Recombinant Enterobacter sp. ATP synthase subunit a (atpB)

What is the structure and function of ATP synthase in Enterobacter species?

ATP synthase in Enterobacter species, like other bacterial F-type ATP synthases, consists of two primary domains: F₀ (membrane-embedded) and F₁ (catalytic). The structure follows the general bacterial pattern with subunit composition α₃β₃γδεab₂c₈-15.

The F₀ domain (containing subunit a) forms the proton channel across the membrane, while the F₁ domain (containing the β subunit encoded by atpB) contains the catalytic sites for ATP synthesis . The enzyme functions as a rotary molecular machine where proton flow through F₀ drives rotation of the c-ring, which couples to the central stalk (γ subunit) to induce conformational changes in the F₁ catalytic sites that synthesize ATP .

Unlike some specialized bacterial ATP synthases that use Na⁺ ions, Enterobacter ATP synthase primarily utilizes H⁺ as the coupling ion . The enzyme can operate bidirectionally, either synthesizing ATP using the proton motive force or hydrolyzing ATP to generate a proton gradient .

What expression systems are recommended for producing recombinant Enterobacter ATP synthase components?

Based on established protocols for bacterial ATP synthases, the preferred expression system for Enterobacter ATP synthase components is Escherichia coli, particularly strain DK8 (lacking endogenous ATP synthase). This approach was successfully employed for ATP synthases from other bacteria .

For expression, the following methodology is recommended:

• Clone the desired ATP synthase genes or operons into expression vectors with inducible promoters (e.g., pTrc99a)

• Transform into E. coli DK8 host cells

• Induce expression at mid-logarithmic growth phase

• Harvest cells and isolate membrane fractions or purify the complete complex depending on experimental needs

For improved purification, adding a His₆-tag to the N-terminus of the β subunit (encoded by atpB) facilitates affinity chromatography without significantly affecting enzyme function . Expression typically yields 2-5 mg of purified protein per liter of culture.

How can I assess the catalytic activity of recombinant Enterobacter ATP synthase in vitro?

Several complementary assays can measure ATP synthase activity:

ATP Synthesis Assay:

• Reconstitute purified ATP synthase into liposomes (protein-to-lipid ratio of 1:50 w/w)

• Create an artificial proton motive force using:

  • pH jump method: Incubate proteoliposomes at pH 5.5, then rapidly transfer to pH 8.4 buffer

  • K⁺ diffusion potential: Generate Δψ (typically 160 mV) using valinomycin in the presence of a K⁺ gradient

• Add ADP and Pi (typically 200 μM ADP and 5 mM Pi)

• Measure ATP formation using luciferase assay or HPLC

ATP Hydrolysis Assay:

• Measure inorganic phosphate release using malachite green or EnzChek phosphate assay

• Couple ATP hydrolysis to NADH oxidation through pyruvate kinase and lactate dehydrogenase

• Monitor change in absorbance at 340 nm

Typical activity values for bacterial ATP synthases are:

• ATP synthesis: 10-100 nmol·min⁻¹·mg protein⁻¹

• ATP hydrolysis: 1-5 μmol·min⁻¹·mg protein⁻¹

Notably, like other enterobacterial ATP synthases, Enterobacter ATP synthase shows latent ATPase activity, which can be regulated by the ε subunit .

What are the key mutations that affect ATP synthase function in Enterobacter species?

Several key residues in Enterobacter ATP synthase subunits significantly impact function when mutated:

In subunit a (F₀):

• Conserved Arg residue in TMH4 (equivalent to Arg-210 in E. coli): Critical for preventing proton short-circuiting and essential for coupling proton translocation to rotation

• Residues forming the proton half-channels: Mutations disrupt proton translocation, completely blocking enzyme function

In subunit β (encoded by atpB):

• Catalytic residues involved in ATP binding and hydrolysis

• Residues at interfaces with the γ subunit that affect coupling efficiency

In subunit ε:

• C-terminal domain residues: Affect the regulatory inhibition of ATP hydrolysis while permitting ATP synthesis

Table 1: Critical Mutations Affecting ATP Synthase Function

How do the mechanisms of inhibition differ between Enterobacter ATP synthase and other bacterial ATP synthases?

Enterobacter ATP synthase shares common inhibition mechanisms with other bacterial ATP synthases but exhibits distinct characteristics:

• ε Subunit Inhibition:

  • Like other enterobacterial ATP synthases, Enterobacter ATP synthase is regulated by the C-terminal domain of the ε subunit

  • The ε subunit adopts an "up" conformation that inhibits ATP hydrolysis while permitting ATP synthesis

  • Unlike Bacillus PS3, where inhibition depends on ATP concentration (inhibition at <0.7 mM ATP, permissive at >1 mM ATP), Enterobacter follows the E. coli pattern where inhibition persists even at high ATP concentrations in the absence of sufficient proton motive force

• Inhibitor Sensitivity:

  • Enterobacter ATP synthase likely shares sensitivity to efrapeptin with other enterobacterial ATP synthases, unlike some thermophilic bacterial enzymes that show resistance

  • It exhibits differential sensitivity to resveratrol and piceatannol compared to thermophilic bacterial ATP synthases

• Regulatory Mechanisms:

  • The ATP biosensor studies indicate that ATP dynamics in Enterobacter and related bacteria show transient ATP accumulation during the transition from exponential to stationary growth phases

  • The ATP dynamics differ based on carbon source, with higher ATP levels observed during growth on acetate compared to glucose, contrary to expected ATP yields per carbon source

What approaches can be used to study proton translocation through the a-subunit in recombinant Enterobacter ATP synthase?

Studying proton translocation through the a-subunit requires sophisticated methodologies:

• Reconstitution and Fluorescence Assays:

  • Reconstitute purified ATP synthase into liposomes containing pH-sensitive fluorophores (ACMA or pyranine)

  • Monitor fluorescence changes upon energization with ATP or an artificial proton gradient

  • Calculate H⁺/ATP ratio by correlating proton uptake with ATP synthesis or hydrolysis

• Site-Directed Mutagenesis Combined with Functional Assays:

  • Generate systematic mutations in conserved residues of the a-subunit

  • Assess impact on proton translocation, ATP synthesis, and ATP hydrolysis

  • Map the proton pathway based on mutational effects

• Molecular Dynamics Simulations:

  • Create atomistic models of the a-subunit and c-ring interface

  • Simulate proton movement through the half-channels

  • Identify key residues and water molecules involved in proton transfer

• Cryo-EM Structural Analysis:

  • Determine high-resolution structures of the intact ATP synthase in different rotational states

  • Identify conformational changes in the a-subunit that facilitate proton translocation

  • Compare with established models from E. coli and other bacterial systems

How can ATP dynamics be monitored in living Enterobacter cells, and what insights does this provide about metabolic regulation?

Advanced techniques for monitoring ATP dynamics in living Enterobacter cells include:

• Genetically Encoded ATP Biosensors:

  • Transform Enterobacter with plasmids expressing ATP biosensors like iATPsnFR1.1

  • These biosensors contain a circularly permuted super-folder green fluorescent protein (cp-sfGFP) integrated within the ATP-binding domain

  • ATP binding induces conformational changes leading to enhanced fluorescence

  • To normalize for expression levels, fuse mCherry to the sensor and calculate GFP/mCherry ratio

• Real-time Monitoring Setup:

  • Use microfluidic devices to control environmental conditions

  • Perform time-lapse fluorescence microscopy with controlled media exchange

  • Monitor cells across growth phases and under different carbon sources

• Data Analysis and Interpretation:

Recent findings using these approaches revealed:

• Transient ATP accumulation during the transition from exponential to stationary growth phases

• Different steady-state ATP levels during growth on different carbon sources

• Counterintuitive higher ATP levels during growth on acetate compared to glucose

• ATP production-consumption imbalances driving the observed dynamics

This methodology provides insights into:

• Metabolic regulation during different growth phases

• Energy allocation during stress responses

• Potential targets for antimicrobial development

• Correlations between ATP levels and production of specific metabolites

How can recombinant Enterobacter ATP synthase be leveraged for drug discovery against multidrug-resistant strains?

ATP synthase represents an attractive target for developing antimicrobials against multidrug-resistant Enterobacter strains, which are part of the ESKAPE pathogens of clinical concern . Research approaches include:

• Structure-Based Drug Design:

  • Utilize high-resolution structures of ATP synthase to identify unique binding sites

  • Focus on the a-subunit and c-ring interface, which shows species-specific features

  • Target ATP synthase regulatory mechanisms specific to Enterobacter species

• Peptide Inhibitor Development:

  • Engineer peptide fragments from ATP synthase interfaces using evolutionary algorithms like ROSE

  • Assess protein-protein interactions using predictors like PPI-Detect

  • Evaluate inhibitory potency through in vitro assays

• Screening Methodology:

  • Establish high-throughput ATP synthesis/hydrolysis assays with recombinant enzyme

  • Screen compound libraries against purified enzyme and whole cells

  • Validate hits through binding assays (ITC, MST) and structural studies

• Differential Targeting Strategy:

  • Identify inhibitors that selectively target bacterial ATP synthases over human mitochondrial counterparts

  • Focus on structural differences in the a-subunit and regulatory elements

What are the technical challenges in expressing and purifying functional recombinant ATP synthase from Enterobacter species?

Researchers face several technical challenges when working with recombinant Enterobacter ATP synthase:

• Expression Challenges:

  • Maintaining the correct stoichiometry of multiple subunits

  • Ensuring proper membrane insertion of the F₀ portion

  • Preventing toxicity to the host from overexpression of membrane proteins

• Purification Challenges:

  • Extracting the intact complex from membranes without disrupting subunit interactions

  • Selecting appropriate detergents that maintain enzyme activity

  • Removing endogenous lipids while preserving structure

• Reconstitution Issues:

  • Achieving uniform orientation in liposomes

  • Maintaining the proton-tight seal necessary for activity

  • Ensuring consistent proteoliposome size and protein incorporation

Recommended Approach:

• Co-express all subunits from a single operon under controlled induction

• Use a two-step purification: Ni-affinity followed by size exclusion chromatography

• Reconstitute using a detergent removal method (e.g., Bio-Beads) rather than dialysis

• Verify function through multiple complementary assays

How does ATP synthase contribute to antimicrobial resistance mechanisms in Enterobacter species?

• Energy Provision for Efflux Pumps:

  • ATP synthase supplies the energy required for ATP-dependent efflux pumps that export antibiotics

  • MDR Enterobacter strains often upregulate both ATP synthase and efflux systems

• Adaptation to Environmental Stresses:

  • ATP synthase regulation allows bacteria to adapt to environmental challenges

  • Transient ATP accumulation during stress responses may contribute to survival during antibiotic exposure

• Metabolic Flexibility:

  • The ability to modulate ATP synthesis rates in response to different carbon sources

  • Contributes to adaptation under antibiotic pressure and host environments

• Biofilm Formation:

  • ATP availability influences biofilm formation, a key resistance mechanism

  • Energy allocation during the transition from planktonic to biofilm growth involves ATP synthase regulation

How might novel biosensor technologies enhance our understanding of ATP synthase function in Enterobacter species?

Future research using emerging biosensor technologies could provide unprecedented insights into Enterobacter ATP synthase:

• Single-Molecule Rotation Assays:

  • Attach fluorescent beads or quantum dots to the rotor portion

  • Immobilize the stator portion on a surface

  • Directly visualize rotation under different conditions

  • Measure torque generation and step sizes during ATP synthesis and hydrolysis

• FRET-Based Conformational Sensors:

  • Introduce FRET pairs at strategic locations in different subunits

  • Monitor real-time conformational changes during catalysis

  • Correlate structural transitions with catalytic events

• Spatiotemporal ATP Imaging:

  • Deploy genetically encoded ATP sensors throughout bacterial cells

  • Map ATP concentration gradients near the membrane and in different cellular compartments

  • Correlate with metabolic state and antimicrobial responses

• Combined Electrical and Optical Measurements:

  • Simultaneously measure proton currents and ATP synthesis/hydrolysis

  • Determine precise H⁺/ATP ratios under different conditions

  • Assess efficiency of energy conversion in wild-type and mutant enzymes

These approaches would allow researchers to address fundamental questions about ATP synthase operation in living Enterobacter cells and potentially identify new targets for antimicrobial development.

What insights can comparative studies between Enterobacter ATP synthase and other bacterial ATP synthases provide for evolutionary biology?

Comparative studies of ATP synthases across bacterial species offer valuable evolutionary insights:

• Phylogenetic Analysis:

  • Compare ATP synthase sequences across Enterobacteriaceae family members

  • Identify conserved functional domains versus species-specific adaptations

  • Trace the evolution of regulatory mechanisms like ε subunit inhibition

• Structural Adaptations:

  • Compare c-ring stoichiometry between species (which affects H⁺/ATP ratio)

  • Analyze differences in proton channels and catalytic sites

  • Identify structural features that reflect adaptation to specific ecological niches

• Functional Divergence:

  • Compare kinetic parameters across species

  • Assess differences in regulatory mechanisms

  • Evaluate environmental responsiveness of ATP synthesis/hydrolysis

For Enterobacter specifically, comparisons with other ESKAPE pathogens could reveal how ATP synthase function correlates with pathogenicity and host adaptation. Current evidence suggests that while core catalytic mechanisms are conserved, regulatory elements like the ε subunit show significant species-specific adaptations that likely reflect different ecological pressures .