Recombinant Acorus americanus NAD (P)H-quinone oxidoreductase subunit 4L, chloroplastic

What is NAD(P)H-quinone oxidoreductase subunit 4L, chloroplastic from Acorus americanus and what is its function?

NAD(P)H-quinone oxidoreductase subunit 4L, chloroplastic (ndhE) is a critical component of the chloroplast NADH dehydrogenase-like (NDH) complex involved in photosynthetic electron transport. This protein functions as part of a multi-subunit complex that catalyzes the oxidation of NAD(P)H and the reduction of quinones or similar compounds as electron acceptors .

The protein participates specifically in cyclic electron transport in chloroplasts, which is essential for balancing the ATP/NADPH ratio during photosynthesis. This process enhances photosynthetic efficiency, particularly under environmental stress conditions . The protein is encoded by the ndhE gene and contains 101 amino acid residues forming a transmembrane structure that contributes to the proton translocation mechanism coupled with electron transport .

The functional role of this protein extends to:

• Photosynthetic light reactions

• ATP synthesis coupled electron transport

• Maintenance of redox balance in chloroplasts

• Stress response mechanisms in plants

How should recombinant NAD(P)H-quinone oxidoreductase be stored and handled for optimal stability?

Proper storage and handling of recombinant NAD(P)H-quinone oxidoreductase is crucial for maintaining its stability and enzymatic activity. Based on established protocols for similar proteins, the following guidelines are recommended :

Storage conditions:

• Store at -20°C or -80°C for extended storage periods

• Maintain the protein in a storage buffer containing Tris-based buffer with 50% glycerol

• Aliquot the protein to avoid repeated freeze-thaw cycles

• Working aliquots can be stored at 4°C for up to one week

Handling recommendations:

• Briefly centrifuge vials before opening to bring contents to the bottom

• Reconstitute lyophilized protein in deionized sterile water to a concentration of 0.1-1.0 mg/mL

• Add glycerol to a final concentration of 50% for long-term storage

• Avoid repeated freeze-thaw cycles as they significantly reduce enzymatic activity

Research has shown that oxidoreductases are generally sensitive to oxidative damage and thermal denaturation. Therefore, the addition of reducing agents such as DTT or β-mercaptoethanol at low concentrations may help maintain the protein in its active state .

What expression systems are commonly used for producing recombinant NAD(P)H-quinone oxidoreductase?

Several expression systems have been successfully employed for the production of recombinant NAD(P)H-quinone oxidoreductase, with Escherichia coli being the most widely used platform. The methodological approaches include:

E. coli expression system:

• Commonly uses BL21(DE3) or similar strains optimized for protein expression

• Typically employs T7 promoter-based expression vectors for controlled induction

• His-tag fusion is frequently utilized to facilitate purification using nickel affinity chromatography

The recombinant expression procedure generally follows this methodology:

• Clone the ndhE gene (full length 1-101 amino acids) into an appropriate expression vector

• Transform into competent E. coli cells

• Culture in a medium containing yeast extract (5-10 g/L), peptone (10-20 g/L), and NaCl (10 g/L)

• Induce protein expression (commonly with IPTG for T7-based systems)

• Harvest cells and lyse using appropriate buffer systems

• Purify using affinity chromatography based on the fusion tag

For optimal expression, research has shown that culture temperature, induction time, and inducer concentration need to be optimized on a case-by-case basis to balance protein yield with proper folding and activity .

What are common assays for measuring NAD(P)H-quinone oxidoreductase activity?

NAD(P)H-quinone oxidoreductase activity can be measured using several established assay methods, with the choice depending on the specific research question. The methodological approaches include:

Spectrophotometric assays:

• DCPIP reduction assay: Measures the reduction of 2,6-dichloroindophenol (DCPIP) at 600 nm

  • Reaction mixture: NAD(P)H, enzyme, and DCPIP in appropriate buffer

  • Activity is calculated from the decrease in absorbance over time

• Menadione reduction assay: Uses menadione as the quinone substrate

  • Reaction mixture: NAD(P)H, enzyme, and menadione

  • Activity is monitored by following NAD(P)H oxidation at 340 nm

• Cytochrome c reduction assay: Measures the reduction of cytochrome c in the presence of a quinone

  • Reaction mixture: NAD(P)H, enzyme, quinone substrate, and cytochrome c

  • Activity is monitored by the increase in absorbance at 550 nm

Advanced enzyme kinetic parameters:
Research on NAD(P)H-quinone oxidoreductases has established the following kinetic parameters for similar enzymes:

These assays provide valuable information about the enzyme's catalytic efficiency and substrate preferences, enabling researchers to characterize both wild-type and mutant forms of the enzyme .

How does the structure-function relationship of NAD(P)H-quinone oxidoreductase subunit 4L affect its catalytic mechanism?

The structure-function relationship of NAD(P)H-quinone oxidoreductase subunit 4L is complex and directly influences its catalytic mechanism. Recent structural studies and functional analyses have revealed several key aspects:

Structural determinants of function:

• The transmembrane domains create a hydrophobic environment crucial for quinone binding

• Conserved residues in the quinone binding pocket coordinate substrate orientation

• The spatial arrangement of the subunit within the NDH complex facilitates electron transfer pathways

• Interaction surfaces with other subunits allow for concerted conformational changes during catalysis

Research using heterodimer approaches has shown that subunits of NAD(P)H-quinone oxidoreductase function independently with two-electron acceptors but dependently with four-electron acceptors . This suggests a cooperative mechanism where structural changes in one subunit influence the activity of adjacent subunits during complex substrate reduction.

For example, when wild-type/mutant heterodimers were studied with H194A mutation, the heterodimer showed:

• With two-electron acceptors: Km values similar to wild-type homodimer but kcat values only about 50% of wild-type

• With four-electron acceptors: Both Km and kcat values similar to the low-efficiency mutant homodimer

This experimental evidence supports a model where structural integrity across multiple subunits is particularly important for handling more complex electron transfer processes, highlighting the intricate structure-function relationship in this enzyme.

What are the challenges and strategies for engineering recombinant NAD(P)H-quinone oxidoreductase for enhanced organic solvent tolerance?

Engineering recombinant NAD(P)H-quinone oxidoreductase for enhanced organic solvent tolerance presents significant challenges but also opportunities for biocatalytic applications. The methodological approaches and considerations include:

Challenges in organic solvent systems:

• Most native oxidoreductases exhibit low activities and stability in non-aqueous systems

• The catalytic efficiency often decreases due to conformational changes in organic solvents

• Substrate and product solubility issues in mixed aqueous-organic systems

• Cofactor regeneration complications in non-aqueous environments

Strategies for enhancing organic solvent tolerance:

• Structure-based design:

  • Analysis of amino acid interaction networks to identify flexible regions

  • Strengthening hydrophobic cores through targeted mutations

  • Introducing disulfide bridges to enhance structural stability

• Computational approaches:

  • Molecular dynamics simulations to assess protein behavior in organic solvents

  • Conservation and co-evolution analysis to identify critical residues

  • Prediction of solvent distribution around the protein structure

• Experimental methods:

  • Directed evolution with screening in organic solvent conditions

  • Semi-rational design targeting surface residues

  • Immobilization techniques to protect the enzyme from direct solvent exposure

Research has shown that oxidoreductases from extremophiles offer valuable templates for engineering solvent tolerance. For example, halophilic alcohol dehydrogenase retained 47% activity in 30% DMSO after 72 hours, while formate dehydrogenase from Burkholderia dolosa showed 170% activity in 30% DMSO compared to aqueous conditions .

The development of organic solvent-tolerant oxidoreductases offers significant advantages for industrial applications, including:

• Effective solvation of hydrophobic reactants

• Minimization of substrate or product inhibition

• Easier product separation

• Potential for novel reaction chemistries

How can site-directed mutagenesis be employed to investigate catalytic mechanisms of NAD(P)H-quinone oxidoreductase?

Site-directed mutagenesis is a powerful approach for investigating the catalytic mechanisms of NAD(P)H-quinone oxidoreductase. By strategically altering specific amino acid residues, researchers can elucidate structure-function relationships and reaction mechanisms.

Methodological approach for site-directed mutagenesis studies:

• Target selection:

  • Identify conserved residues through sequence alignment across species

  • Focus on residues in the NAD(P)H binding site, such as His-194

  • Target residues in quinone binding pockets

  • Examine residues at subunit interfaces for their role in quaternary structure

• Mutagenesis strategies:

  • Point mutations (e.g., His-194→Ala) to alter specific functional groups

  • Conservative substitutions to assess the importance of specific chemical properties

  • Creation of heterodimers with one wild-type and one mutant subunit to study subunit cooperation

• Functional characterization:

  • Compare kinetic parameters (Km, kcat) between wild-type and mutant proteins

  • Test enzyme activity with various substrates to establish substrate specificity profiles

  • Analyze protein stability and folding using circular dichroism and thermal shift assays

Case study findings:
Research on NAD(P)H:quinone oxidoreductase has demonstrated that the His-194→Ala mutation dramatically increases the Km for NADPH, indicating this residue's critical role in cofactor binding . When this mutation was introduced into one subunit of a heterodimer:

These results revealed that the subunits function independently with two-electron acceptors but dependently with four-electron acceptors, providing insights into the cooperative nature of the catalytic mechanism .

What are the implications of polymorphisms in NAD(P)H-quinone oxidoreductase genes for enzyme function and disease associations?

Polymorphisms in NAD(P)H-quinone oxidoreductase genes have significant implications for enzyme function and disease associations, particularly in the context of xenobiotic metabolism and cancer susceptibility.

Key polymorphisms and their functional impacts:

The most well-characterized polymorphism in NQO1 is a C→T substitution at position 609 of the cDNA (C609T), which results in a proline to serine change at position 187 of the protein. Research has established the following functional consequences:

• The mutant protein retains only 2% of the enzymatic activity of the wild-type protein

• Mutant protein is either not translated or rapidly degraded in cells homozygous for the mutation

• Individuals can be categorized as wild-type (C/C), heterozygous (C/T), or homozygous (T/T) for this polymorphism

Population distribution and disease associations:

Studies on lung cancer patients and control populations have revealed significant associations:

*Statistically significant difference (χ² = 5.52, p = 0.019)

When stratified by smoking status, a significant difference was observed between cases and controls (χ² = 3.88, p = 0.048), suggesting an interaction between the genetic polymorphism and environmental factors .

Research implications:
The C609T polymorphism has important implications for:

• Cancer susceptibility, particularly lung cancer

• Efficacy of certain chemotherapeutic agents that require NQO1 for bioactivation

• Detoxification capacity for quinone-containing compounds

• Individual variations in response to xenobiotic exposures

This polymorphism represents an important factor in personalized medicine approaches and highlights the significance of genetic variations in xenobiotic metabolizing enzymes .

How do chloroplast NAD(P)H-quinone oxidoreductase complexes differ structurally and functionally from their mitochondrial counterparts?

Chloroplast NAD(P)H-quinone oxidoreductase complexes (NDH) and mitochondrial respiratory complex I share evolutionary origins but have diverged significantly in structure and function to adapt to their specific cellular roles.

Structural comparison:

Functional differences:

• Primary role:

  • Chloroplast NDH: Participates in cyclic electron flow during photosynthesis, contributing to ATP production without net NADPH oxidation

  • Mitochondrial Complex I: Central to oxidative phosphorylation, coupling NADH oxidation to proton translocation for ATP synthesis

• Energy conservation:

  • Chloroplast NDH: Helps balance the ATP/NADPH ratio during photosynthesis, particularly under stress conditions

  • Mitochondrial Complex I: Primary entry point for electrons into the respiratory chain, essential for aerobic respiration

• Regulatory mechanisms:

  • Chloroplast NDH: Regulated by light intensity and redox state of the chloroplast

  • Mitochondrial Complex I: Regulated by oxygen availability and energy demand of the cell

• Evolutionary adaptations:

  • Chloroplast NDH: Has evolved to function optimally in the unique environment of the chloroplast thylakoid membrane

  • Mitochondrial Complex I: Optimized for the chemiosmotic coupling in mitochondria

The NAD(P)H-quinone oxidoreductase subunit 4L (ndhE) from Acorus americanus is specifically adapted to function within the chloroplast NDH complex, with structural features that facilitate its interaction with other chloroplast-specific subunits and its integration into the thylakoid membrane environment .

What potential applications exist for recombinant NAD(P)H-quinone oxidoreductase in biocatalysis and biotechnology?

Recombinant NAD(P)H-quinone oxidoreductase offers diverse applications in biocatalysis and biotechnology, leveraging its unique catalytic properties and adaptability to various reaction conditions.

Biocatalytic applications:

• Asymmetric reduction reactions:

  • Stereoselective reduction of quinones and other electron acceptors

  • Synthesis of chiral intermediates for pharmaceuticals

  • Production of flavor and fragrance compounds

• Oxidation reactions:

  • Selective oxidation of alcohols to aldehydes

  • Conversion of primary amines to imines

  • Oxidative coupling reactions

• Cofactor regeneration systems:

  • NAD(P)H regeneration coupled to oxidoreductive transformations

  • Integration into multi-enzyme cascade reactions

  • Development of cofactor-recycling biocatalytic systems

Biotechnological applications:

• Biosensors:

  • Detection of quinone-containing compounds

  • Environmental monitoring of pollutants

  • Medical diagnostics for specific metabolites

• Bioremediation:

  • Degradation of azo dyes in industrial effluents

  • Detoxification of quinone-containing pollutants

  • Transformation of environmental contaminants

• Pharmaceutical applications:

  • Activation of prodrugs

  • Detoxification of xenobiotics

  • Metabolism of quinone-containing drugs

Advantages of engineered oxidoreductases in non-aqueous systems:

The development of organic solvent-tolerant variants of NAD(P)H-quinone oxidoreductase offers several benefits for industrial applications:

Research has demonstrated that certain oxidoreductases maintain significant activity in organic solvents. For example, glucose-1-dehydrogenase from Paenibacillus pini retained 80% activity in 50% v/v DMSO, while formate dehydrogenase from Burkholderia dolosa showed 170% activity enhancement in 30% DMSO . These findings highlight the potential for developing robust biocatalysts for non-conventional media.

How can molecular dynamics simulations contribute to understanding NAD(P)H-quinone oxidoreductase behavior in different environments?

Molecular dynamics (MD) simulations offer powerful insights into NAD(P)H-quinone oxidoreductase behavior under various conditions, providing atomic-level details that are difficult to obtain experimentally.

Methodological approaches in MD simulations:

• System preparation:

  • Build protein structure based on crystallographic data or homology modeling

  • Place in appropriate membrane environment for membrane-bound subunits

  • Add explicit solvent (water, organic solvents, or mixtures)

  • Include cofactors and substrates in the binding sites

  • Apply appropriate force fields (e.g., CHARMM, AMBER, GROMOS)

• Simulation protocols:

  • Energy minimization to remove steric clashes

  • System equilibration under NPT conditions (constant pressure and temperature)

  • Production runs on nanosecond to microsecond timescales

  • Replica exchange methods for enhanced sampling

• Analysis techniques:

  • Root mean square deviation (RMSD) and fluctuation (RMSF) analyses

  • Secondary structure stability assessment

  • Hydrogen bond network analysis

  • Essential dynamics and principal component analysis

  • Binding free energy calculations

Applications to NAD(P)H-quinone oxidoreductase research:

MD simulations have been used to investigate various aspects of oxidoreductase function in different environments:

• Organic solvent effects:
  Research has combined experimental approaches with MD simulations to understand how organic solvents affect horse liver alcohol dehydrogenase (HLADH), providing insights applicable to NAD(P)H-quinone oxidoreductase. These studies revealed:

  • Protein location in biphasic media

  • Organic solvent distribution around the protein

  • Effects on enzyme conformation and dynamics

  • Solvent-dependent changes in substrate access channels

• Structure-function relationships:
  MD simulations can reveal:

  • Conformational changes during catalysis

  • Communication pathways between subunits

  • Effects of mutations on protein stability and function

  • Substrate and cofactor binding mechanisms

• Protein engineering guidance:
  Simulations help identify:

  • Flexible regions that might benefit from stabilization

  • Surface residues that interact with solvents

  • Conserved internal networks critical for function

  • Potential sites for introducing beneficial mutations

For example, MD simulations have demonstrated that considering dynamics as an integral part of enzyme function can enable engineering enzymes for industrial and medicinal applications. They can also provide critical insights into how organic solvent properties influence enzyme dynamic conformation, guiding the development of solvent-tolerant variants .

What are the best practices for purifying functionally active recombinant NAD(P)H-quinone oxidoreductase?

Purifying functionally active recombinant NAD(P)H-quinone oxidoreductase requires careful consideration of multiple factors to maintain the enzyme's native conformation and catalytic activity. The following methodological approach represents best practices based on published research:

Expression and extraction:

• Optimize expression conditions (temperature, induction time, media composition)

• Harvest cells carefully at appropriate growth phase

• Use gentle cell lysis methods (e.g., sonication with cooling intervals or enzymatic lysis)

• Include protease inhibitors in all buffers to prevent degradation

• Maintain reducing conditions (e.g., with DTT or β-mercaptoethanol) to protect thiol groups

Purification strategy:

For His-tagged recombinant NAD(P)H-quinone oxidoreductase, the following purification approach has proven effective:

• Immobilized metal affinity chromatography (IMAC):

  • Use nickel nitrilotriacetate (Ni-NTA) resin under non-denaturing conditions

  • Apply stepwise elution with increasing imidazole concentrations

  • Typical buffer: Tris-HCl with NaCl and low imidazole for binding, higher imidazole for elution

• Additional purification steps:

  • Size exclusion chromatography to separate different oligomeric states

  • Ion exchange chromatography for removing contaminants with different charge properties

  • Affinity chromatography with inhibitors (e.g., dicumarol) for specific binding

Quality control and characterization:

• Purity assessment:

  • SDS-PAGE (>90% purity standard)

  • Native PAGE to verify oligomeric state

  • Western blotting for identity confirmation

• Activity verification:

  • Spectrophotometric assays with model substrates

  • Determination of kinetic parameters

  • Stability testing under storage conditions

Case study: Purification of heterodimeric NAD(P)H-quinone oxidoreductase

A particularly innovative approach was developed for purifying wild-type/mutant heterodimers:

• Express a construct where one subunit contained a His-tag and the other had a mutation (H194A)

• Purify using Ni-NTA chromatography with stepwise elution

• Separate the heterodimer (H194A/HNQOR) from homodimers

• Confirm heterodimer composition using SDS and native PAGE plus immunoblot analysis

• Characterize enzyme kinetics with two-electron and four-electron acceptors

This heterodimer approach has general applications for studying functional and structural relationships of subunits in dimeric or oligomeric proteins and represents an advanced purification strategy for complex protein assemblies .

How does the plant species source affect the properties of recombinant NAD(P)H-quinone oxidoreductase?

The plant species source significantly influences the properties of NAD(P)H-quinone oxidoreductase, reflecting evolutionary adaptations to different ecological niches and metabolic requirements. Specifically, comparing Acorus americanus and related species reveals important differences in enzyme characteristics.

Comparative analysis of NAD(P)H-quinone oxidoreductase from different Acorus species:

• Sequence variation:
  While the core catalytic domains are highly conserved, there are subtle amino acid differences between species that affect:

  • Substrate specificity

  • Cofactor preference (NADH vs. NADPH)

  • Temperature and pH optima

  • Stability under varying conditions

• Functional adaptations:
  Different Acorus species have evolved in distinct environments, leading to adaptations in their enzymatic machinery:

  • Acorus calamus shows enhanced antimicrobial properties, suggesting its NAD(P)H-quinone oxidoreductase may interact with unique secondary metabolites

  • Acorus americanus appears adapted to more temperate conditions

  • Acorus tatarinowii demonstrates notable antioxidant properties that may involve its NAD(P)H-quinone oxidoreductase in redox balance

Phytochemical environment:

The natural phytochemical profiles of different Acorus species influence the native interactions and possibly the evolution of their NAD(P)H-quinone oxidoreductases:

Research has demonstrated that extracts from Acorus tatarinowii (at concentrations of 1.5, 5, and 15 μg/mL) prevent hydrogen-peroxide-induced cell injury and inhibit reactive oxygen species accumulation, suggesting involvement of its oxidoreductase systems in redox regulation .

Recombinant expression considerations:

When expressing recombinant NAD(P)H-quinone oxidoreductase from different plant species:

• Codon optimization may be necessary for efficient expression in bacterial systems

• Post-translational modifications may differ between species and affect enzyme properties

• Folding requirements might vary, necessitating different expression conditions

• Native partners or protein-protein interactions could differ, affecting in vitro activity

Understanding these species-specific differences is crucial for selecting the most appropriate enzyme variant for particular biotechnological applications and for predicting how the recombinant protein will behave under various experimental conditions.

What considerations are important when designing experiments to study the role of NAD(P)H-quinone oxidoreductase in photosynthetic electron transport?

Designing experiments to investigate the role of NAD(P)H-quinone oxidoreductase in photosynthetic electron transport requires careful consideration of multiple factors to ensure meaningful and reproducible results.

Experimental design considerations:

• Genetic approaches:

  • Gene knockout/knockdown methodologies to create ndhE-deficient plants

  • Complementation studies with wild-type or mutant versions

  • Site-directed mutagenesis of conserved residues

  • Heterologous expression in model organisms

• Biochemical and biophysical methods:

  • Membrane isolation techniques that preserve NDH complex integrity

  • Activity assays under conditions that mimic physiological environments

  • Spectroscopic methods to monitor electron transport (e.g., chlorophyll fluorescence)

  • In vitro reconstitution of subcomplexes to study specific interactions

• Structural biology approaches:

  • Cryo-electron microscopy of intact NDH complexes

  • X-ray crystallography of individual components or subcomplexes

  • Cross-linking mass spectrometry to map protein-protein interactions

  • Single-particle analysis to capture different conformational states

Critical experimental parameters:

Case study approach:

To comprehensively study chloroplast NAD(P)H-quinone oxidoreductase function in photosynthetic electron transport, a multi-technique approach is recommended:

• In vivo measurements:

  • Chlorophyll fluorescence to assess cyclic electron flow

  • P700 absorbance changes to monitor PSI redox state

  • Gas exchange to measure photosynthetic efficiency

  • Growth phenotyping under varying light and stress conditions

• In vitro assays:

  • Isolated thylakoid membrane electron transport

  • Purified NAD(P)H-quinone oxidoreductase activity with various substrates

  • Reconstitution experiments with defined components

• Integration with structural data:
  The chloroplast NDH complex has been characterized by cryo-EM (EMDB-31307), providing structural context for functional studies . Experiments should be designed to correlate structure with function by:

  • Testing predictions from structural models

  • Examining how mutations in key regions affect both structure and function

  • Using structure-guided approaches to probe electron transfer pathways