Recombinant Shewanella baltica Acetyl-coenzyme A synthetase (acsA), partial

What is Shewanella baltica acetyl-coenzyme A synthetase and what is its function?

Acetyl-coenzyme A synthetase (acsA) from Shewanella baltica is an enzyme that catalyzes the conversion of acetate to acetyl-CoA through a two-step reaction requiring ATP. The full reaction can be represented as:

Acetate + ATP + CoA → Acetyl-CoA + AMP + PPi

The enzyme is also known as Acetate--CoA ligase or Acyl-activating enzyme, with the Enzyme Commission number EC 6.2.1.1 . Functionally, acsA plays a critical role in acetate metabolism, allowing bacteria to utilize acetate as a carbon source by converting it to acetyl-CoA, which can then enter the TCA cycle or other metabolic pathways. In Shewanella baltica, which inhabits redox-fluctuating environments like the Baltic Sea, this enzyme is particularly important for adapting to varying carbon availability conditions .

What are the optimal storage conditions for recombinant S. baltica acsA?

For research applications using recombinant Shewanella baltica acsA, proper storage is critical to maintain enzymatic activity. The recommended storage conditions are:

It's important to note that repeated freezing and thawing cycles significantly reduce enzyme activity and should be avoided . Working aliquots should be prepared and stored at 4°C for up to one week to minimize freeze-thaw cycles. For reconstitution, the protein should be dissolved in deionized sterile water to a concentration of 0.1-1.0 mg/mL, with the addition of 5-50% glycerol (final concentration) for long-term storage .

How is recombinant S. baltica acsA typically produced and purified?

Recombinant Shewanella baltica acsA is commonly produced in yeast expression systems, which provide appropriate eukaryotic post-translational modifications while maintaining high protein yield . The production process typically involves:

• Cloning the partial or complete acsA gene from Shewanella baltica (strain OS155 / ATCC BAA-1091) into an appropriate expression vector

• Transforming the construct into a yeast host

• Inducing protein expression under optimized conditions

• Cell harvest and lysis

• Protein purification using affinity chromatography (based on the tag incorporated during cloning)

The final product typically achieves >85% purity as verified by SDS-PAGE . While the specific tag type may vary depending on the manufacturing process, it's important to consider how the tag might affect enzyme activity in experimental applications. For detection purposes, Western blotting with antibodies against the tag or the protein itself can be used to confirm the identity of the purified protein.

How does acetylation affect the activity of S. baltica acsA and what methods are best for studying this modification?

Lysine acetylation is an evolutionarily conserved protein modification that significantly regulates acetyl-CoA synthetase activity. Research shows that acetylation of a conserved lysine residue in the active site of acsA inactivates the enzyme, creating a regulatory mechanism that controls acetate metabolism . The acetylation status is regulated by:

• Acetyltransferases (such as YfiQ/Pat/AcuA) that acetylate the conserved lysine

• Deacetylases (such as CobB/SrtN) that remove the acetyl group and restore activity

To study acetylation of S. baltica acsA, several complementary methods can be employed:

• Mass spectrometry-based approaches: LC-MS/MS analysis following tryptic digestion can identify acetylated lysine residues and quantify acetylation levels.

• Site-directed mutagenesis: Replacing the target lysine residue with arginine (which maintains the positive charge but cannot be acetylated) or glutamine (which mimics constitutive acetylation) allows assessment of the functional importance of acetylation.

• In vitro acetylation/deacetylation assays: Using purified acetyltransferases and deacetylases to modify the purified enzyme followed by activity measurements.

• Western blotting with anti-acetyllysine antibodies: To detect and semi-quantify acetylation levels.

Studies in related bacteria have shown that acetylation of acsA is particularly prominent in high glucose conditions, suggesting a carbon source-dependent regulation mechanism . This modification serves as a key control point in the overflow metabolism, limiting acetyl-CoA synthesis when preferred carbon sources are available .

What experimental approaches are recommended for assessing S. baltica acsA enzymatic activity in different environmental conditions?

Measuring acsA activity under various environmental conditions relevant to S. baltica's natural habitat requires specialized experimental designs:

• Spectrophotometric coupled assays: The enzymatic reaction can be coupled to the production of AMP, which can be monitored through additional enzymatic reactions that result in NAD+/NADH conversion, measurable at 340 nm.

• Radiometric assays: Using 14C-labeled acetate to track incorporation into acetyl-CoA.

• HPLC-based methods: To separate and quantify reaction products (acetyl-CoA) and substrates (acetate, ATP, CoA).

For environmental relevance, the following conditions should be tested:

When designing these experiments, it's crucial to consider the interactions between these parameters. For example, the temperature optima may shift under different salinity conditions, reflecting adaptations to specific environmental niches.

How can genomic and proteomic approaches be integrated to understand the evolutionary significance of acsA in S. baltica strains?

Integrating genomic and proteomic approaches provides a comprehensive understanding of acsA's evolution and adaptation in S. baltica:

• Comparative genomics: Analyze acsA sequences across different S. baltica strains to identify polymorphisms and potential adaptation signatures. This can be done through:

  • Multilocus sequence typing (MLST) including the acsA gene

  • Whole genome sequencing followed by comparative genomic analysis

  • Identification of selection pressures using dN/dS ratios

• Structural proteomics: Compare the structural features of acsA across strains to understand functional adaptations:

  • Homology modeling based on crystal structures of related enzymes

  • Identification of strain-specific variations in catalytic sites or regulatory domains

• Functional proteomics: Characterize post-translational modifications and protein-protein interactions:

  • Acetylome analysis through enrichment of acetylated peptides followed by MS/MS

  • Co-immunoprecipitation to identify potential regulatory partners

  • Protein turnover rates in different environmental conditions

• Integrative analysis: Correlate genomic variations with phenotypic differences:

  • Growth rates on acetate as sole carbon source

  • Enzyme kinetics parameters (Km, Vmax) for different substrate concentrations

  • Response to environmental stressors

This integrated approach has revealed that S. baltica strains show significant genotypic and phenotypic differentiation, reflecting adaptation to specific environmental niches in the Baltic Sea . The acsA gene likely plays a crucial role in this adaptation by enabling efficient utilization of acetate under specific environmental conditions.

What are the most common challenges when working with recombinant S. baltica acsA and how can they be addressed?

Working with recombinant acsA can present several challenges that researchers should anticipate:

• Protein stability issues: acsA may show reduced stability after purification.

  • Solution: Add stabilizing agents such as glycerol (5-50%) to storage buffers

  • Store in small aliquots to avoid repeated freeze-thaw cycles

  • Consider adding reducing agents like DTT or β-mercaptoethanol if disulfide bond formation is suspected

• Low enzymatic activity: Activity may be lower than expected due to improper folding or post-translational modifications.

  • Solution: Optimize reconstitution conditions (pH, salt concentration)

  • Verify tag position is not interfering with enzyme activity

  • Check for potential inhibitors in the buffer or reaction components

• Substrate specificity variations: S. baltica acsA may have different substrate preferences than better-characterized homologs.

  • Solution: Test activity with various acyl substrates beyond acetate

  • Optimize ATP and CoA concentrations for the specific enzyme variant

  • Consider the presence of metal ions (Mg2+, Mn2+) that might affect activity

• Acetylation during expression: The enzyme may be partially acetylated during recombinant expression, affecting baseline activity.

  • Solution: Co-express with a deacetylase to ensure an active form

  • Use mass spectrometry to assess acetylation status before experiments

  • Consider expression in a system with reduced acetyltransferase activity

Documenting these challenges and solutions provides valuable methodological guidance for other researchers working with this enzyme.

How can researchers design experiments to study the interaction between acsA activity and central metabolism in S. baltica?

To investigate how acsA integrates with central metabolism in S. baltica, consider these experimental approaches:

• Metabolic flux analysis: Use 13C-labeled substrates to track carbon flow through central metabolic pathways with and without active acsA.

  • Incorporate isotope-labeled acetate and measure labeled metabolites using GC-MS or LC-MS

  • Compare wild-type strains with acsA mutants to assess metabolic rewiring

• Transcriptomic profiling: Analyze gene expression changes in response to acetate availability and acsA activity.

  • RNA-seq analysis comparing growth on acetate versus other carbon sources

  • qRT-PCR validation of key metabolic genes in wild-type and acsA mutant strains

• Metabolite profiling: Measure intracellular concentrations of key metabolites affected by acsA activity.

  • Focus on acetyl-CoA, CoA, acetate, and TCA cycle intermediates

  • Compare profiles under different environmental conditions relevant to the Baltic Sea

• Growth phenotyping: Characterize growth parameters on different carbon sources.

  • Measure growth rates, lag phases, and yields on acetate versus other carbon sources

  • Test mixed carbon source utilization to assess catabolite repression effects

• Enzyme activity assays: Measure activities of enzymes connected to acsA in central metabolism.

  • Focus on TCA cycle enzymes, glyoxylate bypass, and gluconeogenesis

  • Compare activities in different growth phases and carbon sources

Research has shown that acetylation of Acs (acsA) limits acetyl-CoA synthesis, leading to acetate accumulation and growth inhibition in related bacteria . This indicates that acsA regulation is a key control point in bacterial carbon metabolism, particularly in environments with fluctuating carbon availability like the Baltic Sea.

How might S. baltica acsA regulation mechanisms inform biotechnological applications?

Understanding the regulation of S. baltica acsA opens several avenues for biotechnological applications:

• Metabolic engineering for acetate utilization: Engineering acsA regulation can improve acetate assimilation in industrial microorganisms.

  • Creating acetylation-resistant variants (e.g., through site-directed mutagenesis of key lysine residues) can enhance growth on acetate-rich substrates

  • This approach has precedent, as a random mutation in Leu-641 of a related acetyl-CoA synthetase made it insensitive to acetylation even in high glucose concentrations

• Biosensor development: acsA activity can serve as the basis for biosensors detecting:

  • Acetate levels in environmental samples

  • Protein acetylation modulators (potential drug candidates)

  • Environmental stressors that affect protein acetylation

• Cold-adapted enzyme applications: As S. baltica is psychrotrophic, its acsA may have unique properties suitable for low-temperature biotechnological processes .

  • Potential applications in food processing, bioremediation in cold environments, or low-temperature biocatalysis

  • Structural features enabling cold adaptation could be transferred to other enzymes

• Biofilm control strategies: If acsA regulation affects biofilm formation (as acetate metabolism often does in bacteria), this could inform new approaches to control bacterial biofilms in industrial or medical settings.

Recent research has shown that mutations affecting acetylation sensitivity of acetyl-CoA synthetase can allow efficient co-utilization of multiple carbon sources, which has significant implications for metabolic engineering applications .

What role might acsA play in the adaptation of S. baltica to environmental changes in the Baltic Sea?

The Baltic Sea represents a unique environment with gradient conditions where S. baltica has evolved specific adaptations:

• Redox adaptation: S. baltica thrives in redox-fluctuating environments where acsA may play a key role in metabolic flexibility .

  • Under changing oxygen conditions, acetate metabolism via acsA could provide an alternative energy source

  • Research questions could focus on how acsA expression and activity change across redox gradients

• Temporal adaptation: Studies sampling S. baltica over 12 years have revealed significant genotypic and phenotypic differentiation .

  • Investigating acsA sequence and activity variations across these temporal samples could reveal evolutionary adaptation mechanisms

  • Correlation with changing environmental parameters (oxygen levels, organic matter composition) would be particularly informative

• Spatial niche specialization: S. baltica strains show differentiation based on depth and location in the Baltic Sea .

  • acsA variations might contribute to specialization for different carbon sources available at different depths

  • Comparative analysis of acsA from strains isolated from different depths could reveal adaptive signatures

• Interspecies interactions: acsA function may influence competitive or cooperative interactions with other microorganisms in the Baltic Sea ecosystem.

  • Co-culture experiments with different Baltic Sea bacteria could reveal how acsA-mediated acetate utilization affects community dynamics

  • Metatranscriptomic approaches could reveal in situ expression patterns in relation to other community members

Research has shown that S. baltica represents the most important H2S-producing organism in certain Baltic Sea environments , suggesting a key role in sulfur cycling that may be linked to its carbon metabolism including acsA function.

How do post-translational modifications beyond acetylation affect S. baltica acsA function in different environmental contexts?

While acetylation is well-studied, other post-translational modifications (PTMs) may also regulate acsA function:

• Phosphorylation: Potential phosphorylation sites in acsA could provide rapid regulation in response to environmental signals.

  • Experimental approach: Phosphoproteomics analysis of S. baltica grown under different conditions

  • Functional validation through phosphomimetic mutations (Ser/Thr to Asp/Glu) or phospho-null mutations (Ser/Thr to Ala)

• S-thiolation: Given S. baltica's role in sulfur cycling , S-thiolation (formation of mixed disulfides) might regulate acsA under oxidative stress.

  • Experimental approach: Redox proteomics to identify potential S-thiolation sites

  • Analysis of acsA activity under different redox conditions

• Succinylation/Malonylation: These emerging PTMs could provide additional regulatory layers connected to the TCA cycle status.

  • Experimental approach: Acylome analysis focusing on succinylation and malonylation

  • Correlation with metabolic status and carbon source availability

• Proteolytic processing: Controlled proteolysis might regulate acsA activity or localization.

  • Experimental approach: N-terminal proteomics to identify potential processing events

  • Western blot analysis with antibodies targeting different epitopes

For studying these modifications in environmental contexts relevant to S. baltica, researchers should consider:

This research direction would provide unprecedented insights into how bacteria like S. baltica use complex PTM networks to adapt to changing environments.

How does S. baltica acsA compare structurally and functionally to acetyl-CoA synthetases from other bacteria?

Comparative analysis of acsA across bacterial species reveals important structural and functional differences:

• Structural comparison:

  • S. baltica acsA likely contains domains similar to those found in other bacterial ACS enzymes, including an N-terminal domain that binds ATP and a C-terminal domain that binds acetate and CoA

  • The active site typically contains conserved histidine residues that are critical for catalysis, as shown in related enzymes where mutations of these residues abolish activity entirely

  • The acetylation site is generally a conserved lysine residue in a region accessible to acetyltransferases

• Functional comparison:

  • Substrate specificity: While primarily acting on acetate, acsA enzymes from different bacteria show varying affinities for other short-chain fatty acids

  • Kinetic parameters: Km and Vmax values often reflect adaptation to typical substrate concentrations in the organism's natural environment

  • Regulatory mechanisms: While acetylation is common, the specific regulatory networks controlling acsA expression and activity vary significantly

• Phylogenetic considerations:

  • S. baltica belongs to a genus characterized by significant phenotypic heterogeneity

  • Within Shewanella species, S. baltica strains show distinct G+C content (46-47 mol%) which may influence codon usage and protein expression

• Environmental adaptation signatures:

  • Cold adaptation: As a psychrotrophic organism, S. baltica acsA likely contains structural features that enable activity at lower temperatures compared to mesophilic counterparts

  • Salinity adaptation: Features enabling function in the brackish Baltic Sea environment may distinguish it from freshwater or marine homologs

This comparative approach provides insights into the evolution of acsA and its adaptation to specific ecological niches, particularly the unique Baltic Sea environment where S. baltica has specialized.

What research techniques have provided the most valuable insights into acsA function across bacterial species?

Several research techniques have proven particularly valuable for understanding acsA function:

• Crystallography and structural analysis:

  • X-ray crystallography has revealed that the catalytic center typically contains a zinc ion coordinated by two aspartic acids, a histidine, and a water molecule

  • Surface analysis shows a narrow pocket that accommodates the acetylated lysine during catalysis

  • These structural insights enable rational design of mutations to test functional hypotheses

• Genetic manipulation approaches:

  • Gene knockout studies (ΔacsA) reveal the physiological importance of acsA in different growth conditions

  • Point mutations targeting catalytic residues or regulatory sites (e.g., acetylation sites) provide mechanistic insights

  • Studies with Δcob (deacetylase) and ΔyfiQ (acetyltransferase) mutants have revealed the importance of acetylation in regulating acsA activity

• Systems biology approaches:

  • Transcriptomics shows how acsA expression changes across conditions

  • Metabolomics reveals the impact of acsA activity on the broader metabolic network

  • Fluxomics using 13C-labeled substrates maps carbon flow through acsA and connected pathways

• Evolutionary and comparative studies:

  • Multilocus sequence typing (MLST) and comparative genomic hybridization (CGH) have revealed genotypic differentiation among S. baltica strains

  • These approaches have identified potential environmental adaptation signatures in acsA sequences

• Environmental sampling and correlation:

  • Studies sampling bacteria along environmental gradients (like the redox gradient in the Baltic Sea) reveal how acsA function correlates with specific conditions

  • Time-series sampling over years has tracked evolutionary changes in acsA in response to changing environmental conditions

These complementary approaches provide a comprehensive understanding of acsA function and evolution that no single technique could achieve alone.