The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Research report: fumC (fumC2; PP_1755; UniProt Q88M20) in Pseudomonas putida KT2440

0) Scope, identity verification, and disambiguation

The target protein UniProt Q88M20 is annotated as class II fumarate hydratase (fumarase C), EC 4.2.1.2, encoded by fumC and mapped to the ordered locus PP_1755 in Pseudomonas putida KT2440.

A KT2440-focused study explicitly enumerates three fumarase loci in this organism—fumA (PP_0897), fumC1 (PP_0944), and fumC2 (PP_1755)—thereby anchoring PP_1755 as fumC2, consistent with the UniProt accession provided and disambiguating it from fumC genes in other bacteria. (carvajal2014nuevosaspectosdel pages 144-147)

1) Key concepts and current understanding (definitions, reaction, pathway role)

1.1 Enzyme class and reaction

Fumarate hydratase (fumarase; EC 4.2.1.2) catalyzes the reversible interconversion of fumarate and L-malate (S-malate)—a canonical step of the tricarboxylic acid (TCA; Krebs; citrate) cycle. (stuttgen2020closedfumarasec pages 1-2, vugtlussenburg2013biochemicalsimilaritiesand pages 1-2)

For class II fumarase (FumC-type) specifically, structural/biochemical work defines the reaction as fumarate ↔ S-malate and places it explicitly in the Krebs cycle, with the enzyme functioning as a homotetramer in bacteria. (stuttgen2020closedfumarasec pages 1-2, stuttgen2020closedfumarasec pages 7-9)

1.2 Class I vs class II fumarases (important for functional annotation)

Bacteria can encode multiple fumarase isoenzymes spanning two biochemical families:

• Class I fumarases (e.g., FumA/FumB in E. coli) are [4Fe–4S] cluster-containing enzymes; they are oxygen/oxidant sensitive because their Fe–S cluster can be oxidized/inactivated. (vugtlussenburg2013biochemicalsimilaritiesand pages 1-2, vugtlussenburg2013biochemicalsimilaritiesand pages 2-3)
• Class II fumarases (FumC-type) are iron-independent (no Fe–S cluster) and are generally oxygen-stable/oxidant-resistant, offering robustness when ROS damages Fe–S enzymes. (lu2019aconservedmotif pages 1-2, lu2019aconservedmotif pages 6-7)

This distinction matters biologically because it motivates isoenzyme switching under oxidative stress and provides a mechanism for maintaining TCA flux when Fe–S enzymes are compromised. (lu2019aconservedmotif pages 6-7)

1.3 Mechanism and structural determinants (substrate specificity / catalysis)

High-resolution structures and kinetics of bacterial FumC (used here as mechanistic inference for the KT2440 homolog) show:

• FumC monomers have multiple domains and assemble into a homotetramer with active sites formed by residues from multiple subunits. (stuttgen2020closedfumarasec pages 1-2)
• The active site contains a dynamic segment termed the “SS Loop” with a conserved sequence motif; distinct open/closed conformations are observed crystallographically, supporting a conformationally gated catalytic mechanism. (stuttgen2020closedfumarasec pages 1-2, stuttgen2020closedfumarasec pages 5-7)
• The proposed class II mechanism proceeds via an anti-1,2 addition–elimination logic for malate/fumarate interconversion; residues in the SS Loop (e.g., Ser318, Lys324, Asn326 in the studied homolog) contribute primarily to turnover (kcat) rather than substrate binding (Km relatively unchanged in alanine variants). (stuttgen2020closedfumarasec pages 1-2, stuttgen2020closedfumarasec pages 7-9)
• Substrate preference can be directionally asymmetric; one study reports higher affinity for fumarate than S-malate (3.8-fold). (stuttgen2020closedfumarasec pages 7-9)

These mechanistic points support a confident functional annotation of Q88M20 as a cytosolic TCA-cycle enzyme catalyzing fumarate↔L-malate, and they contextualize why multiple fumarase isoenzymes may coexist. (stuttgen2020closedfumarasec pages 1-2, stuttgen2020closedfumarasec pages 7-9)

Structural figure evidence (FumC): Open/closed SS Loop conformations, tetramer/active-site organization, and residue-level interactions are shown in retrieved figures from Stuttgen et al. (stuttgen2020closedfumarasec media 2ed2f20e, stuttgen2020closedfumarasec media 427f7b9b, stuttgen2020closedfumarasec media b221f39b)

2) KT2440-specific functional evidence for fumC2/PP_1755 (Q88M20)

2.1 Isoenzyme repertoire and functional redundancy

In a 2024 genome-scale-model (GSMM) guided engineering study in KT2440, the fumarase (FUM) reaction is linked to three genes: PP_0897, fumC1/PP_0944, and fumC2/PP_1755, with PP_1755 (fumC2) described as non-essential under the tested conditions. Specifically, deletion of PP_1755 and/or PP_0944 had no impact on viability in rich media or M9 minimal media with p-coumarate as carbon source, indicating redundancy at the level of supporting the FUM reaction under these contexts. (banerjee2024bottlenecksinthe pages 3-5)

2.2 Stress/perturbation contexts that alter fumarase isoenzyme usage

A KT2440 mutant perturbation study (ΔapaH background) provides direct evidence that fumC2/PP_1755 is upregulated and can contribute to compensatory fumarase activity:

• Proteomics: FumC2 induced ~4-fold; FumA mildly repressed (~1.3-fold). (carvajal2014nuevosaspectosdel pages 144-147)
• Enzymology: Total fumarase activity was similar between wild type and mutant, but FumC-specific activity was significantly higher in the mutant, consistent with isoenzyme compensation by a fumC-type enzyme. (carvajal2014nuevosaspectosdel pages 144-147)

Collectively, these observations support the functional role of PP_1755/Q88M20 as an active fumarase isoenzyme in KT2440 and show that its contribution can increase under specific perturbations. (carvajal2014nuevosaspectosdel pages 144-147)

2.3 Chemical stress proteomics (herbicide metabolites)

In a quantitative proteomics study of KT2440 responses to chlorophenoxy herbicides and metabolites, a specific metabolite (DCC) caused a distinct proteome response and fumarase C abundance was notably reduced. The study used a DCC concentration that produced ~50% growth-rate reduction, reported as 0.07 mM DCC for KT2440, and observed fewer proteome changes under DCC than other compounds (35 proteins changed under DCC). (benndorf2006pseudomonasputidakt2440 pages 3-5)

This indicates that fumarase C levels are responsive to chemical stressors in KT2440, though the directionality (decrease here) likely reflects broader physiological constraints (e.g., ATP limitation due to uncoupling) rather than a simple “oxidative stress induction” rule. (benndorf2006pseudomonasputidakt2440 pages 3-5)

2.4 Oxidative stress regulation context in P. putida

A review focused on oxidative stress in P. putida reports that fumC-1 is induced in KT2440 under superoxide and nitric oxide stress, and that this induction appears independent of SoxR in KT2440 (contrasting enteric SoxRS paradigms). (kim2014oxidativestressresponse pages 5-6)

This supports a broader interpretation that fumarase C-type isoenzymes are integrated into oxidative/nitrosative stress physiology in P. putida; however, the cited review statement refers to fumC-1 (PP_0944) rather than fumC2 (PP_1755), and it does not supply fold-changes in the excerpted text. (kim2014oxidativestressresponse pages 5-6)

2.5 Regulatory linkage via Fur under phenol stress (indirect)

Under phenol exposure, the Fur regulator is reported as upregulated in KT2440, and the authors note (citing prior work) that Fur represses targets including fumC. This provides a plausible regulatory link between phenol-induced iron/oxidative stress responses and fumarase isoenzyme expression, but the excerpt does not provide direct fumC expression quantitation for phenol exposure. (santos2004insightsintopseudomonas pages 9-10)

3) Cellular localization: what can be concluded

Direct KT2440-specific subcellular localization experiments for FumC2/PP_1755 were not found in the retrieved texts. Nevertheless, the relevant functional context is strongly consistent with a cytosolic enzyme:

• FumC is a central carbon metabolism enzyme (TCA cycle), which in bacteria is carried out in the cytosol (enzymes soluble; metabolites diffuse in cytoplasm). (stuttgen2020closedfumarasec pages 1-2, vugtlussenburg2013biochemicalsimilaritiesand pages 1-2)
• KT2440 proteomics studies detecting fumarase C changes were performed using 2D-gel/MS approaches that predominantly analyze cellular protein fractions enriched for soluble/cytoplasmic proteins, consistent with fumarase C being a soluble enzyme; however, explicit fractionation/localization assignments for FumC were not provided in the extracted text. (benndorf2006pseudomonasputidakt2440 pages 3-5)

Thus, the best evidence-supported statement is that FumC2 functions intracellularly as part of central metabolism, with localization most consistent with the cytosol, but direct localization evidence is not available in the current corpus. (stuttgen2020closedfumarasec pages 1-2, benndorf2006pseudomonasputidakt2440 pages 3-5)

4) Pathways and biological processes

4.1 Core pathway role: the citrate/TCA cycle

FumC catalyzes the fumarate↔malate step of the TCA cycle, supporting energy generation, redox balance, and provision of biosynthetic precursors. (stuttgen2020closedfumarasec pages 1-2, vugtlussenburg2013biochemicalsimilaritiesand pages 1-2)

4.2 Oxidative-stress resilience and isoenzyme switching (conceptual framework)

Mechanistic work on fumarase families emphasizes that class II fumarases (FumC-type) are oxidant-resistant because they lack an Fe–S cluster, whereas class I fumarases are oxidant-sensitive; class II fumarases can be induced by oxidative stress and thereby serve as a contingency enzyme to preserve TCA flux when Fe–S enzymes are damaged by peroxide-related chemistry. (lu2019aconservedmotif pages 1-2, lu2019aconservedmotif pages 6-7)

In KT2440, this conceptual framework is consistent with observed induction of a fumarase C isoenzyme under certain perturbations (ΔapaH) and oxidative/nitrosative stress (fumC-1 induction under superoxide/NO). (carvajal2014nuevosaspectosdel pages 144-147, kim2014oxidativestressresponse pages 5-6)

5) Recent developments (prioritizing 2023–2024) and real-world implementations

5.1 2024 GSMM-guided strain design for aromatic bioconversion: fumarase as a bottleneck

A 2024 bioRxiv preprint on implementing GSMM-based designs in P. putida KT2440 (p-coumarate utilization coupled to glutamine/indigoidine production) identifies the fumarate hydratase (FUM) reaction as a key rate-limiting / tightly constrained node in aromatic catabolism.

Key results (2024):

• Gene mapping: The design associates FUM with PP_0897, fumC1/PP_0944, fumC2/PP_1755, with PP_1755 and PP_0944 described as non-essential individually/together under tested conditions. (banerjee2024bottlenecksinthe pages 3-5)
• Model-derived quantitative flux window: Proteomics-constrained, context-specific models predict a narrow permissive range for FUM flux: maximal glutamine production at FUM = 19.88 mmol/gDCW/h with biomass 0.43 h⁻¹, maximal biomass at FUM = 26.44 mmol/gDCW/h with negligible glutamine; constraining to ~20 mmol/gDCW/h yields ~0.45 h⁻¹ biomass and ~8.61 mmol/gDCW/h glutamine (0.86 mol/mol). (banerjee2024bottlenecksinthe pages 7-10)
• Experimental phenotype: In the engineered background, ∆PP_0897 strains show severe defects on p-coumarate minimal media (including failure to grow in some conditions), and complementation restores growth, indicating that even with multiple fumarase genes present, specific isoenzymes (and/or their regulation) can dominate in certain metabolic states. (banerjee2024bottlenecksinthe pages 7-10)
• Proteomics scale: Perturbing PP_0897 during p-coumarate growth yielded substantial proteome shifts: 47 proteins significantly increased (log2FC > 2.5) and 142 significantly decreased (log2FC < −2.5). (banerjee2024bottlenecksinthe pages 12-14)
• Engineering outcome: Promoter replacement to reduce PP_0897 expression could restore growth and improve product performance; one excerpt reports up to 5-fold higher specific indigoidine productivity per cell in promoter variants compared with parent. (banerjee2024bottlenecksinthe pages 3-5)

Implication for fumC2 annotation: While fumC2/PP_1755 is non-essential in these experiments, it is part of a redundant fumarase repertoire that determines whether the FUM reaction can be tuned/retained under strong selection pressures in bioproduction settings. (banerjee2024bottlenecksinthe pages 3-5, banerjee2024bottlenecksinthe pages 7-10)

6) Expert synthesis and analysis (authoritative interpretation)

6.1 What function should be assigned to Q88M20?

Given (i) explicit KT2440 locus mapping of PP_1755 as fumC2, (ii) observed compensation and increased FumC-specific activity in a KT2440 mutant, and (iii) consistent biochemical mechanism of class II fumarases, the most evidence-supported functional annotation is:

• Enzymatic function: fumarate hydratase (class II fumarase), catalyzing fumarate + H2O ⇌ L-malate in the TCA cycle. (stuttgen2020closedfumarasec pages 1-2, carvajal2014nuevosaspectosdel pages 144-147)

6.2 How should substrate specificity be described?

Within the evidence retrieved here, the most defensible substrate description is specificity for fumarate and S-malate in the TCA cycle, with one detailed FumC kinetic/structural study indicating greater affinity for fumarate than malate (3.8-fold), and mutational analyses showing SS Loop residues primarily affect turnover. (stuttgen2020closedfumarasec pages 7-9)

6.3 What is the likely physiological “reason” for multiple fumarases?

The strongest mechanistic rationale, supported by authoritative mechanistic work, is that iron-dependent class I enzymes are susceptible to oxidative inactivation, whereas class II fumarases are oxidant-resistant and can sustain metabolic flux during oxidative stress; this is a plausible driver for maintaining fumC-type isoenzymes. KT2440 data are consistent with inducible fumarase C-type behavior under perturbations. (lu2019aconservedmotif pages 6-7, carvajal2014nuevosaspectosdel pages 144-147, kim2014oxidativestressresponse pages 5-6)

6.4 Localization and pathway integration

Although direct localization experiments were not retrieved, the combined evidence supports that Q88M20 operates as a soluble intracellular enzyme in central metabolism. The presence of the enzyme in 2D-gel proteomic datasets and its role in the cytosolic TCA cycle are consistent with a cytosolic localization, but this remains an inference within the current evidence set. (stuttgen2020closedfumarasec pages 1-2, benndorf2006pseudomonasputidakt2440 pages 3-5)

7) Evidence summary table

Table: This table summarizes the strongest available evidence for fumarase isoenzymes in Pseudomonas putida KT2440, emphasizing fumC2/PP_1755 (UniProt Q88M20). It highlights identity verification, biochemical role, stress-response behavior, and recent metabolic engineering findings relevant to functional annotation.

8) Key citations with URLs and publication dates (where available)

• Banerjee et al. 2024-03. “Bottlenecks in the Implementation of Genome Scale Metabolic Model Based Designs for Bioproduction from Aromatic Carbon Sources.” bioRxiv. https://doi.org/10.1101/2024.03.15.585139 (banerjee2024bottlenecksinthe pages 1-3, banerjee2024bottlenecksinthe pages 7-10)
• Stuttgen et al. 2020-01. “Closed fumarase C active-site structures reveal SS Loop residue contribution in catalysis.” FEBS Letters. https://doi.org/10.1002/1873-3468.13603 (stuttgen2020closedfumarasec pages 1-2, stuttgen2020closedfumarasec media 2ed2f20e)
• Lu & Imlay 2019-09. “A conserved motif liganding the [4Fe–4S] cluster in [4Fe–4S] fumarases…” Redox Biology. https://doi.org/10.1016/j.redox.2019.101296 (lu2019aconservedmotif pages 1-2, lu2019aconservedmotif pages 6-7)
• Kim & Park 2014-06. “Oxidative stress response in Pseudomonas putida.” Applied Microbiology and Biotechnology. https://doi.org/10.1007/s00253-014-5883-4 (kim2014oxidativestressresponse pages 5-6)
• Benndorf et al. 2006-06. “Pseudomonas putida KT2440 responds specifically to chlorophenoxy herbicides…” PROTEOMICS. https://doi.org/10.1002/pmic.200500781 (benndorf2006pseudomonasputidakt2440 pages 3-5)

9) Gaps and limitations (what remains uncertain)

• Direct experimental localization (e.g., cytosolic vs membrane association) for FumC2/PP_1755 in KT2440 was not found in the retrieved texts.
• Direct KT2440 biochemical characterization (purified PP_1755 kinetics, Km/kcat, substrate range) was not retrieved; mechanistic and kinetic details above rely on well-supported homolog inference from bacterial class II fumarases. (stuttgen2020closedfumarasec pages 7-9)
• The KT2440 oxidative stress induction evidence in the retrieved review excerpt refers to fumC-1 (PP_0944) rather than fumC2 (PP_1755); fumC2 induction is supported under ΔapaH perturbation, not directly under ROS in the extracted KT2440 datasets. (kim2014oxidativestressresponse pages 5-6, carvajal2014nuevosaspectosdel pages 144-147)

References

1. (carvajal2014nuevosaspectosdel pages 144-147): L Agulló Carvajal. Nuevos aspectos del control del metabolismo en" pseudomonas putida" kt2440: metabolismo del ácido fenilacético y papel del gen apah. Unknown journal, 2014.

2. (stuttgen2020closedfumarasec pages 1-2): Gage M. Stuttgen, Julian D. Grosskopf, Colton R. Berger, John F. May, Basudeb Bhattacharyya, and Todd M. Weaver. Closed fumarase c active‐site structures reveal ss loop residue contribution in catalysis. FEBS Letters, 594:337-357, Jan 2020. URL: https://doi.org/10.1002/1873-3468.13603, doi:10.1002/1873-3468.13603. This article has 8 citations and is from a peer-reviewed journal.

3. (vugtlussenburg2013biochemicalsimilaritiesand pages 1-2): Barbara M. A. van Vugt-Lussenburg, Laura van der Weel, Wilfred R. Hagen, and Peter-Leon Hagedoorn. Biochemical similarities and differences between the catalytic [4fe-4s] cluster containing fumarases fuma and fumb from escherichia coli. PLoS ONE, 8:e55549, Feb 2013. URL: https://doi.org/10.1371/journal.pone.0055549, doi:10.1371/journal.pone.0055549. This article has 45 citations and is from a peer-reviewed journal.

4. (stuttgen2020closedfumarasec pages 7-9): Gage M. Stuttgen, Julian D. Grosskopf, Colton R. Berger, John F. May, Basudeb Bhattacharyya, and Todd M. Weaver. Closed fumarase c active‐site structures reveal ss loop residue contribution in catalysis. FEBS Letters, 594:337-357, Jan 2020. URL: https://doi.org/10.1002/1873-3468.13603, doi:10.1002/1873-3468.13603. This article has 8 citations and is from a peer-reviewed journal.

5. (vugtlussenburg2013biochemicalsimilaritiesand pages 2-3): Barbara M. A. van Vugt-Lussenburg, Laura van der Weel, Wilfred R. Hagen, and Peter-Leon Hagedoorn. Biochemical similarities and differences between the catalytic [4fe-4s] cluster containing fumarases fuma and fumb from escherichia coli. PLoS ONE, 8:e55549, Feb 2013. URL: https://doi.org/10.1371/journal.pone.0055549, doi:10.1371/journal.pone.0055549. This article has 45 citations and is from a peer-reviewed journal.

6. (lu2019aconservedmotif pages 1-2): Zheng Lu and James A. Imlay. A conserved motif liganding the [4fe–4s] cluster in [4fe–4s] fumarases prevents irreversible inactivation of the enzyme during hydrogen peroxide stress. Sep 2019. URL: https://doi.org/10.1016/j.redox.2019.101296, doi:10.1016/j.redox.2019.101296. This article has 35 citations and is from a domain leading peer-reviewed journal.

7. (lu2019aconservedmotif pages 6-7): Zheng Lu and James A. Imlay. A conserved motif liganding the [4fe–4s] cluster in [4fe–4s] fumarases prevents irreversible inactivation of the enzyme during hydrogen peroxide stress. Sep 2019. URL: https://doi.org/10.1016/j.redox.2019.101296, doi:10.1016/j.redox.2019.101296. This article has 35 citations and is from a domain leading peer-reviewed journal.

8. (stuttgen2020closedfumarasec pages 5-7): Gage M. Stuttgen, Julian D. Grosskopf, Colton R. Berger, John F. May, Basudeb Bhattacharyya, and Todd M. Weaver. Closed fumarase c active‐site structures reveal ss loop residue contribution in catalysis. FEBS Letters, 594:337-357, Jan 2020. URL: https://doi.org/10.1002/1873-3468.13603, doi:10.1002/1873-3468.13603. This article has 8 citations and is from a peer-reviewed journal.

9. (stuttgen2020closedfumarasec media 2ed2f20e): Gage M. Stuttgen, Julian D. Grosskopf, Colton R. Berger, John F. May, Basudeb Bhattacharyya, and Todd M. Weaver. Closed fumarase c active‐site structures reveal ss loop residue contribution in catalysis. FEBS Letters, 594:337-357, Jan 2020. URL: https://doi.org/10.1002/1873-3468.13603, doi:10.1002/1873-3468.13603. This article has 8 citations and is from a peer-reviewed journal.

10. (stuttgen2020closedfumarasec media 427f7b9b): Gage M. Stuttgen, Julian D. Grosskopf, Colton R. Berger, John F. May, Basudeb Bhattacharyya, and Todd M. Weaver. Closed fumarase c active‐site structures reveal ss loop residue contribution in catalysis. FEBS Letters, 594:337-357, Jan 2020. URL: https://doi.org/10.1002/1873-3468.13603, doi:10.1002/1873-3468.13603. This article has 8 citations and is from a peer-reviewed journal.

11. (stuttgen2020closedfumarasec media b221f39b): Gage M. Stuttgen, Julian D. Grosskopf, Colton R. Berger, John F. May, Basudeb Bhattacharyya, and Todd M. Weaver. Closed fumarase c active‐site structures reveal ss loop residue contribution in catalysis. FEBS Letters, 594:337-357, Jan 2020. URL: https://doi.org/10.1002/1873-3468.13603, doi:10.1002/1873-3468.13603. This article has 8 citations and is from a peer-reviewed journal.

12. (banerjee2024bottlenecksinthe pages 3-5): Deepanwita Banerjee, Javier Menasalvas, Yan Chen, Jennifer W. Gin, Edward E. K. Baidoo, Christopher J. Petzold, Thomas Eng, and Aindrila Mukhopadhyay. Bottlenecks in the implementation of genome scale metabolic model based designs for bioproduction from aromatic carbon sources. bioRxiv, Mar 2024. URL: https://doi.org/10.1101/2024.03.15.585139, doi:10.1101/2024.03.15.585139. This article has 0 citations.

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14. (kim2014oxidativestressresponse pages 5-6): Jisun Kim and Woojun Park. Oxidative stress response in pseudomonas putida. Applied Microbiology and Biotechnology, 98:6933-6946, Jun 2014. URL: https://doi.org/10.1007/s00253-014-5883-4, doi:10.1007/s00253-014-5883-4. This article has 143 citations and is from a domain leading peer-reviewed journal.

15. (santos2004insightsintopseudomonas pages 9-10): Pedro M. Santos, Dirk Benndorf, and Isabel Sá‐Correia. Insights into pseudomonas putida kt2440 response to phenol‐induced stress by quantitative proteomics. PROTEOMICS, 4:2640-2652, Sep 2004. URL: https://doi.org/10.1002/pmic.200300793, doi:10.1002/pmic.200300793. This article has 282 citations and is from a peer-reviewed journal.

16. (banerjee2024bottlenecksinthe pages 7-10): Deepanwita Banerjee, Javier Menasalvas, Yan Chen, Jennifer W. Gin, Edward E. K. Baidoo, Christopher J. Petzold, Thomas Eng, and Aindrila Mukhopadhyay. Bottlenecks in the implementation of genome scale metabolic model based designs for bioproduction from aromatic carbon sources. bioRxiv, Mar 2024. URL: https://doi.org/10.1101/2024.03.15.585139, doi:10.1101/2024.03.15.585139. This article has 0 citations.

17. (banerjee2024bottlenecksinthe pages 12-14): Deepanwita Banerjee, Javier Menasalvas, Yan Chen, Jennifer W. Gin, Edward E. K. Baidoo, Christopher J. Petzold, Thomas Eng, and Aindrila Mukhopadhyay. Bottlenecks in the implementation of genome scale metabolic model based designs for bioproduction from aromatic carbon sources. bioRxiv, Mar 2024. URL: https://doi.org/10.1101/2024.03.15.585139, doi:10.1101/2024.03.15.585139. This article has 0 citations.

18. (banerjee2024bottlenecksinthe pages 5-7): Deepanwita Banerjee, Javier Menasalvas, Yan Chen, Jennifer W. Gin, Edward E. K. Baidoo, Christopher J. Petzold, Thomas Eng, and Aindrila Mukhopadhyay. Bottlenecks in the implementation of genome scale metabolic model based designs for bioproduction from aromatic carbon sources. bioRxiv, Mar 2024. URL: https://doi.org/10.1101/2024.03.15.585139, doi:10.1101/2024.03.15.585139. This article has 0 citations.

19. (banerjee2025addressinggenomescale pages 5-5): Deepanwita Banerjee, Javier Menasalvas, Yan Chen, Jennifer W. Gin, Edward E. K. Baidoo, Christopher J. Petzold, Thomas Eng, and Aindrila Mukhopadhyay. Addressing genome scale design tradeoffs in pseudomonas putida for bioconversion of an aromatic carbon source. NPJ Systems Biology and Applications, Jan 2025. URL: https://doi.org/10.1038/s41540-024-00480-z, doi:10.1038/s41540-024-00480-z. This article has 13 citations.

20. (banerjee2024bottlenecksinthe pages 1-3): Deepanwita Banerjee, Javier Menasalvas, Yan Chen, Jennifer W. Gin, Edward E. K. Baidoo, Christopher J. Petzold, Thomas Eng, and Aindrila Mukhopadhyay. Bottlenecks in the implementation of genome scale metabolic model based designs for bioproduction from aromatic carbon sources. bioRxiv, Mar 2024. URL: https://doi.org/10.1101/2024.03.15.585139, doi:10.1101/2024.03.15.585139. This article has 0 citations.

Artifacts

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Citations

1. carvajal2014nuevosaspectosdel pages 144-147
2. lu2019aconservedmotif pages 6-7
3. stuttgen2020closedfumarasec pages 1-2
4. stuttgen2020closedfumarasec pages 7-9
5. banerjee2024bottlenecksinthe pages 3-5
6. kim2014oxidativestressresponse pages 5-6
7. santos2004insightsintopseudomonas pages 9-10
8. banerjee2024bottlenecksinthe pages 7-10
9. banerjee2024bottlenecksinthe pages 12-14
10. vugtlussenburg2013biochemicalsimilaritiesand pages 1-2
11. vugtlussenburg2013biochemicalsimilaritiesand pages 2-3
12. lu2019aconservedmotif pages 1-2
13. stuttgen2020closedfumarasec pages 5-7
14. banerjee2024bottlenecksinthe pages 5-7
15. banerjee2025addressinggenomescale pages 5-5
16. banerjee2024bottlenecksinthe pages 1-3
17. 4Fe–4S
18. 4fe-4s
19. 4fe–4s
20. https://doi.org/10.1007/s00253-014-5883-4
21. https://doi.org/10.1002/pmic.200500781
22. https://doi.org/10.1016/j.redox.2019.101296
23. https://doi.org/10.1002/1873-3468.13603
24. https://doi.org/10.1101/2024.03.15.585139
25. https://doi.org/10.1101/2024.03.15.585139;
26. https://doi.org/10.1038/s41540-024-00480-z
27. https://doi.org/10.1002/1873-3468.13603,
28. https://doi.org/10.1371/journal.pone.0055549,
29. https://doi.org/10.1016/j.redox.2019.101296,
30. https://doi.org/10.1101/2024.03.15.585139,
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Functional Annotation Report: fumC (Q88M20, PP_1755) — Fumarate Hydratase Class II from Pseudomonas putida KT2440

UniProt accession: Q88M20 · Gene: fumC (synonym fumC-2) · Locus: PP_1755
Organism: Pseudomonas putida (strain ATCC 47054 / DSM 6125 / NCIMB 11950 / KT2440)
EC: 4.2.1.2 · Length: 464 aa · Family: class-II fumarase/aspartase superfamily, fumarase subfamily

Summary

The gene fumC (UniProt Q88M20; ordered locus PP_1755; synonym fumC-2) of Pseudomonas putida strain KT2440 encodes fumarate hydratase class II ("fumarase C"; EC 4.2.1.2), a soluble cytoplasmic metabolic enzyme. Its primary and defining function is to catalyze the reversible, stereospecific hydration/dehydration of fumarate to L-(S)-malate — the reaction (S)-malate ⇌ fumarate + H₂O. This is a canonical step of the tricarboxylic acid (TCA / Krebs) cycle, in which fumarate produced by succinate dehydrogenase is hydrated to malate, which is subsequently oxidized to oxaloacetate. The enzyme is annotated by the curated HAMAP rule MF_00743 and belongs to the class-II fumarase/aspartase superfamily.

Class II fumarases (the FumC type) are mechanistically and evolutionarily distinct from the class I fumarases (FumA/FumB in E. coli). Class II enzymes are iron-independent — they contain no [Fe-S] cluster — and are heat-stable, whereas class I enzymes rely on a labile [4Fe-4S] cluster for catalysis. This iron-independence underpins the alternative UniProt names "aerobic fumarase" and "iron-independent fumarase" and reflects a physiological role as an oxidant-resistant, aerobically-favored fumarase. The enzyme functions as a homotetramer with 222 symmetry; its active sites are built from residues contributed by three of the four subunits, so the oligomeric assembly is obligatory for catalysis. Catalysis proceeds by general acid/base–catalyzed anti (E2-like) addition/elimination of water across the fumarate double bond, an enormously rate-accelerated reaction, with a conserved His/Ser acid-base pair (His186/Ser316 in Q88M20 numbering) at the catalytic center.

The functional assignment for Q88M20 is strongly supported by multiple lines of evidence: (i) curated UniProt/HAMAP annotation specifying the catalytic activity, pathway, subunit structure, and cytoplasmic localization; (ii) ~61% sequence identity to the biochemically characterized E. coli FumC (P05042), far above the threshold for confident function transfer; (iii) 100% conservation of the class II fumarase signature motif GSSIMPGKVN and of the two annotated catalytic residues; and (iv) structural and mechanistic studies of orthologs (E. coli FumC, M. tuberculosis Rv1098c). Among the three fumarase isozymes encoded by KT2440, Q88M20 is the closest ortholog of canonical E. coli FumC, marking it as the principal class II aerobic fumarase of this organism. No literature ambiguity was encountered: the gene symbol, organism, protein family, and domain architecture are all mutually consistent with a class II fumarase.

Key Findings

Finding 1 — FumC catalyzes the reversible fumarate ⇌ L-malate interconversion (EC 4.2.1.2)

The primary function of Q88M20 (PP_1755) is the stereospecific, reversible hydration of fumarate to L-(S)-malate, the canonical fumarate→malate step of the TCA cycle. UniProt annotation, derived from the curated HAMAP rule MF_00743, assigns Q88M20 to the class-II fumarase/aspartase family with EC 4.2.1.2 and specifies the catalytic activity (S)-malate = fumarate + H₂O. The reaction is highly substrate-specific: the class II fumarase family acts on the trans-dicarboxylic olefin fumarate and its hydration product (S)-malate, and does not accept the cis isomer (maleate) or other dicarboxylates as productive substrates.

This assignment is grounded in direct experimental characterization of the ortholog. Weaver and colleagues, studying E. coli fumarase C, state that "Fumarase C catalyzes the stereospecific interconversion of fumarate to L-malate as part of the metabolic citric acid or Kreb's cycle" (PMID: 8909293). Because Q88M20 shares this family membership and the full catalytic constellation with E. coli FumC (see Finding 6), the same reaction and substrate specificity apply.

Finding 2 — Class II fumarase is iron-independent, heat-stable, and adapted to aerobic/oxidative-stress conditions

The FumC-type (class II) fumarase is biochemically distinguished from the class I enzymes by being iron-independent and heat-stable. In E. coli, the fumC gene product is a heat-stable fumarase that does not require iron for activity — unlike the [Fe-S]-cluster class I enzymes FumA and FumB. Park and Gunsalus report that "the fumC gene product is a heat-stable fumarase which does not require iron for activity" (PMID: 7592392). This property explains the UniProt alternate names "aerobic fumarase" and "iron-independent fumarase" for Q88M20.

Physiologically, class II FumC functions as an oxidant-resistant backup to the iron-dependent fumarases. Because it lacks a solvent-exposed, oxidation-sensitive [Fe-S] cluster, FumC remains active when superoxide or iron limitation would inactivate the class I enzymes. The same study demonstrates regulatory logic consistent with this role: "Superoxide radicals also caused increased fumC gene expression; fumA expression was unaffected. Both the superoxide control and the iron control of fumC expression required the SoxR regulatory protein" (PMID: 7592392). Thus FumC is induced under oxidative stress and iron limitation, providing continuity of the TCA-cycle fumarate-hydration step under aerobic and oxidative conditions.

Finding 3 — FumC is a cytoplasmic homotetramer with a shared inter-subunit active site and acid-base catalytic machinery

The class II fumarase/aspartase superfamily forms 222-symmetric homotetramers in which each subunit adopts a three-domain, largely α-helical fold, with a central bundle of five long helices contributing to a 20-helix core bundle at the tetramer center. This fold was first defined by the crystal structure of avian δ-crystallin (a "hijacked" argininosuccinate lyase), which "is distantly related to the class II fumarases, aspartases, adenylosuccinases" and whose active-site cleft "is located on the boundary between three subunits of the tetramer" (PMID: 7634077).

In E. coli FumC the catalytic "A" site is assembled from side chains contributed by multiple subunits, and a second nearby anion-binding "B" site exists. Weaver and Banaszak describe "a binding site for anions which is generated by side chains from three of the four subunits within the tetramer" (PMID: 8909293) — meaning the homotetrameric assembly is obligatory for a functional active site. Studies of the essential class II fumarase Rv1098c from M. tuberculosis reveal the catalytic dynamics: "substrate binding promotes the closure of the active site through conformational changes involving the catalytic SS-loop and the C-terminal domain," and site-directed mutagenesis identifies "Ser318 as one of the two acid-base catalysts" (PMID: 22561013). The chemistry itself is a general acid/base–catalyzed anti (E2-like) addition/elimination of water across the fumarate double bond — an extraordinarily rate-enhanced hydration in which fumarate hydratase decreases the reaction free-energy barrier by ~30 kcal/mol (PMID: 36595439).

Finding 4 — UniProt confirms the single-step TCA reaction, homotetramer, cytoplasmic localization, and catalytic residues

Direct curated annotation of Q88M20 provides a self-consistent functional picture. The protein is 464 amino acids; its catalytic activity is (S)-malate = fumarate + H₂O (EC 4.2.1.2); its pathway assignment is "Carbohydrate metabolism; tricarboxylic acid cycle; (S)-malate from fumarate: step 1/1"; its subunit structure is homotetramer; its subcellular location is cytoplasm; and its similarity is to the "class-II fumarase/aspartase family, Fumarase subfamily". The "step 1/1" designation confirms FumC constitutes the sole enzymatic step converting fumarate to (S)-malate in this pathway module.

The annotated catalytic machinery mirrors the characterized E. coli FumC: Active site 186 (proton donor/acceptor) and Active site 316, with binding-site residues at positions 96, 127 (site B), 137, 185, 317, 322, and residue 329 flagged as important for catalytic activity. This residue-level correspondence with the experimentally validated E. coli enzyme reinforces the confidence of the functional transfer.

Finding 5 — KT2440 encodes three fumarase isozymes plus fumarase-superfamily paralogs; FumC specificity is set by active-site residues, not fold

P. putida KT2440 possesses metabolic redundancy at the fumarate-hydration step. A UniProt proteome survey (organism 160488) returns two class II fumarases — the target fumC/Q88M20 (PP_1755, 464 aa) and fumC-I/Q88PA6 (459 aa) — plus a class I fumarase (Q88PF3, 507 aa). The presence of two fumC genes is what the "fumC-2" synonym for the target reflects.

The genome also encodes several fumarase-superfamily relatives that share the tetrameric fumarase/aspartase fold but catalyze entirely different reactions: argH (argininosuccinate lyase), aspA (aspartate ammonia-lyase, Q88C45), purB (adenylosuccinate lyase, Q88FR7), pcaB (3-carboxy-cis,cis-muconate cycloisomerase, Q88N37), and hmgB (fumarylacetoacetase, Q88E48). Because these superfamily members share the fold yet diverge in catalytic residues, substrate specificity is determined by the active-site constellation, not the overall fold. The P. putida 3-carboxy-cis,cis-muconate lactonizing enzyme (CMLE) illustrates this directly: "PpCMLE is a homotetramer and belongs to the fumarase class II superfamily" (PMID: 15301541), yet it catalyzes a cycloisomerization, not fumarate hydration. FumC's fidelity for fumarate/(S)-malate therefore rests on its conserved class II fumarase catalytic residues (the His/Ser acid-base pair and the SS-loop), which distinguish it from the aspartase/lyase members of the same superfamily.

Finding 6 — Bioinformatic support: 61% identity to characterized E. coli FumC with conserved signature and catalytic residues

A global Needleman–Wunsch alignment of Q88M20 (464 aa) against the experimentally characterized E. coli FumC (P05042, 467 aa) yields 60.7% identity over 484 aligned positions (≈63% over full length). This is far above the ~30–40% identity threshold generally considered sufficient for confident transfer of enzyme function. The class II fumarase / fumarate-lyase family signature motif GSSIMPGKVN is present and 100% identical in Q88M20 (residues 315–324), E. coli FumC (317–326), and the paralog fumC-I/Q88PA6 (312–321). The two UniProt-annotated catalytic residues are conserved: His186 (within the …GRTH… motif; the proton donor/acceptor) and Ser316 (the first serine of the GSSIMPGKVN motif, equivalent to the E. coli/Rv1098c Ser318 acid-base catalyst). This convergence of high overall identity, an intact family signature, and conserved catalytic residues provides strong bioinformatic confirmation of fumarase function.

Finding 7 — Q88M20 is the closest ortholog of canonical E. coli FumC among KT2440's fumarases

Pairwise global identity comparisons place Q88M20 unambiguously as the principal class II fumarase of KT2440:

Among the three KT2440 fumarases, the target is the most similar to the biochemically characterized E. coli FumC and only distantly related (28%) to the mechanistically unrelated class I fumarase. This identifies Q88M20 as the organism's principal aerobic class II fumarase, with fumC-I serving as a second, more divergent class II paralog.

Mechanistic Model / Interpretation

The catalyzed reaction and its metabolic context

FumC operates at a specific junction of central carbon metabolism. In the oxidative TCA cycle, it catalyzes:

succinate
   │  (succinate dehydrogenase, Complex II)
   ▼
FUMARATE  ──────────────►  L-(S)-MALATE  ──────►  oxaloacetate
    FumC (class II)      │  (malate dehydrogenase)
    + H₂O                ▼
    EC 4.2.1.2       to citrate synthase
      (reversible, stereospecific hydration)

FumC constitutes the sole enzymatic step for the fumarate→(S)-malate conversion in this pathway module ("step 1/1"). The reaction is freely reversible; its net direction is set by metabolic flux and the concentrations of fumarate and malate.

Structure–function logic

   Class II fumarase (FumC) — homotetramer, 222 symmetry
   ┌───────────────────────────────────────────────┐
   │  4 subunits × 3 α-helical domains each         │
   │  central 20-helix core bundle                  │
   │  Active site A: built from 3 of 4 subunits     │
   │     ─ His186 (proton donor/acceptor)           │
   │     ─ Ser316 (acid-base catalyst; SS-loop)     │
   │     ─ signature motif GSSIMPGKVN (315–324)     │
   │  Site B: second anion-binding site             │
   │  NO [Fe-S] cluster → iron-independent, aerobic │
   └───────────────────────────────────────────────┘
     │
     ▼  general acid/base anti (E2-like) addition of water
fumarate + H₂O  ⇌  (S)-malate   (ΔΔG‡ ≈ 30 kcal/mol rate enhancement)

Three structural features are decisive for function. First, the obligatory homotetramer: because the active site is assembled from residues of three different subunits, no monomer or dimer can be catalytically competent. Second, the conserved acid-base pair (His186/Ser316) and the mobile SS-loop, which close over the bound substrate and execute the anti addition/elimination of water. Third, the absence of an iron-sulfur cluster, which distinguishes class II from class I fumarases and confers heat stability and resistance to oxidative/iron-limiting stress.

Why the class assignment matters physiologically

Class I and class II fumarases are convergent solutions to the same chemical problem but are not homologous. Class I enzymes (e.g., E. coli FumA/FumB) use a [4Fe-4S] cluster and are inactivated by superoxide and iron starvation; class II enzymes (FumC) are iron-free and robust under these conditions. This dichotomy is underscored by structural work on parasite fumarases, where inhibitor selectivity for class I FHs "is due to direct coordination of the inhibitor to the unique Fe of the catalytic [4Fe-4S] cluster … but is absent from class II human FH" (PMID: 30645090). The regulatory data from E. coli — SoxR-dependent induction of fumC by superoxide and iron limitation — indicate that class II fumarase acts as a stress-resistant safeguard for TCA-cycle continuity. In an obligate aerobe like P. putida KT2440, which experiences substantial oxidative load during aerobic metabolism and the degradation of aromatic and other compounds, an iron-independent, heat-stable fumarase is a physiologically sensible principal isozyme.

Distinguishing FumC from its superfamily relatives

The fumarase class II fold is a metabolic "chassis" reused across several reactions (aspartate ammonia-lyase, argininosuccinate lyase, adenylosuccinate lyase, CMLE, δ-crystallin). The aspartase/fumarase superfamily shares "a monomer that is composed of three domains oriented in an elongated S-shape" with "active sites located in clefts between the subunits" (PMID: 10800598). Specificity for fumarate/(S)-malate is not conferred by the fold but by the exact set of active-site residues. Q88M20 carries the fumarase-specific His/Ser catalytic pair and the intact GSSIMPGKVN signature, and its high identity to the validated E. coli FumC rules out the possibility that it is an aspartase or lyase misannotated as a fumarase.

Evidence Base

Consistency assessment: All ten reviewed papers are mutually consistent with the assignment of Q88M20 as a class II fumarase. No literature was found describing a different gene product under the fumC symbol that would create ambiguity. The gene symbol (fumC), organism (P. putida KT2440), protein family (class-II fumarase/aspartase), and InterPro domains (Fum_hydII IPR005677; Fumarase_C_C IPR018951; Fumarate_lyase_CS IPR020557) all align.

Limitations and Knowledge Gaps

1. No direct biochemical characterization of Q88M20 itself. The functional assignment rests on curated annotation, sequence homology, and structural/mechanistic studies of orthologs (E. coli FumC, M. tuberculosis Rv1098c). No published kinetic parameters (kcat, Km for fumarate/L-malate), no purified-protein assay, and no crystal structure specific to the P. putida KT2440 enzyme were identified. Function is inferred by homology, albeit at high confidence (~61% identity, intact catalytic residues).

2. Localization is inferred, not experimentally demonstrated. Cytoplasmic localization is the UniProt/HAMAP annotation and is consistent with the soluble, non-membrane nature of the class II fumarase fold and its TCA-cycle role, but no proteomic or fluorescence-based localization study specific to PP_1755 was found.

3. Physiological division of labor between the isozymes is uncharacterized in P. putida. KT2440 encodes two class II fumarases (Q88M20 and fumC-I/Q88PA6) plus a class I fumarase (Q88PF3). The specific growth conditions under which each is expressed, their relative flux contributions, and any regulatory hierarchy (analogous to the SoxR/ArcA/FNR control in E. coli) have not been directly established for P. putida.

4. Regulatory inference is cross-species. The oxidative-stress/iron-limitation induction and SoxR dependence are documented in E. coli. Whether the same regulatory logic governs P. putida PP_1755 remains to be verified experimentally.

5. Substrate-specificity boundaries not directly tested for the P. putida enzyme. Family-level specificity (fumarate/(S)-malate; exclusion of maleate and non-cognate dicarboxylates) is inferred from the conserved active site and from characterized orthologs, not measured for Q88M20.

Proposed Follow-up Experiments / Actions

1. Recombinant expression and steady-state kinetics. Clone PP_1755, express and purify the His-tagged protein, and measure kcat and Km for fumarate hydration and L-malate dehydration. Confirm stereospecificity ((S)- vs (R)-malate) and test candidate non-substrates (maleate, D-malate, aspartate) to delimit specificity.

2. Iron-independence and thermostability assays. Verify absence of [Fe-S] cluster (UV-vis, iron content, EPR) and measure thermal stability, directly confirming the class II designation for the P. putida enzyme.

3. Structural determination. Solve the crystal or cryo-EM structure of Q88M20 (apo and with malate/inhibitor) to confirm the homotetramer, the inter-subunit active site, and the His186/Ser316 catalytic geometry. Alternatively, validate an AlphaFold model against the E. coli FumC template using structural-biology tooling.

4. Site-directed mutagenesis of predicted catalytic residues. Mutate His186 and Ser316 (and site-B residues) and measure the loss of activity to experimentally confirm their catalytic roles in the P. putida enzyme.

5. Genetic dissection of isozyme roles. Construct single and combinatorial deletions of PP_1755, fumC-I (Q88PA6), and the class I fumarase (Q88PF3), and assess growth on TCA-cycle-dependent carbon sources under aerobic, oxidative-stress (paraquat), and iron-limited conditions to define each isozyme's physiological niche.

6. Expression profiling under stress. Use RT-qPCR or RNA-seq to test whether PP_1755 is induced by superoxide generators and iron chelation, probing whether the E. coli-like SoxR/oxidative-stress regulation is conserved in P. putida.

Conclusion

The gene fumC (Q88M20, PP_1755) of Pseudomonas putida KT2440 encodes fumarate hydratase class II (fumarase C, EC 4.2.1.2), a cytoplasmic, iron-independent, heat-stable homotetramer that catalyzes the reversible, stereospecific hydration of fumarate to L-(S)-malate as the fumarate→malate step of the TCA cycle. Its active sites are shared across subunits and employ a conserved His186/Ser316 acid-base pair and a mobile SS-loop to perform general acid/base anti addition of water. As a class II enzyme it functions as an aerobic, oxidant-resistant fumarase. The assignment is supported by curated annotation, ~61% identity to the biochemically characterized E. coli FumC with 100% conservation of the class II catalytic motif and residues, and by structural/mechanistic studies of orthologs — while acknowledging that no direct biochemical characterization of the P. putida enzyme itself has yet been published.

Artifacts

• OpenScientist final report
• OpenScientist final report

Citations

1. PMID:8909293
2. PMID:7592392
3. PMID:7634077
4. PMID:22561013
5. PMID:36595439
6. PMID:15301541
7. PMID:30645090
8. PMID:10800598