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.