXPS) catalyzes the first reaction of the MEP pathway and is an attractive target for the development of new antibiotics. We report here the successful use of a loop truncation to crystallize and solve the first DXPS structures of a pathogen, namely M. tuberculosis (MtDXPS). The main difference found to other DXPS structures is in the active site where a highly coordinated water was found, showing a new mechanism for the enamine-intermediate stabilization. Unlike other DXPS structures, a “fork-like” motif could be identified in the enamine structure, using a different residue for the interaction with the cofactor, potentially leading to a decrease in the stability of the intermediate. In addition, electron density suggesting a phosphate group could be found close to the active site, provides new evidence for the D-GAP binding site. These results provide the opportunity to improve or develop new inhibitors specific for MtDXPS through structure-based drug design.

Reassessing the evolution of the 1-deoxy-D-xylulose 5-phosphate synthase family suggests a possible novel function for the DXS class 3 proteins

The methylerythritol 4-phosphate (MEP) pathway is of paramount importance for generating plastidial isoprenoids. The first enzyme of the MEP pathway, 1-deoxy-D-xylulose-5-phosphate synthase (DXS), catalyzes a flux-controlling step. In plants the DXS gene family is composed of three distinct classes with non-redundant functions. Although the DXS1 and DXS2 subfamilies have been well characterized, the DXS3 subfamily has been considerably understudied. Here, we carried out in silico and functional analyses to better understand the DXS3 class. Our phylogenetic analysis showed high variation in copy number among the different DXS classes, with the apparent absence of DXS1 class in some species. We found that DXS3 subfamily emerged later than DXS1 and DXS2 and it is under less intense purifying selection. Furthermore, in the DXS3 subfamily critical amino acids positions in the thiamine pyrophosphate binding pocket are not conserved. We demonstrated that the DXS3 proteins from Arabidopsis, Maize, and Rice lack functional DXS activity. Moreover, the Arabidopsis DXS3 protein displayed distinctive sub-organellar chloroplast localization not observed in any DXS1 or DXS2 proteins. Co-expression analysis of the DXS3 from Arabidopsis showed that, unlike DXS1 and DXS2 proteins, it co-expresses with genes related to post-embryonic development and reproduction and not with primary metabolism and isoprenoid synthesis.

The Thiamin-Requiring 3 Mutation of Arabidopsis 5-Deoxyxylulose-Phosphate Synthase 1 Highlights How the Thiamin Economy Impacts the Methylerythritol 4-Phosphate Pathway

The thiamin-requiring mutants of Arabidopsis have a storied history as a foundational model for biochemical genetics in plants and have illuminated the central role of thiamin in metabolism. Recent integrative genetic and biochemical analyses of thiamin biosynthesis and utilization imply that leaf metabolism normally operates close to thiamin-limiting conditions. Thus, the mechanisms that allocate thiamin-diphosphate (ThDP) cofactor among the diverse thiamin-dependent enzymes localized in plastids, mitochondria, peroxisomes, and the cytosol comprise an intricate thiamin economy. Here, we show that the classical thiamin-requiring 3 (th3) mutant is a point mutation in plastid localized 5-deoxyxylulose synthase 1 (DXS1), a key regulated enzyme in the methylerythritol 4-phosphate (MEP) isoprene biosynthesis pathway. Substitution of a lysine for a highly conserved glutamate residue (E323) located at the subunit interface of the homodimeric enzyme conditions a hypomorphic phenotype that can be rescued by supplying low concentrations of thiamin in the medium. Analysis of leaf thiamin vitamers showed that supplementing the medium with thiamin increased total ThDP content in both wild type and th3 mutant plants, supporting a hypothesis that the mutant DXS1 enzyme has a reduced affinity for the ThDP cofactor. An unexpected upregulation of a suite of biotic-stress-response genes associated with accumulation of downstream MEP intermediate MEcPP suggests that th3 causes mis-regulation of DXS1 activity in thiamin-supplemented plants. Overall, these results highlight that the central role of ThDP availability in regulation of DXS1 activity and flux through the MEP pathway.

Structural basis of substrate recognition and thermal protection by a small heat shock protein

Small heat shock proteins (sHsps) bind unfolding proteins, thereby playing a pivotal role in the maintenance of proteostasis in virtually all living organisms. Structural elucidation of sHsp-substrate complexes has been hampered by the transient and heterogeneous nature of their interactions, and the precise mechanisms underlying substrate recognition, promiscuity, and chaperone activity of sHsps remain unclear. Here we show the formation of a stable complex between Arabidopsis thaliana plastid sHsp, Hsp21, and its natural substrate 1-deoxy-D-xylulose 5-phosphate synthase (DXPS) under heat stress, and report cryo-electron microscopy structures of Hsp21, DXPS and Hsp21-DXPS complex at near-atomic resolution. Monomeric Hsp21 binds across the dimer interface of DXPS and engages in multivalent interactions by recognizing highly dynamic structural elements in DXPS. Hsp21 partly unfolds its central α-crystallin domain to facilitate binding of DXPS, which preserves a native-like structure. This mode of interaction suggests a mechanism of sHsps anti-aggregation activity towards a broad range of substrates.

Revealing Donor Substrate-Dependent Mechanistic Control on DXPS, an Enzyme in Bacterial Central Metabolism

The thiamin diphosphate-dependent enzyme 1-deoxy-d-xylulose 5-phosphate synthase (DXPS) catalyzes the formation of DXP from pyruvate (donor) and d-glyceraldehyde 3-phosphate (d-GAP, acceptor). DXPS is essential in bacteria but absent in human metabolism, highlighting it as a potential antibacterial drug target. The enzyme possesses unique structural and mechanistic features that enable development of selective inhibition strategies and raise interesting questions about DXPS function in bacterial pathogens. DXPS distinguishes itself within the ThDP enzyme class by its exceptionally large active site and random sequential mechanism in DXP formation. In addition, DXPS displays catalytic promiscuity and relaxed acceptor substrate specificity, yet previous studies have suggested a preference for pyruvate as the donor substrate when d-GAP is the acceptor substrate. However, such donor specificity studies are potentially hindered by a lack of knowledge about specific, alternative donor-acceptor pairs. In this study, we exploited the promiscuous oxygenase activity of DXPS to uncover alternative donor substrates for DXPS. Characterization of glycolaldehyde, hydroxypyruvate, and ketobutyrate as donor substrates revealed differences in stabilization of enzyme-bound intermediates and acceptor substrate usage, illustrating the influence of the donor substrate on reaction mechanism and acceptor specificity. In addition, we found that DXPS prevents abortive acetyl-ThDP formation from a DHEThDP carbanion/enamine intermediate, similar to transketolase, supporting the potential physiological relevance of this intermediate on DXPS. Taken together, these results offer clues toward alternative roles for DXPS in bacterial pathogen metabolism.

Identification of a 1-deoxy-D-xylulose-5-phosphate synthase (DXS) mutant with improved crystallographic properties

In this report, we describe a truncated Deinococcus radiodurans 1-deoxy-D-xylulose-5-phosphate synthase (DXS) protein that retains enzymatic activity, while slowing protein degradation and showing improved crystallization properties. With modern drug-design approaches relying heavily on the elucidation of atomic interactions of potential new drugs with their targets, the need for co-crystal structures with the compounds of interest is high. DXS itself is a promising drug target, as it catalyzes the first reaction in the 2-C-methyl-D-erythritol 4-phosphate (MEP)-pathway for the biosynthesis of the universal precursors of terpenes, which are essential secondary metabolites. In contrast to many bacteria and pathogens, which employ the MEP pathway, mammals use the distinct mevalonate-pathway for the biosynthesis of these precursors, which makes all enzymes of the MEP-pathway potential new targets for the development of anti-infectives. However, crystallization of DXS has proven to be challenging: while the first X-ray structures from Escherichia coli and D. radiodurans were solved in 2004, since then only two additions have been made in 2019 that were obtained under anoxic conditions. The presented site of truncation can potentially also be transferred to other homologues, opening up the possibility for the determination of crystal structures from pathogenic species, which until now could not be crystallized. This manuscript also provides a further example that truncation of a variable region of a protein can lead to improved structural data.

Molecular Cloning and Differential Gene Expression Analysis of 1-Deoxy-D-xylulose 5-Phosphate Synthase (DXS) in Andrographis paniculata (Burm. f) Nees

Andrographis paniculata 1-deoxy-D-xylulose-5-phosphate synthase (ApDXS) gene (GenBank Accession No MG271749.1) was isolated and cloned from leaves for the first time. Expression of ApDXS gene was carried out in Escherichia coli Rosetta cells. Tissue-specific ApDXS gene expression by quantitative RT-PCR (qRT-PCR) revealed maximum fold expression in the leaves followed by stem and roots. Further, the differential gene expression profile of Jasmonic acid (JA)-elicited in vitro adventitious root cultures showed enhanced ApDXS expression compared to untreated control cultures. A. paniculata 3-hydroxy-3-methylglutaryl-coenzyme A reductase (ApHMGR) gene expression was also studied where it was up-regulated by JA elicitation but showed lower expression compared to ApDXS. The highest expression of both genes was found at 25 µm JA elicitation followed by 50 µm. HPLC data indicated that the transcription levels were correlated with increased andrographolide accumulation. The peak level of andrographolide accumulation was recorded at 25 μM JA (9.38-fold) followed by 50 µM JA (7.58-fold) in elicitation treatments. The in silico generated ApDXS 3D model revealed 98% expected amino acid residues in the favored and 2% in the allowed regions of the Ramachandran plot with 92% structural reliability. Further, prediction of conserved domains and essential amino acids [Arg (249, 252, 255), Asn (307) and Ser (247)] involved in ligand/inhibitor binding was carried out by in silico docking studies. Our present findings will generate genomic information and provide a blueprint for future studies of ApDXS and its role in diterpenoid biosynthesis in A. paniculata.

The expression of TwDXS in the MEP pathway specifically affects the accumulation of triptolide

1-Deoxy-d-xylulose-5-phosphate synthase (DXS) is the first enzyme in the plant 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway of terpenoid synthesis. TwDXS is a prominent protein in the Tripterygium wilfordii proteome, with especially high expression in the root periderm. It is significantly regulated by methyl jasmonate. Here, we studied the influence of TwDXS expression on bioactive terpenoids in T. wilfordii. Specific fragments of TwDXS (GenBank: AKP20998.1) with lengths of 2148 and 437 bp were amplified to construct the overexpression (OE) and RNA-interference (RNAi) vectors, respectively. After transformation of suspension cells, the expression of TwDXS and genes related to the terpenoid biosynthetic pathway was measured using qRT-PCR. TwDXS mRNA level was 153 and 43% of the control in the OE and RNAi lines. Related genes in the 2-C-methyl-d-erythritol 4-phosphate (MEP), mevalonic acid (MVA) and downstream pathways showed similar trends to the changes of TwDXS expression. Ultra Performance Liquid Chromatography (UPLC) was employed to measure the accumulation of terpenoids. Importantly, the triptolide content showed significant differences in both the TwDXS OE (222.35% of the control) and RNAi (34.86% of the control). However, there were no obvious changes in the celastrol content. In this study, we verified that the expression of TwDXS affects triptolide but not celastrol in T. wilfordii via both TwDXS OE and RNAi experiments.

Active Site Histidines Link Conformational Dynamics with Catalysis on Anti-Infective Target 1-Deoxy-d-xylulose 5-Phosphate Synthase

The product of 1-deoxy-d-xyluose 5-phosphate (DXP) synthase, DXP, feeds into the bacterial biosynthesis of isoprenoids, thiamin diphosphate (ThDP), and pyridoxal phosphate. DXP is essential for human pathogens but not utilized by humans; thus, DXP synthase is an attractive anti-infective target. The unique ThDP-dependent mechanism and structure of DXP synthase offer ideal opportunities for selective targeting. Upon reaction with pyruvate, DXP synthase uniquely stabilizes the predecarboxylation intermediate, C2α-lactylThDP (LThDP), in a closed conformation. Subsequent binding of d-glyceraldehyde 3-phosphate induces an open conformation that is proposed to destabilize LThDP, triggering decarboxylation. Evidence for the closed and open conformations has been revealed by hydrogen-deuterium exchange mass spectrometry and X-ray crystallography, which indicate that H49 and H299 are involved in conformational dynamics and movement of the fork and spoon motifs away from the active site is important for the closed-to-open transition. Interestingly, H49 and H299 are critical for DXP formation and interact with the predecarboxylation intermediate in the closed conformation. H299 is removed from the active site in the open conformation of the postdecarboxylation state. In this study, we show that substitution at H49 and H299 negatively impacts LThDP formation by shifting the conformational equilibrium of DXP synthase toward an open conformation. We also present a method for monitoring the dynamics of the spoon motif that uncovered a previously undetected role for H49 in coordinating the closed conformation. Overall, our results suggest that H49 and H299 are critical for the closed, predecarboxylation state providing the first direct link between catalysis and conformational dynamics.