Recombinant Sulfurihydrogenibium sp. 6-phosphofructokinase (pfkA)

What is the structure and function of Sulfurihydrogenibium sp. pfkA?

Sulfurihydrogenibium sp. pfkA, like other phosphofructokinases, catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate. Based on structural studies of other PFK1 enzymes, pfkA likely adopts a tetrameric quaternary structure with distinct catalytic and regulatory domains. Each monomer contains active sites for substrate binding and multiple allosteric sites for regulatory molecules.

Similar to human PFKL, the catalytic mechanism involves ATP binding and phosphoryl transfer to fructose-6-phosphate (F6P). The enzyme likely exists in active R-state and inactive T-state conformations that differ in the arrangement of catalytic domains, as observed in other PFK1 enzymes . These conformational states are central to the allosteric regulation mechanism, with the transition between states affecting the positioning of active site residues responsible for F6P binding.

How should researchers optimize the expression of recombinant Sulfurihydrogenibium sp. pfkA?

For optimal expression of recombinant Sulfurihydrogenibium sp. pfkA, consider the following methodological approach:

• Vector selection: Use expression vectors with strong promoters (T7, tac) and appropriate fusion tags (His6, FLAG) to facilitate purification.

• Expression system: E. coli BL21(DE3) strains are recommended for thermostable proteins, with expression at 30°C after induction with 0.5-1.0 mM IPTG.

• Buffer optimization: Based on pH studies of other PFK enzymes, use buffers in the pH range of 7.5-8.5 for maximal enzyme activity during purification .

• Stabilization additives: Include glycerol (10-15%), reducing agents like 2-mercaptoethanol (15 mM), and chelating agents such as EDTA (4 mM) in purification buffers, similar to those used for other PFK preparations .

• Purification strategy: Implement a multi-step purification process including heat treatment (exploiting thermostability), followed by affinity chromatography and size-exclusion chromatography to separate tetrameric forms from potential filamentous assemblies .

What assay methods are most reliable for measuring Sulfurihydrogenibium sp. pfkA activity?

The most reliable assay for pfkA activity is a coupled spectrophotometric assay that monitors NADH oxidation, similar to methods used for other PFK enzymes. The methodological approach should include:

• Reaction conditions: 50 mM Tris-HCl buffer (pH 7.4-8.5), 10 mM MgCl₂, 0.2 mM NADH, with varying concentrations of ATP and fructose-6-phosphate .

• Coupled enzymes: Add aldolase (0.25 U/mL), triose phosphate isomerase (1 U/mL), and α-glycerophosphate dehydrogenase (4 U/mL) to the reaction mixture .

• Measurement parameters: Monitor NADH oxidation at 340 nm for 10 minutes, identifying the linear phase for each experiment. Use a molar extinction coefficient of 6,220 × 10⁶ M⁻¹cm⁻¹ for calculating fructose-1,6-bisphosphate concentration .

• Controls: Include measurements at different pH values (6.5-9.0) to determine optimal assay conditions, as PFK activity is highly pH-dependent .

• Data analysis: Apply allosteric kinetic models to analyze substrate-velocity curves, as PFK typically shows sigmoidal kinetics with respect to fructose-6-phosphate .

How does the allosteric regulation of Sulfurihydrogenibium sp. pfkA compare to other bacterial and eukaryotic PFKs?

The allosteric regulation of Sulfurihydrogenibium sp. pfkA likely shares similarities with other bacterial PFKs but may exhibit unique features due to its thermophilic origin. Based on studies of other PFK enzymes, we can anticipate the following regulatory mechanisms:

• Allosteric effectors: Like other PFK enzymes, Sulfurihydrogenibium sp. pfkA is probably regulated by ATP, ADP, AMP, and potentially fructose-2,6-bisphosphate . Based on structural studies of human PFKL, the enzyme likely contains multiple nucleotide binding sites beyond the catalytic site, including:

  • An activating allosteric site (site 1) at the interface between catalytic and regulatory domains

  • A second site (site 2) in the catalytic domain

  • A third site (site 3) that may serve dual activating/inhibitory functions depending on occupancy

• Conformational changes: The enzyme likely shifts between R-state (active) and T-state (inactive) conformations, with the transition modulated by effector binding. In the T-state, rotation between pairs of catalytic domains disrupts F6P binding pockets without affecting ATP binding in active sites .

• Regulatory differences: Unlike human PFK, which is strongly inhibited by citrate, bacterial PFKs (including potentially Sulfurihydrogenibium sp. pfkA) may be less sensitive to this inhibitor . Similarly, Sulfurihydrogenibium sp. pfkA may exhibit different responses to fructose-2,6-bisphosphate compared to eukaryotic enzymes.

• ATP regulation: While ATP is both a substrate and allosteric inhibitor for most PFK enzymes, the specific concentration thresholds and binding mechanisms may differ in Sulfurihydrogenibium sp. pfkA, potentially as an adaptation to thermophilic conditions.

What structural features contribute to the thermostability of Sulfurihydrogenibium sp. pfkA?

As a protein from a thermophilic organism, Sulfurihydrogenibium sp. pfkA likely employs several structural strategies to maintain stability at elevated temperatures:

• Increased ionic interactions: Additional salt bridges, particularly at subunit interfaces, would stabilize the quaternary structure. Analysis of the tetramer assembly interfaces may reveal ion pairing networks absent in mesophilic homologs.

• Enhanced hydrophobic core packing: The interior of the protein likely features optimized van der Waals contacts and a more densely packed hydrophobic core compared to mesophilic counterparts.

• Reduced flexibility in loop regions: Surface loops may be shorter and more rigid, potentially with proline residues at strategic positions to reduce conformational entropy.

• Higher secondary structure content: Increased α-helical and β-sheet propensity, with optimized helix capping and stronger hydrogen bonding networks in secondary structure elements.

• C-terminal stabilization: The C-terminal region, which is important for allosteric regulation in PFK enzymes , may feature additional stabilizing interactions. In human PFKL, the C-terminal tail stabilizes the T-state conformation and regulates enzyme activity .

• Oligomerization interfaces: The tetrameric assembly interfaces may contain specific residues that enhance thermostability. For instance, comparison with the human PFKL interface, which features residue N702 at the filament-forming interface , might reveal substitutions that affect both thermostability and oligomerization potential.

A methodical approach to investigating these features would involve site-directed mutagenesis of residues at key interfaces, followed by thermal stability assays (differential scanning calorimetry or thermal shift assays) and structural studies.

How do substrate binding kinetics of Sulfurihydrogenibium sp. pfkA differ at elevated temperatures?

The substrate binding kinetics of Sulfurihydrogenibium sp. pfkA at elevated temperatures likely reflect adaptations to its thermophilic lifestyle. A methodological investigation would involve:

• Temperature-dependent kinetic analysis: Measure enzyme activity across a temperature range (25-95°C) to determine temperature optimum and construct Arrhenius plots to calculate activation energies.

• Substrate affinity changes: At elevated temperatures, Sulfurihydrogenibium sp. pfkA may show different Km values for F6P and ATP compared to mesophilic enzymes. Data from PFK studies in other organisms suggest that:

  • F6P binding may have a higher Km (lower affinity) than mesophilic PFKs at standard temperatures (which showed Km values of 0.81 ± 0.11 mM for muscle and 1.61 ± 0.25 mM for fat body in R. prolixus )

  • The enzyme likely exhibits optimal affinity at temperatures near its physiological optimum

• Cooperative binding: Analyze the cooperativity coefficient (Hill coefficient) for F6P binding at different temperatures. Like other PFKs, Sulfurihydrogenibium sp. pfkA likely displays sigmoidal kinetics with respect to F6P concentration , but the degree of cooperativity may be temperature-dependent.

• pH dependence: Investigate how pH optima shift with temperature, as ionization constants are temperature-dependent. Most PFKs show optimal activity at pH 8.0-8.5 , but this may differ for thermophilic variants at their physiological temperatures.

What methodological approaches are most effective for studying the R-state to T-state transition in Sulfurihydrogenibium sp. pfkA?

Investigating the R-state to T-state conformational transition of Sulfurihydrogenibium sp. pfkA requires a multi-faceted methodological approach:

• Structural studies:

  • Cryo-electron microscopy (cryo-EM) to capture different conformational states, similar to approaches used for human PFKL

  • X-ray crystallography with various ligands to trap different allosteric states

  • Hydrogen-deuterium exchange mass spectrometry (HDX-MS) to map conformational changes upon effector binding

• Spectroscopic techniques:

  • Circular dichroism (CD) spectroscopy to measure secondary structure changes

  • Fluorescence spectroscopy with intrinsic tryptophan or extrinsic fluorophores to detect conformational transitions

  • Nuclear magnetic resonance (NMR) on selectively labeled enzyme to monitor specific residue environments

• Molecular dynamics simulations:

  • Simulate the transition between R and T states at different temperatures

  • Calculate energy landscapes for the conformational changes

  • Predict the effects of thermophilic adaptations on state stability

• Mutagenesis studies:

  • Target residues predicted to be involved in the R/T transition based on homology with human PFKL, where residues R201 and R292 play key roles in F6P binding and are affected by T-state formation

  • Create mutations at the interfaces between catalytic domains that may stabilize either the R or T state

  • Investigate the role of the C-terminal tail, which is known to be critical for proper regulation of PFK1 enzymes

• Kinetic approaches:

  • Pre-steady-state kinetics to capture transient states during the allosteric transition

  • Enzyme activity measurements in the presence of various concentrations of allosteric effectors

  • Analysis of cooperativity as an indicator of R/T state population distribution

How can site-directed mutagenesis be used to enhance specific properties of Sulfurihydrogenibium sp. pfkA?

Site-directed mutagenesis represents a powerful approach for engineering Sulfurihydrogenibium sp. pfkA with enhanced properties. Based on structural insights from other PFK enzymes, the following methodological strategies are recommended:

• Enhancing catalytic efficiency:

  • Target active site residues corresponding to those in human PFKL that interact with F6P, such as homologs of R201 and R292

  • Modify residues at the interface between catalytic domains to stabilize the R-state conformation

  • Introduce mutations that optimize the position of catalytic residues for phosphoryl transfer

• Modifying allosteric regulation:

  • Create mutations at site 3 (ATP inhibitory site) to reduce inhibition while maintaining catalytic activity

  • Introduce mutations at the activator binding site (site 1) to enhance sensitivity to activators like ADP

  • Modify the C-terminal region, which is critical for allosteric regulation of PFK1 enzymes

• Further enhancing thermostability:

  • Increase proline content in loop regions to reduce conformational entropy

  • Introduce additional salt bridges at subunit interfaces

  • Optimize hydrophobic core packing through conservative substitutions

• Altering oligomerization properties:

  • Target the equivalent of residue N702 in human PFKL, which is critical for filament formation

  • Modify interface residues to either enhance tetrameric stability or promote controlled higher-order assembly

• Methodological approach:

  • Design mutations based on sequence alignment with homologous PFKs of known structure

  • Use computational modeling to predict effects before experimental testing

  • Implement a systematic screening workflow including thermal stability assays, kinetic measurements, and structural characterization

What is the potential role of Sulfurihydrogenibium sp. pfkA in metabolic engineering applications?

Sulfurihydrogenibium sp. pfkA offers unique advantages for metabolic engineering applications due to its thermostability and potentially distinct regulatory properties:

• Thermostable biocatalysis:

  • Integration into multi-enzyme reaction systems operating at elevated temperatures

  • Development of immobilized enzyme systems with extended operational stability

  • Creation of cell-free metabolic pathways for bioproduction at higher temperatures, which can reduce contamination risk

• Glycolytic flux control:

  • Engineering Sulfurihydrogenibium sp. pfkA with altered regulatory properties could enable precise control over glycolytic flux in host organisms

  • Introduction into mesophilic hosts as a less-regulated alternative to endogenous PFK could increase carbon flux through glycolysis

  • Potential for creating temperature-responsive metabolic switches in engineered organisms

• Methodological considerations for pathway engineering:

  • Expression optimization must account for potential differences in codon usage and protein folding machinery between the thermophilic source and mesophilic hosts

  • Integration with other glycolytic enzymes may require enzyme engineering to ensure compatible kinetic parameters

  • Activity at different temperatures should be characterized to define the operational range

• Research approach:

  • Construct synthetic operons containing Sulfurihydrogenibium sp. pfkA and complementary thermostable glycolytic enzymes

  • Evaluate performance in model organisms across temperature ranges

  • Implement directed evolution approaches to optimize activity in specific host environments

How can structural comparisons between Sulfurihydrogenibium sp. pfkA and human PFK inform drug design for metabolic disorders?

Structural comparison between Sulfurihydrogenibium sp. pfkA and human PFK isoforms could provide valuable insights for developing therapeutics targeting metabolic disorders:

• Structural analysis methodology:

  • Generate homology models of Sulfurihydrogenibium sp. pfkA based on existing PFK structures

  • Perform structural alignments with human PFKL, PFKM, and PFKP isoforms

  • Identify conserved features and divergent regions, particularly at allosteric binding sites

• Allosteric site targeting:

  • Compare site 3, which serves dual activating/inhibitory functions in human PFKL

  • Identify unique features of the activating allosteric site (site 1) that could be exploited for isoform-specific targeting

  • Analyze differences in the F1,6BP binding site, which regulates the R/T state transition

• Specificity determinants:

  • Analyze the structural basis for differences in substrate specificity and allosteric regulation

  • Identify residues that could confer selectivity for inhibitors targeting specific human PFK isoforms

  • Use thermophilic adaptations to inform stability-enhancing modifications to drug candidates

• Applications to metabolic disorders:

  • Design of PFK activators for glycogen storage diseases

  • Development of isoform-specific inhibitors for cancer metabolism targeting, considering that cancer cells often upregulate glycolysis

  • Creation of stabilized PFK variants for enzyme replacement therapies

What are the major challenges in purifying active recombinant Sulfurihydrogenibium sp. pfkA and how can they be addressed?

Purification of active recombinant Sulfurihydrogenibium sp. pfkA presents several challenges that require specific methodological solutions:

• Expression system compatibility:

  • Challenge: Codon usage bias between thermophilic Sulfurihydrogenibium sp. and mesophilic expression hosts

  • Solution: Optimize codons for expression host or use strains with expanded tRNA repertoires (e.g., Rosetta)

• Protein folding at lower temperatures:

  • Challenge: Thermophilic proteins may fold incorrectly at standard expression temperatures

  • Solution: Express at elevated temperatures (30-37°C) or co-express with chaperones; consider heat shock treatment during cell growth

• Preventing oligomerization heterogeneity:

  • Challenge: PFK enzymes can form different oligomeric states, including filaments as observed with human PFKL

  • Solution: Implement size-exclusion chromatography to separate tetrameric forms; consider mutations equivalent to N702T in human PFKL, which prevents filament formation

• Maintaining activity during purification:

  • Challenge: Loss of activity due to dissociation of tetramers or loss of essential metal ions

  • Solution: Include stabilizing agents (glycerol, reducing agents) and divalent cations (Mg²⁺) in all buffers; maintain pH in the optimal range (8.0-8.5)

• Accurate activity measurements:

  • Challenge: Temperature-dependent activity profiles complicate standardized assays

  • Solution: Develop a temperature-controlled assay system; establish standard activity measurements at multiple temperatures

How can researchers distinguish between effects on catalytic activity versus allosteric regulation when studying Sulfurihydrogenibium sp. pfkA mutants?

Distinguishing between effects on catalytic activity and allosteric regulation is a complex challenge that requires a systematic methodological approach:

• Kinetic separation of effects:

  • Measure kinetic parameters (Km, Vmax) at varying concentrations of substrates F6P and ATP

  • Analyze cooperativity (Hill coefficient) for F6P binding as an indicator of allosteric behavior

  • Compare kinetic parameters in the presence and absence of known allosteric effectors

• Structural approaches:

  • Use hydrogen-deuterium exchange mass spectrometry (HDX-MS) to map conformational changes upon effector binding

  • Employ site-directed spin labeling and electron paramagnetic resonance (EPR) to detect conformational changes in specific regions

  • Perform crystallography or cryo-EM under different liganded states to directly visualize R and T states

• Mutagenesis strategy:

  • Create specific mutations targeting:

    • Catalytic site only (affect catalysis but not allosteric transitions)

    • Allosteric effector binding sites only (affect regulation but not catalysis)

    • Interface between catalytic and regulatory domains (affect communication between sites)

• Data analysis framework:

  • Apply allosteric models (MWC, KNF) to distinguish between effects on R/T state equilibrium versus catalytic efficiency

  • Use statistical coupling analysis to identify networks of residues involved in allosteric communication

  • Implement global fitting of kinetic data to models incorporating both catalytic and regulatory parameters

What computational approaches can advance our understanding of the evolution of thermostability in Sulfurihydrogenibium sp. pfkA?

Computational approaches offer powerful tools for investigating the evolutionary basis of thermostability in Sulfurihydrogenibium sp. pfkA:

• Phylogenetic analysis:

  • Construct comprehensive phylogenetic trees of PFK enzymes across thermophilic and mesophilic organisms

  • Implement ancestral sequence reconstruction to identify key mutations in the evolutionary trajectory

  • Apply statistical methods to identify coevolving residue networks associated with thermostability

• Molecular dynamics simulations:

  • Perform temperature replica exchange molecular dynamics (T-REMD) to sample conformational space at different temperatures

  • Calculate dynamical properties (RMSF, correlation matrices) to identify regions with altered flexibility

  • Simulate unfolding pathways to identify weak points in protein structure and compare with mesophilic homologs

• Energy landscape analysis:

  • Calculate folding free energy landscapes at different temperatures

  • Identify metastable states and energy barriers between R and T conformations

  • Compare energetic contributions of different types of interactions (hydrogen bonds, salt bridges, hydrophobic)

• Machine learning approaches:

  • Develop neural network models to predict thermostability from sequence features

  • Implement feature importance analysis to identify sequence patterns associated with thermostability

  • Train classifiers to distinguish thermophilic from mesophilic PFK sequences

• Network analysis:

  • Apply protein structure networks to identify critical nodes for structural integrity

  • Compare allosteric communication pathways between thermophilic and mesophilic PFKs

  • Identify residue interaction networks responsible for maintaining quaternary structure at elevated temperatures

How can research on Sulfurihydrogenibium sp. pfkA contribute to our understanding of extremophile metabolism?

Research on Sulfurihydrogenibium sp. pfkA provides a window into extremophile metabolism with several important research directions:

• Metabolic flux analysis:

  • Investigate how thermophilic adaptations in pfkA affect glycolytic flux control

  • Compare flux distribution between glycolysis and alternative pathways at different temperatures

  • Develop models for predicting metabolic responses to environmental changes in extremophiles

• Comparative enzymology:

  • Systematically compare kinetic parameters and regulatory properties of pfkA with other glycolytic enzymes from Sulfurihydrogenibium sp.

  • Determine whether thermophilic adaptations are coordinated across metabolic pathways

  • Identify potential metabolic bottlenecks in thermophilic glycolysis

• Adaptation to multiple extremes:

  • Investigate how pfkA adaptations balance thermostability with other extreme conditions (pH, pressure)

  • Examine how redox sensitivity is tuned in environments with varying oxygen availability

  • Analyze metal ion requirements and tolerance in relation to geothermal environment chemistry

• Methodological approaches:

  • Develop in vitro reconstitution of thermophilic glycolytic pathways

  • Implement isotope-based metabolic flux analysis at elevated temperatures

  • Create genetic tools for manipulating Sulfurihydrogenibium sp. metabolism in vivo

• Ecological context:

  • Relate pfkA properties to the ecological niche of Sulfurihydrogenibium sp.

  • Investigate how metabolic adaptations contribute to fitness in extreme environments

  • Compare pfkA properties across Sulfurihydrogenibium species from different thermal environments