Recombinant Akkermansia muciniphila NADH-quinone oxidoreductase subunit K (nuoK)

What is Akkermansia muciniphila NADH-quinone oxidoreductase subunit K (nuoK)?

NADH-quinone oxidoreductase subunit K (nuoK) from Akkermansia muciniphila is a protein component of the NADH dehydrogenase I complex (also known as NDH-1 subunit K), which plays a crucial role in the electron transport chain of this bacterium. According to available data, nuoK is encoded by the gene Amuc_1607 in A. muciniphila strain ATCC BAA-835 . The protein consists of 105 amino acid residues with the sequence: MIPLTHYLILSGVLFAIGLMGVIVRRDIIVIFMCLEMMLSAANLSLVAFSRAQGTMGLPNYDGQALSIFILTIAAAEVAIGLALIVSLYRARRTASTQDLNTLKD .

The nuoK protein functions as part of Complex I in the respiratory chain and is involved in energy metabolism within A. muciniphila. This function is particularly significant given that A. muciniphila is a gut symbiont associated with numerous health benefits, including improved metabolic responses and enhanced gut barrier function through modulation of mucus layer thickness . The protein likely contributes to the bacterium's specialized metabolism, which centers around mucin degradation as its primary carbon and nitrogen source .

What are the structural characteristics of nuoK in A. muciniphila?

Based on the amino acid sequence provided for nuoK (Amuc_1607), the protein exhibits characteristics typical of membrane-embedded subunits of respiratory complexes . Analysis of the sequence MIPLTHYLILSGVLFAIGLMGVIVRRDIIVIFMCLEMMLSAANLSLVAFSRAQGTMGLPNYDGQALSIFILTIAAAEVAIGLALIVSLYRARRTASTQDLNTLKD reveals multiple hydrophobic regions consistent with transmembrane domains, which is expected for a protein functioning within the bacterial membrane.

The protein has a molecular weight of approximately 11-12 kDa based on its 105 amino acid length . Its structure likely includes:

• Multiple membrane-spanning helices

• Hydrophobic domains for membrane integration

• Regions involved in quinone binding

• Interfaces for interaction with other Complex I subunits

Researchers studying nuoK should consider these structural properties when designing experiments, particularly for protein expression and purification protocols, as membrane proteins present unique challenges compared to soluble proteins.

What are the optimal storage conditions for recombinant nuoK protein?

According to the product information in search result , recombinant A. muciniphila NADH-quinone oxidoreductase subunit K should be stored under the following conditions:

• Short-term storage: 4°C for up to one week

• Medium-term storage: -20°C

• Long-term storage: -80°C (for extended preservation)

The protein is typically stored in a Tris-based buffer containing 50% glycerol, specifically optimized for this protein . It is important to note that repeated freeze-thaw cycles should be avoided as they can lead to protein denaturation and loss of activity. The documentation explicitly states: "Repeated freezing and thawing is not recommended" .

For experimental work, researchers should prepare small working aliquots to minimize freeze-thaw events and validate protein stability and activity after storage using appropriate functional assays specific to NADH-quinone oxidoreductase activity.

How does nuoK contribute to A. muciniphila's metabolic processes?

The nuoK protein (Amuc_1607) functions as subunit K of NADH-quinone oxidoreductase (Complex I), a key enzyme in the respiratory electron transport chain. This complex plays several critical roles in bacterial metabolism:

• Electron Transport: Transfers electrons from NADH to quinones in the membrane

• Energy Conservation: Couples electron transfer to proton translocation across the membrane

• Redox Balance: Maintains NAD+/NADH ratios crucial for metabolic processes

• ATP Generation: Contributes to the proton motive force used for ATP synthesis

In A. muciniphila specifically, these processes are particularly important because this bacterium has a specialized metabolism focused on mucin degradation. A. muciniphila uniquely utilizes mucin as its primary carbon and nitrogen source, as mentioned in search result . The energy derived from the electron transport chain, to which nuoK contributes, would be essential for powering this specialized metabolic activity that enables the bacterium to thrive in the mucosal environment of the gut.

What expression systems are recommended for producing recombinant nuoK?

Producing functional recombinant NADH-quinone oxidoreductase subunit K presents challenges due to its hydrophobic nature and integral membrane characteristics. Based on common practices for similar proteins, researchers should consider the following expression strategies:

Bacterial Expression Systems:

• Modified E. coli strains (e.g., C41(DE3) or C43(DE3)) specifically designed for membrane protein expression

• Systems with titratable promoters to control expression levels and reduce toxicity

• Fusion tags that enhance solubility (e.g., MBP, SUMO, or thioredoxin)

Cell-Free Expression Systems:

• May be advantageous for membrane proteins like nuoK

• Allows direct incorporation into liposomes or nanodiscs

• Reduces toxic effects associated with overexpression in living cells

Expression Optimization Table:

A recommended approach would be to start with multiple expression constructs with different fusion tags and solubility enhancers, then screen for expression and solubility in small-scale trials before scaling up production.

What techniques are most effective for purifying active recombinant nuoK protein?

Purification of active recombinant NADH-quinone oxidoreductase subunit K presents significant challenges due to its hydrophobic nature and membrane association. Researchers should consider the following comprehensive purification strategy:

Step 1: Membrane Protein Extraction

• Cell lysis under gentle conditions (e.g., French press or sonication)

• Membrane fraction isolation via ultracentrifugation (100,000 × g)

• Detergent screening to identify optimal solubilization conditions

Step 2: Affinity Chromatography

• Utilize affinity tags incorporated into the recombinant construct

• Common options include His-tag, Strep-tag II, or FLAG-tag

• Conduct purification in the presence of the selected detergent

Step 3: Size Exclusion Chromatography

• Further purify the protein and assess its oligomeric state

• Separate monomeric from aggregated protein

• Evaluate protein-detergent complex size

Detergent Screening Table:

This methodological approach provides researchers with a framework for obtaining purified, active nuoK protein suitable for biochemical and structural studies. Throughout the purification process, it's essential to monitor protein integrity and activity to ensure the final product retains its native functionality.

How can site-directed mutagenesis be used to study the functional domains of nuoK?

Site-directed mutagenesis represents a powerful approach for investigating the structure-function relationships of the nuoK protein. For NADH-quinone oxidoreductase subunit K, researchers should consider the following methodological approach:

Target Selection Strategy:

• Sequence Conservation Analysis:

  • Align nuoK sequences across bacterial species to identify highly conserved residues

  • Focus on residues conserved specifically in Verrucomicrobia or more broadly across bacterial Complex I

  • Prioritize residues in predicted functional regions

• Structural Prediction:

  • Use computational tools to predict transmembrane regions and functional motifs

  • Target residues at predicted quinone-binding sites or proton translocation pathways

  • Identify potential amino acids involved in subunit interactions

Mutagenesis Approach:

• Types of Mutations to Consider:

  • Conservative substitutions (e.g., Leu→Ile) to test specific chemical properties

  • Alanine scanning of predicted functional regions

  • Charge reversal mutations (e.g., Asp→Lys) for residues in charged regions

  • Cysteine substitutions for accessibility studies and cross-linking experiments

• Functional Analysis of Mutants:

  • Express wild-type and mutant proteins under identical conditions

  • Assess protein stability and expression levels

  • Conduct NADH oxidation assays using artificial electron acceptors

  • Measure proton pumping efficiency if reconstituted into liposomes

What experimental approaches are most effective for studying nuoK's role in electron transport chains?

Investigating the specific role of nuoK in the electron transport chain requires specialized bioenergetic and biochemical techniques. Researchers should consider the following methodological approaches:

Biochemical and Biophysical Methods:

• Enzyme Activity Assays:

  • Measure NADH oxidation rates in membrane preparations

  • Assess quinone reduction kinetics

  • Determine the effects of specific inhibitors (e.g., rotenone, piericidin A)

  • Compare wild-type and nuoK-modified variants

• Proton Pumping Measurements:

  • Reconstitute purified Complex I into proteoliposomes

  • Monitor pH changes using pH-sensitive fluorescent dyes (e.g., ACMA)

  • Measure proton/electron stoichiometry

  • Assess the impact of mutations in nuoK on proton translocation efficiency

Genetic and Cellular Approaches:

Experimental Design Table:

This comprehensive experimental approach would provide detailed insights into nuoK's specific contribution to the electron transport chain function in A. muciniphila, potentially revealing unique adaptations related to this bacterium's specialized ecological niche in the gut environment.

How does the function of nuoK in A. muciniphila compare to homologous proteins in other bacteria?

Comparative analysis of nuoK across different bacterial species provides evolutionary context and potential functional insights. Researchers should approach this question using both computational and experimental methods:

Computational Comparative Analysis:

• Sequence Homology Analysis:

  • Perform BLAST searches to identify nuoK homologs

  • Generate multiple sequence alignments to identify conserved residues

  • Calculate conservation scores for each position

  • Identify A. muciniphila-specific sequence features

• Phylogenetic Analysis:

  • Construct phylogenetic trees of nuoK sequences

  • Compare with species phylogeny to identify co-evolutionary patterns

  • Analyze evolutionary rates to identify functionally important regions

  • Determine if nuoK from A. muciniphila shows unique evolutionary history

Experimental Comparative Approach:

• Heterologous Expression:

  • Express nuoK from different species in a model organism

  • Test functional complementation in nuoK-deficient strains

  • Assess biochemical properties of different nuoK proteins

  • Determine if A. muciniphila nuoK has unique functional characteristics

Comparative Data Table Example:

This comparative approach would reveal evolutionary adaptations in nuoK and potentially identify unique features of the A. muciniphila protein that contribute to this bacterium's specialized lifestyle in the mucin-rich gut environment.

How does the structure of nuoK compare across different strains of A. muciniphila?

Investigating structural variations in nuoK across different A. muciniphila strains can provide insights into the protein's evolution and functional adaptation. Researchers should consider the following methodological approach:

Comparative Genomic Analysis:

• Sequence Collection and Alignment:

  • Gather nuoK sequences from all available A. muciniphila genome assemblies

  • Include clinical isolates and environmental strains

  • Generate multiple sequence alignments

  • Calculate sequence identity and similarity matrices

• Polymorphism Identification:

  • Identify single nucleotide polymorphisms (SNPs)

  • Detect insertion/deletion variants

  • Determine if variations are synonymous or non-synonymous

  • Map variations to predicted functional domains

Strain Comparison Framework:

• Strain Collection Strategy:

  • Include the reference strain (ATCC BAA-835)

  • Sample strains from diverse geographical regions

  • Include clinical isolates from various health states

  • Consider strains with different metabolic capabilities

• Experimental Validation:

  • Express variant nuoK proteins from different strains

  • Compare biochemical properties and activity

  • Assess structural differences using biophysical methods

  • Correlate structure with functional differences

This systematic approach would reveal the degree of conservation of nuoK across A. muciniphila strains and identify any strain-specific adaptations that might correlate with metabolic or ecological differences. Such information could provide insights into the evolution of this bacterium as it adapts to different host environments.

What protein-protein interactions has nuoK been shown to participate in?

Understanding the protein-protein interactions of nuoK is crucial for elucidating its functional role within Complex I and potentially identifying novel interactions. Researchers investigating this aspect should consider the following methodological approaches:

Techniques for Studying nuoK Interactions:

• Co-Immunoprecipitation (Co-IP):

  • Generate specific antibodies against nuoK or use epitope-tagged versions

  • Pull down nuoK and identify interacting partners via mass spectrometry

  • Confirm interactions with targeted Western blotting

  • Distinguish between direct and indirect interactions

• Crosslinking Studies:

  • Use chemical crosslinkers of various arm lengths to capture interactions

  • Apply mass spectrometry to identify crosslinked peptides

  • Map interaction interfaces between nuoK and partner proteins

  • Validate with site-directed mutagenesis of interface residues

Expected Interactions Based on Homology:

As nuoK functions as a subunit of Complex I, it likely interacts with other Complex I components. Based on structural studies of bacterial Complex I:

• Core Interactions:

  • Direct interactions with adjacent Complex I subunits (likely nuoJ and nuoL)

  • Association with lipids within the membrane environment

  • Potential interaction with quinone substrates

• Assembly Factors:

  • Temporary interactions with Complex I assembly factors

  • Potential chaperone interactions during membrane insertion

This systematic approach would provide a comprehensive view of nuoK's interaction network within A. muciniphila, potentially revealing unique aspects of respiratory chain organization in this gut symbiont.

What is the relationship between nuoK and other A. muciniphila proteins involved in host immune modulation?

While the search results primarily focus on Amuc_1100 (a pili-like protein) as a key immune-modulatory protein of A. muciniphila , researchers might investigate potential relationships between nuoK and immune modulation through the following methodological approaches:

Experimental Design for Investigating Potential Immune Relationships:

• Recombinant Protein Studies:

  • Purify recombinant nuoK and test direct effects on immune cells

  • Compare immune responses to nuoK with known immunomodulatory proteins like Amuc_1100

  • Measure cytokine production in peripheral blood mononuclear cells (PBMCs)

  • Assess activation of Toll-like receptors (particularly TLR2 and TLR4)

• Gene Expression Correlation:

  • Analyze transcriptomic data for co-regulation patterns

  • Determine if nuoK expression correlates with known immune-modulating factors

  • Investigate if similar environmental signals regulate both nuoK and immune factors

Potential Experimental Results Table:

This systematic investigation would determine whether nuoK, despite its primary role in energy metabolism, might have moonlighting functions in host-microbe interactions, similar to what has been observed for other bacterial metabolic enzymes. The approach mirrors the methodology used to characterize the immune properties of Amuc_1100, which was shown to enhance trans-epithelial resistance and induce specific cytokine profiles through TLR2/TLR4 activation .

How can nuoK be used as a marker for A. muciniphila in microbiome studies?

Researchers interested in using nuoK as a potential marker for detecting and quantifying A. muciniphila in microbiome samples should consider the following methodological approaches:

Marker Development Strategy:

• Sequence Specificity Analysis:

  • Compare nuoK (Amuc_1607) sequences across bacterial species

  • Identify regions unique to A. muciniphila

  • Design primers or probes targeting these unique regions

  • Validate specificity against closely related species

• PCR-Based Detection Methods:

  • Design conventional PCR assays for qualitative detection

  • Develop quantitative PCR (qPCR) methods for enumeration

  • Optimize droplet digital PCR (ddPCR) for absolute quantification

  • Design multiplexed assays including nuoK and other markers

• Next-Generation Sequencing Applications:

  • Create nuoK-specific amplicon sequencing approaches

  • Develop bioinformatic pipelines for nuoK identification in metagenomic data

  • Compare sensitivity to 16S rRNA gene-based detection methods

Performance Comparison Table Example:

This methodological framework would enable researchers to develop and validate nuoK-based detection systems for A. muciniphila, potentially offering advantages in specificity over current methods. Given the correlation between A. muciniphila abundance and various health parameters , improved detection methods could have significant implications for microbiome-based diagnostics.

What computational approaches can be used to predict nuoK function and interactions?

Researchers can employ various computational methods to predict functional aspects of the nuoK protein and its interaction network:

Structural Prediction Approaches:

• Homology Modeling:

  • Identify suitable templates from solved Complex I structures

  • Generate models using tools like SWISS-MODEL, I-TASSER, or AlphaFold

  • Refine models using molecular dynamics simulations

  • Validate predictions with experimental data when available

• Transmembrane Topology Prediction:

  • Employ algorithms like TMHMM, HMMTOP, or TOPCONS

  • Predict membrane-spanning regions and orientation

  • Identify potential functional loops

  • Map conservation patterns onto topological models

Functional Prediction Methods:

• Protein Function Annotation:

  • Employ tools like InterProScan to identify functional domains

  • Use Gene Ontology annotation to predict biological processes

  • Apply KEGG pathway mapping for metabolic context

  • Identify potential catalytic or binding sites

• Network-Based Approaches:

  • Construct protein-protein interaction networks based on homology

  • Predict functional associations using STRING database

  • Analyze co-expression networks from transcriptomic data

  • Identify potential functional partners through guilt-by-association

This multi-layered computational approach would provide researchers with a comprehensive prediction of nuoK function and interactions, guiding experimental design and hypothesis generation for further laboratory investigations into this important component of A. muciniphila's respiratory system.