Recombinant Mycobacterium tuberculosis Fumarate reductase subunit C (frdC)

What is the structural composition of the fumarate reductase complex in Mycobacterium tuberculosis?

The fumarate reductase (FRD) in Mycobacterium tuberculosis is a heterotetrameric complex composed of four distinct subunits with specific functions:

• A catalytic subunit (FrdA)

• An iron-sulfur cluster subunit (FrdB)

• Two transmembrane subunits (FrdC and FrdD)

FrdC specifically functions in anchoring the catalytic components of the fumarate reductase complex to the cytoplasmic membrane, as part of the FrdC family of proteins . The complete complex is involved in anaerobic respiration and plays a crucial role in maintaining membrane potential in Mycobacteria under oxygen-limiting conditions .

How is the frdABCD operon organized in the Mycobacterium tuberculosis genome?

In M. tuberculosis H37Rv, the frdABCD operon (comprising genes Rv1552-Rv1555) encodes the complete fumarate reductase complex. The genomic coordinates for this region are approximately 1759435-1760555 in the M. tuberculosis H37Rv reference genome (accession NC_000962.3) . The operon arrangement ensures coordinated expression of all components needed for the functional enzyme complex.

Interestingly, in M. bovis BCG Moreau (the Brazilian vaccine strain), sequence analysis reveals a mutation in a poly-G homopolymeric region at the end of frdB when compared to M. tuberculosis, resulting in a sequence that codes for a hypothetical fused FrdBC protein . This finding demonstrates evolutionary differences in this operon among mycobacterial species.

What is the primary function of fumarate reductase in Mycobacterium tuberculosis metabolism?

Fumarate reductase plays multiple critical roles in M. tuberculosis metabolism:

• Anaerobic respiration: FRD catalyzes the reduction of fumarate to succinate, which is particularly important under oxygen-limiting conditions .

• Redox homeostasis: The enzyme helps maintain redox balance by restoring reducing equivalents under microaerophilic conditions .

• Membrane potential maintenance: FRD contributes to generating succinate that can be secreted to maintain membrane potential in oxygen-limiting, non-replicative Mycobacteria .

• Adaptation to hypoxia: During oxygen limitation, M. tuberculosis shows significant upregulation of frdA (212-fold increase in hypoxic non-replicating cultures at 0.1 mmHg oxygen compared to aerobic conditions), indicating its importance in adaptation to hypoxic environments .

• Early infection stages: Research suggests that the frdABCD operon is important for proper bacterial development during both axenic growth and macrophage infection .

How does the fumarate reductase complex contribute to Mycobacterium tuberculosis survival under hypoxic conditions?

Under hypoxic conditions, M. tuberculosis undergoes significant metabolic adaptations to survive. Experimental data shows that a drop in dissolved oxygen concentration from 50 mmHg to 0.42 mmHg leads to a 2.3-fold decrease in intracellular ATP levels with an almost 70-fold increase in the NADH/NAD+ ratio . This indicates that re-oxidation of NADH becomes limiting in the absence of a terminal electron acceptor.

The fumarate reductase complex plays a critical role in this adaptation through:

• Reversal of the TCA cycle: Gene expression analysis shows that enzymes involved in the oxidative direction of the TCA cycle (citrate synthase, aconitase, α-ketoglutarate dehydrogenase) are downregulated under hypoxia, while frdA is significantly upregulated .

• Succinate production and secretion: Under hypoxic conditions, M. tuberculosis shows significant accumulation of succinate in the extracellular milieu. Isotope labeling studies with 13C-labeled precursors confirmed that this succinate is produced by a reversal of the TCA cycle in the non-oxidative direction with net CO2 incorporation .

• Fermentative metabolism: The reductive branch of the TCA cycle is coupled to succinate secretion, creating a fermentative process that helps maintain an energized membrane under oxygen limitation .

This metabolic adaptation represents a potential target for treating latent tuberculosis, as it may be essential for bacterial survival in the hypoxic environments found within granulomas.

What structural and functional differences exist between the fumarate reductase complexes in M. tuberculosis and M. bovis BCG Moreau?

Comparative analysis of the fumarate reductase complexes from M. tuberculosis and M. bovis BCG Moreau reveals several key differences:

These findings suggest that while there are structural differences between the fumarate reductase complexes in these two mycobacterial species, their functional properties may be largely conserved.

What is the impact of frdABCD knockout on M. tuberculosis growth and infection?

Studies with knockout strains have provided important insights into the functional significance of the frdABCD operon:

• Axenic growth: Knockout of the frdABCD operon results in delayed growth under laboratory conditions, suggesting its importance even under aerobic conditions .

• Macrophage infection: The Δfrd knockout strain shows a lower internalization rate in macrophage infection assays compared to wild-type strains .

• In vivo pathogenesis: Interestingly, when C57Bl/6 mice were aerosol infected with an frdA knockout mutant, no difference was observed in the rate of replication or persistence compared to the wild-type strain, indicating that frdA is not essential for murine pathogenesis .

• Compensatory mechanisms: The absence of a strong phenotype in the frdA knockout mutant is likely due to the presence of compensatory mechanisms. M. tuberculosis encodes three potential fumarate reductase/succinate dehydrogenase complexes, and the other two homologs (sdhCDAB and Rv0247c-Rv0249c) may complement fumarate reductase activity under anaerobic conditions in the knockout .

• Enzyme activity measurements: Analysis of membrane-associated fumarate/succinate dehydrogenase activities showed similar specific enzyme activities in membrane fractions from both the frdA knockout and the parental wild-type strain under aerobic and anaerobic conditions, supporting the existence of functional compensation .

These findings highlight the metabolic plasticity of M. tuberculosis and its ability to adapt to genetic modifications through compensatory pathways.

How does transcriptional slippage affect expression of fumarate reductase components in mycobacteria?

Transcriptional slippage represents an important regulatory mechanism affecting fumarate reductase expression in mycobacteria:

• Mechanism: Transcriptional slippage occurs when mutations arise in repetitive regions (such as poly-G/C sequences) within a coding sequence. This process allows the production of both native and variant forms of a protein, potentially reducing the impact of mutations .

• Observed isoforms: In both M. tuberculosis and BCG Moreau, a 52 kDa protein band was detected in addition to the expected FrdB (27 kDa) or fused FrdBC (40 kDa) proteins. This larger isoform can be explained by a transcriptional slippage event .

• Evolutionary significance: This phenomenon is thought to be an evolutionary mechanism that reduces the impact of mutations by allowing the production of both the native and variant forms of proteins. In the case of the fumarate reductase complex, this may help maintain its functionality despite genetic alterations .

• Prevalence in mycobacteria: The data suggests that transcriptional slippage may be a more common phenomenon in mycobacteria than previously recognized, particularly in loci containing homopolymeric sequences .

This mechanism may contribute to the functional resilience of mycobacterial metabolism despite genetic mutations, potentially supporting adaptation to different environmental conditions.

What are the current approaches for studying fumarate reductase activity in M. tuberculosis?

Several methodological approaches have been developed to study fumarate reductase activity in M. tuberculosis:

• Recombinant protein production and purification:

  • The sequence corresponding to M. tuberculosis frdB can be cloned into expression vectors (e.g., pET28a(+)), adding a C-terminal 6-His tag

  • Purification can be performed using nickel affinity chromatography

  • Elution with imidazole buffer followed by dialysis against PBS

• Antibody production for detection:

  • Purified recombinant proteins can be used to immunize mice

  • The resulting polyclonal antibodies can be used in western blotting assays to detect FrdB-containing proteins

• Enzyme activity assays:

  • Membrane-associated fumarate/succinate dehydrogenase activities can be measured in membrane fractions from both wild-type and knockout strains

  • These assays can be performed under both aerobic and anaerobic conditions to assess activity differences

• Metabolomic analysis:

  • Isotope labeling with 13C-glucose can be used to track metabolic flux through the TCA cycle

  • Analysis of isotopomers of secreted succinate can help determine the direction of the TCA cycle and the contribution of fumarate reductase activity

• Hollow Fiber System of Tuberculosis (HFS-TB):

  • This system allows the study of M. tuberculosis under dynamic drug concentrations that mimic those observed in patients

  • It can be used to test the effect of potential fumarate reductase inhibitors on M. tuberculosis growth and metabolism

• Genetic knockout studies:

  • Creation of knockout strains by operon excision

  • Complementation with alleles from different mycobacterial species to assess functional conservation

These methodological approaches provide valuable tools for understanding the structure, function, and importance of fumarate reductase in M. tuberculosis metabolism.

Is the fumarate reductase complex a viable drug target for tuberculosis treatment?

The fumarate reductase complex shows significant potential as a drug target for tuberculosis treatment:

• Essentiality: While knockout studies in mice did not show a strong phenotype due to compensatory mechanisms, this does not diminish the potential of fumarate reductase as a drug target. As noted by researchers, "the compensatory adaptations that occur in genetic knockout mutants have many generations to become established which would not be the case during sudden interruption of a pathway during chemical inhibition" .

• Role in latent TB: The reductive branch of the TCA cycle coupled to succinate secretion represents a fermentative process that may be essential for survival during latent tuberculosis. Inhibiting this process could potentially target bacteria in their dormant state .

• Membrane anchors as targets: The membrane anchors of the fumarate reductase complex (FrdC and FrdD) have been described as druggable targets with predicted interactions with known drugs, such as sildenafil (Viagra), tadalafil (Cialis), and vardenafil (Levitra) .

• Considerations for drug design: Effective drug design focusing on fumarate reductase must take into account the need to inhibit products of all three potential fumarate reductase/succinate dehydrogenase complexes (sdh1, sdh2, and frd) in M. tuberculosis to overcome their possible functional redundancy .

• Hollow Fiber System testing: The HFS-TB model, which has been validated by both FDA and EMA for drug development, could be used to test potential fumarate reductase inhibitors. This model has shown 94% accuracy in predicting optimal doses for clinical use, optimal PK/PD drug exposures, susceptibility breakpoints, and emergence of drug resistance .

The unique features of mycobacterial fumarate reductase compared to human homologs make it an attractive target for selective inhibition, potentially leading to new antituberculosis drugs with novel mechanisms of action.

How do succinate dehydrogenase (SDH) inhibitors compare to potential fumarate reductase inhibitors for M. tuberculosis treatment?

Recent research has identified succinate dehydrogenase (SDH) inhibitors as promising next-generation antimicrobials against M. tuberculosis. These findings have relevance for fumarate reductase inhibitor development due to the structural and functional similarities between these enzyme complexes:

• Structural similarities: Fumarate reductase is structurally and mechanistically similar to succinate dehydrogenase (SDH), the respiratory chain complex II. These enzymes catalyze the redox interconversion of succinate and fumarate, requiring similar coenzymes and prosthetic groups .

• SDH inhibitor findings:

  • Lead SDH inhibitors demonstrate bacteriostatic activity against wild-type and drug-resistant strains of M. tuberculosis

  • Specific inhibition of SDH activity dysregulates mycobacterial metabolism and respiration

  • Inhibition results in the secretion of intracellular succinate

  • Chemical inhibition of SDH potentiates the activity of other bioenergetic inhibitors and prevents the emergence of resistance to various drugs

• Structural differences: M. tuberculosis Sdh1 has recently been identified as a new class of respiratory complex II (type F) with a novel electron transfer pathway and unique substrate-binding site. These features make it significantly different from the human counterpart, supporting its potential as a drug target .

• Combined targeting strategy: Given the functional redundancy among sdh1, sdh2, and frd in M. tuberculosis, an effective targeting strategy might involve developing inhibitors that affect all three enzyme complexes simultaneously .

These findings suggest that insights from SDH inhibitor development could inform the design of effective fumarate reductase inhibitors, and that a comprehensive approach targeting both enzyme complexes might be most effective for tuberculosis treatment.

What techniques can be used to express and purify recombinant M. tuberculosis frdC for structural and functional studies?

For researchers working with recombinant M. tuberculosis frdC, the following expression and purification protocol can be adapted from related work with frdB:

Expression System:

• Vector selection: pET28a(+) or similar expression vectors that allow for addition of affinity tags

• Tagging strategy: Addition of a C-terminal 6-His tag for purification

• Host strain: E. coli BL21(DE3) or similar strains optimized for recombinant protein expression

Cloning Strategy:

• Template: M. tuberculosis H37Rv genomic DNA

• Primers: Design with appropriate restriction sites (e.g., NcoI and XhoI) for directional cloning

• PCR amplification: Use high-fidelity polymerase (e.g., Platinum SuperFi DNA polymerase)

Purification Protocol:

• Cell lysis: Sonication in lysis buffer containing protease inhibitors

• Affinity chromatography:

  • Nickel affinity chromatography using Ni-NTA resin

  • Loading buffer: 20 mM Tris-HCl pH 8.0, 500 mM NaCl, 5 mM imidazole

  • Washing: 5-10 column volumes of loading buffer

  • Elution: Step gradient (10%, 20%, 30%, 40%, and 100%) with elution buffer containing 500 mM imidazole

• Analysis: 12% SDS-PAGE to confirm purity of fractions

• Post-purification processing: Pooling of pure fractions followed by dialysis against PBS

Validation:

• Western blotting: Using antibodies against the His-tag or specific antibodies against frdC

• Mass spectrometry: To confirm protein identity and integrity

• Circular dichroism: To assess secondary structure content and proper folding

Functional Assays:

• Membrane incorporation studies: To assess the ability of recombinant frdC to properly integrate into membranes

• Complex formation: Analysis of interactions with other fumarate reductase subunits (FrdA, FrdB, and FrdD)

• Enzyme activity assays: When assembled with other complex components

This approach would allow researchers to obtain purified recombinant frdC for structural studies (e.g., crystallography or cryo-EM) and functional analyses to better understand its role in the fumarate reductase complex.

What controls should be included when studying the effects of genetic manipulation of the frd operon in M. tuberculosis?

When studying the effects of genetic manipulation of the frd operon in M. tuberculosis, the following controls should be included to ensure robust and interpretable results:

1. Strain Controls:

• Wild-type parental strain: Essential baseline for comparison

• Complemented strain: Knockout strain with the wild-type gene reintroduced to confirm phenotype reversal

• Vector-only control: To account for effects of the vector itself

• Complementation with orthologous genes: From related species (e.g., M. bovis BCG) to assess functional conservation

2. Growth Condition Controls:

• Aerobic and anaerobic conditions: As fumarate reductase function is oxygen-dependent

• Different carbon sources: To assess metabolic flexibility

• Various stress conditions: Including acid stress, oxidative stress, and nutrient limitation

3. Molecular Verification Controls:

• RT-PCR of neighboring genes: To ensure no polar effects on adjacent genes

• Protein expression analysis: Western blotting to confirm absence of target protein

• Enzyme activity assays: To confirm functional consequences of genetic manipulation

4. Compensatory Mechanism Assessment:

• Expression analysis of homologous genes: Such as sdh1 and sdh2, to detect compensatory upregulation

• Enzymatic activity measurements: Of related pathways that might compensate for the loss of function

5. In vitro and in vivo Models:

• Macrophage infection assays: Both resting and activated macrophages

• Animal models: Including both standard laboratory mice and more relevant models that develop hypoxic granulomas

6. Metabolomic Analyses:

• 13C labeling studies: To track carbon flux through central metabolism

• Extracellular metabolite analysis: Particularly succinate secretion

• Intracellular metabolite analysis: To assess changes in TCA cycle intermediates

These comprehensive controls will help to accurately interpret the effects of genetic manipulation of the frd operon and account for the metabolic plasticity of M. tuberculosis that might otherwise confound results.

How can researchers distinguish between the roles of the three potential fumarate reductase/succinate dehydrogenase complexes in M. tuberculosis?

Distinguishing between the roles of the three potential fumarate reductase/succinate dehydrogenase complexes in M. tuberculosis (frdABCD, sdhCDAB, and Rv0247c-Rv0249c) requires a multi-faceted approach:

1. Single and Multiple Gene Knockouts:

• Create single knockouts of each complex

• Generate double knockouts in various combinations

• Attempt triple knockout (if viable) or use conditional expression systems if essential

• Compare phenotypes across various growth conditions

2. Gene Expression Analysis:

• Quantitative RT-PCR to measure expression levels under different conditions

• RNA-seq for genome-wide expression patterns

• Analysis of expression in response to knockout of other complexes to detect compensatory mechanisms

3. Protein-Specific Antibodies:

• Develop antibodies specific to each complex

• Use Western blotting to monitor protein levels

• Immunoprecipitation to study complex formation and interactions

4. Enzyme Activity Assays:

• Measure both succinate dehydrogenase and fumarate reductase activities

• Use membrane fractions to maintain native environment

• Compare activities under aerobic and anaerobic conditions

• Assess directionality of reaction (oxidative vs. reductive)

5. Metabolic Flux Analysis:

• Use 13C-labeled substrates to track carbon flow

• Analyze labeling patterns in metabolites to determine pathway utilization

• Quantify flux through oxidative and reductive pathways of the TCA cycle

6. Specific Inhibitors:

• Develop or identify inhibitors with selectivity for each complex

• Use chemical genetics approach to complement genetic studies

• Assess metabolic consequences of specific inhibition

7. Structural Biology:

• Determine structures of each complex

• Identify unique features that might suggest specialized functions

• Use information for structure-based drug design

8. In vivo Relevance:

• Assess expression of each complex in various infection models

• Test knockout strains in animal models

• Evaluate impact during different phases of infection (acute vs. chronic)

9. Comparative Table of Activities:

This comprehensive approach would allow researchers to delineate the specific roles of each complex and understand their contributions to M. tuberculosis metabolism under various conditions.

What are the emerging approaches for targeting fumarate reductase as part of combination therapy for tuberculosis?

Several promising approaches are emerging for targeting fumarate reductase as part of combination therapy for tuberculosis:

• Bioenergetic inhibitor combinations: Research has shown that inhibition of succinate dehydrogenase potentiates the activity of other bioenergetic inhibitors. Similar strategies could be applied to fumarate reductase inhibitors, targeting multiple components of mycobacterial energy metabolism simultaneously .

• Resistance prevention: Studies indicate that chemical inhibition of related enzymes can prevent the emergence of resistance to various drugs. Including fumarate reductase inhibitors in combination therapies might reduce the development of resistance to companion drugs .

• Fixed-dose combinations (FDC): A study of 32,239 newly diagnosed patients with drug-susceptible TB showed that FDC treatment improved patient's treatment completion without increasing risks of TB recurrence or drug resistance development. Incorporating fumarate reductase inhibitors into such combinations could be beneficial .

• Hollow Fiber System optimization: The validated HFS-TB model could be used to optimize drug combinations including fumarate reductase inhibitors, determining ideal dosing regimens and drug exposure levels before clinical trials .

• Targeting dormant bacteria: Since fumarate reductase is important under hypoxic conditions typical of granulomas, inhibitors could be particularly effective against dormant bacteria, complementing drugs that target actively replicating organisms .

• Membrane anchor targeting: Focusing on the membrane anchors of the fumarate reductase complex (FrdC and FrdD) might provide selective inhibition with minimal off-target effects on human enzymes .

• Comprehensive SDH/FRD inhibition: Given the functional redundancy among the three potential fumarate reductase/succinate dehydrogenase complexes, developing broad-spectrum inhibitors that target all three might be more effective than selective inhibitors .

These approaches represent promising avenues for incorporating fumarate reductase inhibitors into effective combination therapies for tuberculosis, potentially addressing both active disease and latent infection.

How might structural differences between human and mycobacterial fumarate reductase be exploited for selective drug design?

The structural differences between human and mycobacterial fumarate reductase offer significant opportunities for selective drug design:

• Unique electron transfer pathway: Mycobacterial Sdh1 (a related complex) has been identified as a new class of respiratory complex II (type F) with a novel electron transfer pathway. Similar unique features in the fumarate reductase complex could be targeted for selective inhibition .

• Unique substrate-binding site: Structural studies have revealed distinct substrate-binding sites in mycobacterial complexes compared to human counterparts. These differences could be exploited to design inhibitors with high selectivity .

• Rieske-type [2Fe-2S] cluster: The presence of a rarely observed Rieske-type [2Fe-2S] cluster embedded in the transmembrane region of mycobacterial Sdh1 suggests similar unique features might exist in fumarate reductase. These distinctive prosthetic groups could be targeted specifically .

• Membrane anchor targeting: The membrane anchors of the fumarate reductase complex (FrdC and FrdD) have been identified as druggable targets. Since these components show significant differences from human homologs, they represent promising targets for selective inhibition .

• Fused protein products: The fused FrdBC protein observed in some mycobacterial species (e.g., M. bovis BCG) represents a structural feature not found in human enzymes. This unique architecture could be exploited for selective targeting .

• Quinone binding sites: The quinone binding sites in mycobacterial respiratory complexes differ from those in human enzymes. These differences could be leveraged to design inhibitors that specifically disrupt electron transfer in mycobacterial fumarate reductase .

• Transcriptional slippage products: The unique protein isoforms produced by transcriptional slippage in mycobacteria could potentially be targeted specifically, as these variants are not present in human cells .

By focusing drug design efforts on these distinctive structural features, researchers could develop inhibitors with high selectivity for mycobacterial fumarate reductase, minimizing off-target effects on human enzymes and potentially reducing side effects in patients.