Recombinant Teredinibacter turnerae Na (+)-translocating NADH-quinone reductase subunit E

What is Teredinibacter turnerae and why is it significant in studying NADH-quinone reductase?

Teredinibacter turnerae is a cultivable cellulolytic Gammaproteobacterium belonging to the Cellvibrionaceae family that commonly occurs as an intracellular endosymbiont in the gills of wood-eating bivalves of the family Teredinidae (shipworms). This bacterium is significant for studying NADH-quinone reductase because it represents a unique ecological niche where energy conservation mechanisms may have evolved distinct properties. The bacterium has been isolated from different shipworm hosts collected worldwide and contains several genomic regions encoding enzymes for secondary metabolite biosynthesis alongside its primary metabolism genes .

How does the bacterial Na(+)-translocating NADH-quinone reductase differ from mitochondrial Complex I?

The bacterial Na(+)-translocating NADH-quinone reductase differs from mitochondrial Complex I in several key aspects:

• Structural complexity: Mammalian mitochondrial NADH-quinone oxidoreductase (Complex I) contains more than 40 subunits, whereas the bacterial counterpart (NDH-1) in organisms like Paracoccus denitrificans and Thermus thermophilus consists of only 14 subunits .

• Ion specificity: While mitochondrial Complex I translocates protons (H+), the bacterial Na(+)-translocating variant specifically couples electron transfer to sodium ion translocation.

• Conservation of functional domains: Despite these differences, both enzymes share homologous core subunits, such as the PSST subunit in mitochondria and its bacterial equivalent (NQO6), which play key roles in electron transfer by coupling iron-sulfur clusters to quinone .

What is the general function of subunit E in Na(+)-translocating NADH-quinone reductase?

Subunit E in Na(+)-translocating NADH-quinone reductase likely plays a crucial role in the respiratory chain by participating in the coupling mechanism between electron transfer and sodium ion translocation. While the search results don't specifically detail subunit E function, we can infer from related research on NADH-quinone oxidoreductases that this subunit may be involved in:

• Maintaining structural integrity of the enzyme complex

• Contributing to the sodium ion channel formation

• Participating in the electron transfer pathway from NADH to quinone

• Potentially interacting with other subunits to facilitate energy coupling

Similar to how the PSST subunit couples electron transfer from iron-sulfur cluster N2 to quinone in standard NADH-quinone oxidoreductases, subunit E likely has a specialized role in the sodium-translocating variant's energy conservation mechanism .

What are the optimal conditions for heterologous expression of recombinant T. turnerae Na(+)-translocating NADH-quinone reductase subunit E?

For optimal heterologous expression of recombinant T. turnerae Na(+)-translocating NADH-quinone reductase subunit E, researchers should consider:

• Expression system selection: E. coli BL21(DE3) is typically suitable for initial expression trials, given the bacterial origin of the protein.

• Growth conditions:

  • Medium: LB or TB supplemented with appropriate antibiotics

  • Temperature: Initial induction at 30°C followed by expression at lower temperatures (16-18°C) to enhance protein solubility

  • Induction: 0.1-0.5 mM IPTG when culture reaches OD600 of 0.6-0.8

• Construct design considerations:

  • Codon optimization for the expression host

  • Fusion tags to enhance solubility (MBP, SUMO, or Thioredoxin)

  • Inclusion of TEV or PreScission protease sites for tag removal

• Special considerations:

  • Co-expression with chaperones may improve folding

  • Supplementation with potential cofactors like iron and quinone derivatives

When designing experiments, researchers should be aware that membrane proteins like those involved in respiratory complexes often require specialized expression and purification strategies compared to soluble proteins.

What purification strategy provides the highest yield and activity for recombinant Na(+)-translocating NADH-quinone reductase subunit E?

A comprehensive purification strategy for obtaining high yield and activity of recombinant Na(+)-translocating NADH-quinone reductase subunit E should include:

• Cell lysis options:

  • Mechanical disruption (sonication or French press) in buffer containing 50 mM Tris-HCl pH 8.0, 300 mM NaCl, 10% glycerol, and protease inhibitors

  • For membrane-associated forms, include 0.5-1% non-denaturing detergent (DDM or CHAPS)

• Initial capture:

  • IMAC (Immobilized Metal Affinity Chromatography) for His-tagged constructs

  • Loading buffer: 50 mM Tris-HCl pH 8.0, 300 mM NaCl, 10% glycerol, 20 mM imidazole

  • Elution buffer: Same with 250-300 mM imidazole gradient

• Secondary purification:

  • Ion exchange chromatography (IEX) using either anion or cation exchange depending on the protein's pI

  • Size exclusion chromatography for final polishing and buffer exchange

• Activity preservation:

  • Include stabilizing agents like glycerol (10-20%)

  • Consider adding reducing agents like DTT or β-mercaptoethanol (1-5 mM)

  • Maintain sodium ions (100-150 mM NaCl) throughout purification

• Quality control assessment:

  • SDS-PAGE for purity evaluation

  • Western blotting for identity confirmation

  • Activity assays monitoring NADH oxidation spectrophotometrically

This strategic approach balances yield and activity preservation throughout the purification process.

What are the established methods for measuring Na(+)-translocating NADH-quinone reductase activity in vitro?

Several established methods can be employed to measure Na(+)-translocating NADH-quinone reductase activity in vitro:

• Spectrophotometric NADH oxidation assay:

  • Monitor decrease in absorbance at 340 nm (NADH oxidation)

  • Reaction buffer: 50 mM Tris-HCl (pH 7.5), 100 mM NaCl, 0.2 mM NADH

  • Initiate reaction with ubiquinone analog (50-100 μM)

  • Calculate activity using extinction coefficient of NADH (ε = 6,220 M⁻¹ cm⁻¹)

• Sodium ion translocation measurement:

  • Using sodium-sensitive fluorescent dyes (SBFI)

  • Reconstitute enzyme in liposomes with low internal sodium

  • Monitor sodium influx upon addition of NADH and quinone

• Membrane potential measurements:

  • Using voltage-sensitive dyes (DiSC3(5) or Oxonol VI)

  • Monitor fluorescence changes upon enzyme activation

• Oxygen consumption assay:

  • Clark-type electrode to measure coupled respiration

  • Compare rates in presence/absence of sodium ions

  • Use specific inhibitors (rotenone, piericidin A) for verification

Each method provides unique insights into different aspects of enzyme function, and combining multiple approaches yields more comprehensive functional characterization.

How can researchers distinguish between Na(+) and H(+) coupling in NADH-quinone reductase experiments?

Distinguishing between Na(+) and H(+) coupling in NADH-quinone reductase experiments requires systematic approaches:

• Ion dependence assays:

  • Measure activity in buffers where Na+ is replaced with K+ or choline

  • True Na+-dependent enzymes will show significantly reduced activity in Na+-free conditions

  • Titrate Na+ concentrations (0-300 mM) to determine Na+ dependency

• pH dependence analysis:

  • Compare activity profiles across pH range (5.5-9.0)

  • Na+-translocating enzymes typically show less pH sensitivity than H+-coupled ones

  • H+-coupled enzymes often exhibit optimal activity at specific pH values

• Specific inhibitor profiles:

  • Na+-specific ionophores (monensin) will dissipate sodium gradients selectively

  • H+-specific ionophores (CCCP, nigericin) affect only proton-coupled enzymes

• Isotope exchange experiments:

  • Use 22Na+ to directly measure sodium transport

  • Compare with experiments using pH-sensitive probes

• Data analysis and presentation:

This comprehensive approach enables researchers to conclusively determine the primary coupling ion for their NADH-quinone reductase variant.

What structural features distinguish subunit E from other components of Na(+)-translocating NADH-quinone reductase?

Subunit E of Na(+)-translocating NADH-quinone reductase possesses several distinctive structural features:

• Transmembrane topology:

  • Likely contains multiple transmembrane helices that participate in forming the sodium channel

  • Specific residues (aspartate, glutamate) positioned to coordinate Na+ ions

• Conserved motifs:

  • Contains signature sequences associated with sodium binding (D/E-XX-D/E)

  • May possess unique structural elements compared to proton-translocating counterparts

• Interaction interfaces:

  • Contains surfaces that interact with other subunits, particularly those involved in electron transfer

  • May share structural similarities with the PSST/NQO6 subunits found in standard NADH-quinone oxidoreductases, which are known to couple electron transfer from iron-sulfur cluster N2 to quinone

• Potential cofactor binding sites:

  • May coordinate with iron-sulfur clusters similar to the N2 cluster interactions reported in conventional NADH-quinone oxidoreductases

  • Could contain quinone-binding regions essential for electron transfer

While detailed structural information specific to T. turnerae Na(+)-translocating NADH-quinone reductase subunit E is limited in the search results, these features represent likely characteristics based on related bacterial respiratory complexes.

What is the proposed mechanism for Na(+) translocation coupled to electron transfer in this enzyme?

The proposed mechanism for Na(+) translocation coupled to electron transfer in Na(+)-translocating NADH-quinone reductase likely follows these steps:

• Initial electron entry:

  • NADH binds to the hydrophilic domain

  • Electrons are transferred to FMN and then through a series of iron-sulfur clusters

• Terminal electron transfer:

  • Similar to conventional NADH-quinone oxidoreductases, electrons likely flow through an iron-sulfur cluster (analogous to N2) to quinone

  • This step may involve a subunit similar to PSST/NQO6, which has been identified as coupling iron-sulfur cluster N2 to quinone in conventional systems

• Conformational coupling:

  • Electron transfer to quinone triggers conformational changes in transmembrane helices

  • These conformational changes alter the coordination environment of sodium binding sites

• Sodium translocation:

  • Sodium ions bound at the periplasmic side enter low-affinity sites

  • Following conformational change, these sites convert to high-affinity states

  • Subsequent conformational changes push sodium ions toward the cytoplasmic side

  • Final release occurs when sites return to low-affinity states facing the cytoplasm

• Energy transduction efficiency:

  • Typically, translocation of 1-2 Na+ ions occurs per electron pair transferred

  • This stoichiometry determines the bioenergetic efficiency of the process

This mechanism represents a working model based on current understanding of related respiratory complexes, with the specific details for T. turnerae enzyme awaiting further experimental verification.

How does the T. turnerae Na(+)-translocating NADH-quinone reductase compare evolutionarily to similar enzymes in other bacterial species?

The evolutionary relationship of T. turnerae Na(+)-translocating NADH-quinone reductase to similar enzymes in other bacteria reveals interesting patterns:

• Phylogenetic positioning:

  • As a member of the Gammaproteobacteria, T. turnerae's enzyme likely shares closer homology with other marine bacteria than with terrestrial species

  • May represent adaptations specific to the symbiotic lifestyle within shipworms

• Comparative features with characterized systems:

  • The bacterial NADH-quinone oxidoreductase (NDH-1) in Paracoccus denitrificans and Thermus thermophilus consists of 14 subunits, providing a baseline for comparison

  • The presence of four TonB genes in T. turnerae with dual roles in both iron transport and carbohydrate utilization suggests integrated metabolic adaptations

• Adaptations related to marine environment:

  • Sodium coupling may reflect adaptation to marine habitats where sodium gradients are naturally abundant

  • May show specific adaptations related to the shipworm endosymbiotic lifestyle

• Conservation of functional domains:

  • Critical functional regions, such as those involved in quinone binding, likely show high conservation similar to the way the inhibitor-binding site is conserved between the PSST subunit of mitochondria and NQO6 of bacteria

  • Other regions may show higher variability reflecting specific ecological adaptations

These evolutionary relationships provide context for understanding the specialized functions of T. turnerae's respiratory complex adaptations.

What ecological advantages might the Na(+)-coupled NADH-quinone reductase provide to T. turnerae in its shipworm symbiotic environment?

The Na(+)-coupled NADH-quinone reductase likely provides several ecological advantages to T. turnerae in its shipworm symbiotic environment:

• Bioenergetic efficiency in marine settings:

  • In the high-sodium marine environment, a Na+-coupled system allows direct utilization of abundant sodium gradients

  • May provide energetic advantages in the specialized shipworm gill environment

• Integration with wood-degrading metabolism:

  • The dual role of TonB systems in both iron acquisition and cellulose utilization in T. turnerae suggests integrated regulatory networks

  • Na+-coupled respiration may similarly be coordinated with wood-degradation pathways

• Adaptation to variable oxygen conditions:

  • Na+-coupled systems can function efficiently under microaerobic conditions that might be encountered in shipworm tissues

  • Provides metabolic flexibility in the fluctuating oxygen environments of marine wood-boring habitats

• Coordination with iron acquisition systems:

  • T. turnerae produces the siderophore turnerbactin for iron acquisition under limiting conditions

  • Na+-coupled bioenergetics may help maintain energy for siderophore production and transport even under iron limitation

• Potential role in host-symbiont interactions:

  • The unique respiratory system may contribute to signaling or recognition between host and symbiont

  • Could provide selective advantages in establishing and maintaining the specific gill symbiosis

This specialized respiratory system likely represents one of many adaptations that enable T. turnerae to thrive in its unique ecological niche.

How can recombinant T. turnerae Na(+)-translocating NADH-quinone reductase be utilized as a model system for studying respiratory complexes?

Recombinant T. turnerae Na(+)-translocating NADH-quinone reductase offers several advantages as a model system for studying respiratory complexes:

• Structural simplicity advantages:

  • Bacterial respiratory complexes contain fewer subunits than mammalian counterparts (14 vs. >40)

  • Provides a more manageable system for structure-function studies

  • Allows clearer interpretation of mutagenesis results

• Experimental applications:

  • Serves as a platform for investigating ion selectivity mechanisms

  • Enables comparative studies between Na+ and H+ coupling systems

  • Provides insights into the evolution of bioenergetic systems

• Methodological approaches:

  • Reconstitution in liposomes for controlled bioenergetic experiments

  • Site-directed mutagenesis to probe specific residues involved in ion translocation

  • Chimeric constructs with proton-coupled enzymes to identify ion selectivity determinants

• Integration with membrane vesicle studies:

  • T. turnerae produces membrane vesicles enriched with TonB-dependent receptors essential for carbohydrate and iron acquisition

  • These natural systems could be compared with recombinant enzyme studies

This model system bridges the gap between simpler bacterial respiratory complexes and the more elaborate eukaryotic systems, providing valuable insights into fundamental bioenergetic mechanisms.

What technical challenges must be overcome when working with membrane-bound components of NADH-quinone reductase complexes?

Researchers face several technical challenges when working with membrane-bound components of NADH-quinone reductase complexes:

• Expression and solubilization issues:

  • Membrane proteins often show toxicity when overexpressed

  • Require careful optimization of detergent types and concentrations

  • Often need specialized expression hosts or systems

• Purification complexities:

  • Maintaining native lipid interactions during extraction

  • Avoiding aggregation during concentration steps

  • Preserving multisubunit interactions throughout purification

• Functional assay limitations:

  • Detergent micelles may not replicate native membrane environment

  • Activity may be compromised during purification

  • Reconstitution in proteoliposomes introduces experimental variability

• Structural characterization challenges:

  • Difficulty in obtaining crystals for X-ray crystallography

  • Challenges in cryo-EM sample preparation and image processing

  • Limited NMR applications due to size constraints

• Practical solutions table:

These challenges require systematic troubleshooting and method optimization for successful experimental outcomes.

What are the potential interactions between T. turnerae Na(+)-translocating NADH-quinone reductase and the cellulose utilization pathways?

The interplay between T. turnerae Na(+)-translocating NADH-quinone reductase and cellulose utilization pathways represents a fascinating area for investigation:

• Energetic coupling mechanisms:

  • The Na(+)-translocating NADH-quinone reductase likely provides energy for active transport systems involved in cellulose utilization

  • Two of T. turnerae's four TonB genes (tonB1b and tonB2) function both for iron transport and carbohydrate utilization when cellulose is the sole carbon source

• Regulatory networks:

  • Unlike typical TonB systems regulated by iron availability, T. turnerae TonB genes appear to be expressed even under iron-rich conditions, possibly due to their role in cellulose utilization

  • This suggests integrated regulatory networks connecting respiratory function and carbon metabolism

• Membrane vesicle involvement:

  • T. turnerae produces membrane vesicles containing carbohydrate-active enzymes (CAZymes) with activities against cellulose, hemicellulose, and pectin

  • These vesicles are enriched with TonB-dependent receptors essential for carbohydrate acquisition

  • The respiratory complex may energize these transport systems

• Experimental approaches to study these interactions:

  • Transcriptomic analysis comparing expression profiles under different carbon sources

  • Metabolic flux analysis using isotope-labeled substrates

  • Construction of conditional mutants to probe functional relationships

This integrated respiratory and nutritional system likely represents a key adaptation enabling T. turnerae's unique lifestyle as a shipworm symbiont.

What methodologies are most effective for studying inhibitor interactions with Na(+)-translocating NADH-quinone reductase?

Several sophisticated methodologies can effectively study inhibitor interactions with Na(+)-translocating NADH-quinone reductase:

• Photoaffinity labeling approaches:

  • Design photoreactive analogs of known inhibitors (similar to the [³H]TDP approach used for Complex I)

  • Incorporate tritium or other isotopic labels for detection

  • Use competitive binding studies to identify specific binding sites

• Saturation transfer difference (STD) NMR:

  • Provides direct evidence of inhibitor-protein interactions

  • Can map binding epitopes of inhibitors

  • Requires milligram quantities of purified protein

• Microscale thermophoresis (MST):

  • Enables determination of binding constants (Kd)

  • Requires minimal protein amounts

  • Can detect subtle conformational changes upon inhibitor binding

• Computational approaches:

  • Molecular docking to predict binding sites

  • Molecular dynamics simulations to evaluate binding stability

  • Virtual screening to identify novel inhibitor candidates

• Functional assays with structure-activity relationship analysis:

  • Test diverse inhibitor analogs against enzyme activity

  • Correlate structural features with inhibition parameters

  • Generate comprehensive inhibition profiles (IC50, Ki values)

The most comprehensive understanding comes from combining multiple approaches—for example, computational predictions validated by photoaffinity labeling and functional assays. This multi-method strategy has successfully identified the PSST subunit as a key target for inhibitors in conventional NADH-quinone oxidoreductases .

What are the promising avenues for studying the role of Na(+)-translocating NADH-quinone reductase in T. turnerae symbiosis with shipworms?

Several promising research avenues could elucidate the role of Na(+)-translocating NADH-quinone reductase in the T. turnerae-shipworm symbiosis:

• In vivo symbiosis models:

  • Develop genetic manipulation systems for T. turnerae symbionts

  • Create conditional mutants of Na(+)-translocating NADH-quinone reductase genes

  • Monitor effects on colonization efficiency and stability within shipworm gills

• Metabolic interaction studies:

  • Investigate connections between respiratory function and the production of turnerbactin, a siderophore required for bacterial survival under iron-limiting conditions

  • Examine how Na+ bioenergetics interacts with cellulose degradation pathways

  • Study whether Na(+)-translocating NADH-quinone reductase activity influences antibiotic production, like the boronated tartrolon antibiotics produced by T. turnerae

• Host-microbe signaling research:

  • Investigate whether Na+ flux generated by the respiratory complex serves as a signaling mechanism between symbiont and host

  • Study potential coordination with membrane vesicle production, which may facilitate lignocellulose degradation in the host

• Comparative genomics approaches:

  • Compare respiratory complex genes across T. turnerae strains isolated from different shipworm species

  • Identify co-evolutionary patterns between host and symbiont respiratory systems

These research directions would significantly advance our understanding of this unique symbiotic relationship and the specialized adaptations enabling it.

How might structural biology approaches advance our understanding of Na(+)-translocating respiratory complexes?

Advanced structural biology approaches offer significant potential to enhance our understanding of Na(+)-translocating respiratory complexes:

• Cryo-electron microscopy (cryo-EM):

  • Can resolve membrane protein structures without crystallization

  • Enables visualization of different conformational states

  • Could reveal the complete architecture of the Na(+)-translocating complex and the specific arrangement of subunit E

  • Allows comparison with the PSST/NQO6 subunits already identified as key components in electron transfer coupling in conventional NADH-quinone oxidoreductases

• Integrative structural biology:

  • Combining X-ray crystallography of soluble domains with cryo-EM of the complete complex

  • Cross-linking mass spectrometry to map subunit interactions

  • Hydrogen-deuterium exchange to identify dynamic regions

• Time-resolved structural approaches:

  • Time-resolved cryo-EM to capture intermediate states

  • Serial crystallography using X-ray free-electron lasers to visualize catalytic intermediates

  • Correlation of structural changes with electrophysiological measurements

• Structure-guided functional studies:

  • Site-directed mutagenesis of predicted sodium coordination sites

  • Chimeric constructs between Na+ and H+ translocating variants

  • Introduction of spectroscopic probes at key positions identified from structures

The ultimate goal would be to create a molecular movie of the coupling mechanism between electron transfer and sodium translocation, revealing how energy is conserved in this specialized respiratory complex.