Recombinant Shewanella sediminis Nucleoside diphosphate kinase (ndk)

What is the basic structure and function of Shewanella sediminis Nucleoside diphosphate kinase?

Nucleoside diphosphate kinase (NDK) from Shewanella sediminis is a ubiquitous enzyme that catalyzes the transfer of phosphoryl groups from nucleoside triphosphates to nucleoside diphosphates. Based on structural studies of homologous NDKs, the enzyme's structure consists of a four-stranded anti-parallel β-sheet partially covered with six α-helices . The protein contains 143 amino acid residues with a sequence that includes critical active site residues for phosphoryl transfer activity. The full amino acid sequence includes MAIERTFSIIKPDAVAKNHIGAIYNRFETAGLKIIASKMLHLSKEQAEGFYAEHSERPFFGALVEFMTSGPICVQVLEGENAVLANREIMGATNPAEAARGTIRSDFA DSIDENAVHGSDAVASAEREIAYFFSTEELCPRTR . Like other NDKs, it likely functions as a hexamer, as observed in homologous plant NDKs where hexameric molecular packing occurs in both crystal and solution states .

What are the optimal conditions for expressing recombinant S. sediminis NDK in E. coli?

For optimal expression of recombinant S. sediminis NDK in E. coli, researchers should consider the following methodological approach:

• Vector Selection: Use compatible expression vectors designed for Shewanella proteins. Based on similar recombinant protein work, vectors containing T7 promoters often yield good expression levels .

• Expression Conditions: Optimal conditions typically include:

  • Growth temperature: 30°C for initial growth, followed by induction at 18-25°C to reduce inclusion body formation

  • Media: LB or 2xYT supplemented with appropriate antibiotics

  • Induction: 0.1-0.5 mM IPTG at OD600 of 0.6-0.8

  • Post-induction growth time: 4-6 hours at lower temperatures or overnight at 18°C

• Codon Optimization: Consider codon optimization for E. coli expression, as Shewanella species may have different codon usage patterns .

• Fusion Tags: According to product information, NDK may be produced with various tags to facilitate purification, with the specific tag determined during the manufacturing process .

What are the recommended methods for purifying recombinant S. sediminis NDK?

Purification of recombinant S. sediminis NDK can be achieved through the following methodological approach:

• Cell Lysis: Sonication or pressure-based lysis in a buffer containing 50 mM Tris-HCl (pH 8.0), 300 mM NaCl, 10% glycerol, and protease inhibitors.

• Initial Purification:

  • If His-tagged: Use Ni-NTA affinity chromatography with imidazole gradient elution

  • For other tags: Use appropriate affinity purification methods based on the specific tag

• Secondary Purification: Size exclusion chromatography using a Superdex 75 or 200 column to separate hexameric NDK from aggregates and other proteins.

• Quality Control: SDS-PAGE analysis should confirm a purity of >85% as indicated in product specifications .

• Storage Buffer: The purified protein should be stored in 50 mM Tris-HCl (pH 7.5), 100 mM NaCl, with 5-50% glycerol for stability. The recommended final glycerol concentration is 50% .

How can NDK activity be measured in laboratory settings?

NDK activity can be measured using several established methods:

• Coupled Enzyme Assay:

  • Principle: Links NDK-catalyzed production of ATP to reactions catalyzed by hexokinase and glucose-6-phosphate dehydrogenase, with NADPH production measured at 340 nm.

  • Reaction mixture: 50 mM Tris-HCl (pH 7.5), 5 mM MgCl2, 1 mM dithiothreitol, 0.1 mM TDP, 0.5 mM GTP, 1 mM glucose, 0.2 mM NADP+, 2 units of hexokinase, and 1 unit of glucose-6-phosphate dehydrogenase.

  • Initiate the reaction by adding purified NDK and measure the increase in absorbance at 340 nm.

• Direct Assay Using HPLC:

  • Principle: Directly measures the conversion of nucleoside diphosphates to triphosphates.

  • Reaction mixture: 50 mM Tris-HCl (pH 7.5), 5 mM MgCl2, 1 mM nucleoside diphosphate substrate, and 2 mM ATP.

  • Terminate the reaction at various time points with EDTA and quantify products by HPLC.

• Luciferase-Based ATP Detection:

  • For reactions where ATP is produced, luminescence-based detection provides high sensitivity.

What are the kinetic parameters of S. sediminis NDK?

While specific kinetic parameters for S. sediminis NDK have not been directly reported in the search results, they can be approximated based on related NDKs:

• Substrate Specificity: NDKs typically show broad substrate specificity toward various nucleoside di- and triphosphates. Based on homologous enzymes, S. sediminis NDK likely catalyzes phosphoryl transfer between various nucleotides including ATP, GTP, CTP, UTP as donors and their corresponding diphosphates as acceptors.

• Optimal pH and Temperature: Given the psychrophilic nature of Shewanella sediminis, which is found in marine sediments, the enzyme likely shows activity at lower temperatures (10-25°C) than mesophilic counterparts, with optimal pH likely in the range of 7.0-8.0 .

• Cofactor Requirements: Like other NDKs, the enzyme requires divalent metal ions (typically Mg²⁺ or Mn²⁺) for activity, similar to what has been observed with other nucleic acid-modifying enzymes from Shewanella species .

How can recombinant S. sediminis NDK be used in nucleotide metabolism studies?

Recombinant S. sediminis NDK offers several advantages for nucleotide metabolism studies:

• Comparative Biochemistry: S. sediminis NDK can be used to investigate evolutionary adaptations in nucleotide metabolism enzymes from marine sediment bacteria. This allows researchers to compare enzyme properties between psychrophilic and mesophilic organisms to understand environmental adaptations .

• Metabolic Pathway Analysis: As NDK plays a crucial role in maintaining nucleotide pools, the recombinant enzyme can be employed in studies exploring how marine bacteria regulate their nucleotide metabolism under different environmental conditions, particularly in anaerobic marine sediments.

• Substrate Specificity Studies: By testing various nucleotide combinations, researchers can determine if S. sediminis NDK has evolved unique substrate preferences compared to homologous enzymes from other environments.

• Synthetic Biology Applications: The enzyme can be incorporated into engineered metabolic pathways for the production of modified nucleotides or as part of synthetic gene circuits in bacterial systems .

What role does NDK play in the biology of Shewanella sediminis?

NDK plays several critical roles in Shewanella sediminis biology:

• Nucleotide Pool Maintenance: NDK maintains balanced nucleotide pools by catalyzing the interconversion of nucleoside di- and triphosphates, which is essential for DNA replication, RNA synthesis, and various metabolic processes.

• Energy Metabolism: In marine sediment environments where Shewanella sediminis thrives, NDK likely contributes to energy homeostasis under low-oxygen or anaerobic conditions, working alongside the complex electron transfer networks involving cytochromes that are characteristic of Shewanella species .

• Stress Response: While not directly evidenced for S. sediminis, studies in related organisms suggest NDK may be involved in stress responses. For example, in rice, NDK expression is up-regulated under anaerobic stress , suggesting that the enzyme might play a similar role in helping S. sediminis adapt to changing environmental conditions in marine sediments.

• Potential Role in Extracellular Functions: Given that some Shewanella species utilize extracellular DNA in biofilm formation and nutrient acquisition , NDK might be involved in nucleotide recycling pathways related to these processes, although direct evidence for this specific role is not provided in the search results.

How can genetic engineering approaches be used to study NDK function in Shewanella sediminis?

Several genetic engineering strategies can be employed to study NDK function in Shewanella sediminis:

• Gene Deletion and Complementation: Create in-frame deletion mutants of the ndk gene following protocols similar to those used for other Shewanella genes. This involves:

  • PCR amplification of approximately 750 bp upstream and downstream of the ndk gene

  • Joining these fragments via complementary tags

  • Ligation into suitable vectors such as pDS3.0

  • Transformation into S. sediminis using established conjugation protocols

  • Complementation with wild-type ndk to confirm phenotype specificity

• Expression Systems: Utilize the synthetic plasmid toolkit developed for Shewanella species to create expression constructs with:

  • Various promoters of different strengths (constitutive or inducible)

  • Compatible replication origins with characterized copy numbers

  • Appropriate antibiotic resistance markers

• Reporter Gene Fusions: Create translational or transcriptional fusions between ndk and reporter genes like gfp to:

  • Monitor expression patterns under different growth conditions

  • Determine subcellular localization of the NDK protein

  • Assess protein-protein interactions through techniques like FRET

• CRISPR-Based Technologies: As demonstrated in Shewanella oneidensis, CRISPR interference (CRISPRi) and small regulatory RNA (sRNA) approaches could be adapted to regulate ndk expression in S. sediminis for functional studies .

How does recombinant S. sediminis NDK compare functionally with the native enzyme?

When comparing recombinant and native S. sediminis NDK, researchers should consider several factors:

• Post-translational Modifications: The recombinant enzyme expressed in E. coli may lack post-translational modifications that might be present in the native enzyme. While NDKs are not heavily modified proteins, potential differences could include:

  • Phosphorylation states, which may affect activity

  • Disulfide bond formation, which may impact stability

• Hexameric Assembly: The native enzyme likely functions as a hexamer, as observed in homologous NDKs . Researchers should verify that the recombinant form also assembles into the correct oligomeric state using techniques such as:

  • Size exclusion chromatography

  • Dynamic light scattering

  • Native PAGE

• Activity Parameters: Comparison studies should include:

  • Specific activity measurements under identical conditions

  • Substrate preference profiles

  • Temperature and pH optima

  • Metal ion dependence

• Stability Characteristics: The recombinant protein may show different stability profiles compared to the native enzyme, particularly considering that S. sediminis is a psychrophilic organism from marine sediments . Thermal stability and denaturation studies can help identify any differences.

What are the optimal storage conditions for maintaining S. sediminis NDK activity?

To maintain optimal activity of recombinant S. sediminis NDK, the following storage conditions are recommended:

• Short-term Storage (up to one week):

  • Store working aliquots at 4°C in appropriate buffer systems

  • Avoid repeated freeze-thaw cycles that can lead to activity loss

• Long-term Storage:

  • Store at -20°C or preferably -80°C in buffer containing 50% glycerol as a cryoprotectant

  • Prepare small aliquots to avoid repeated freeze-thaw cycles

• Reconstitution Protocols:

  • If lyophilized, reconstitute in deionized sterile water to a concentration of 0.1-1.0 mg/mL

  • Consider adding glycerol to a final concentration of 5-50% for stability, with 50% being the recommended default

• Shelf Life Expectations:

  • Liquid form: Approximately 6 months at -20°C/-80°C

  • Lyophilized form: Approximately 12 months at -20°C/-80°C

What buffer systems are optimal for enzymatic assays with S. sediminis NDK?

For optimal activity in enzymatic assays, the following buffer systems are recommended:

• Standard Reaction Buffer:

  • 50 mM Tris-HCl (pH 7.5-8.0)

  • 5 mM MgCl2 or MnCl2 (divalent metal ions are required for activity)

  • 1 mM DTT or β-mercaptoethanol (to maintain reduced state)

  • Optional: 50 mM KCl or NaCl for ionic strength

• Assay-Specific Considerations:

  • For coupled enzyme assays: Ensure buffer compatibility with auxiliary enzymes

  • For direct spectrophotometric assays: Avoid components with absorption that would interfere with measurements

  • For low-temperature studies: Consider using buffers with minimal pH change at lower temperatures (HEPES or PIPES)

• pH Optimization:

  • Test a range of pH values (6.5-8.5) to determine the optimum for S. sediminis NDK

  • Given the marine sediment origin of S. sediminis, the enzyme might show activity across a broader pH range compared to mesophilic counterparts