Organism: Pseudomonas putida KT2440 (NCBITaxon:160488, PSEPK)
Gene: kguE | Locus: PP_3379 | UniProt: Q88HH8
Hypothesis under review: kguE has phosphogluconate 2-dehydrogenase activity (GO:0008114).
Focus type: function_assignment | Current annotation evidence: IEA (GO_REF:0000003)
Verdict: REFUTED / over-annotated.
The seed hypothesis that kguE (Q88HH8) directly possesses phosphogluconate 2-dehydrogenase activity (GO:0008114, EC 1.1.1.43) is not supported and should be removed from the gene review. The annotation is a stale, electronically-inferred ec2go carry-over (GO_REF:0000003) that originated from a self-contradictory EMBL/UniProt submission: the protein is given the submission name "Epimerase" yet simultaneously carries EC 1.1.1.43 (6-phospho-D-gluconate:NAD(P)⁺ 2-oxidoreductase). GO_REF:0000003 mechanically converts the submitted EC number into the matching GO molecular-function term, with no evidence that the enzyme actually catalyzes that reaction.
Three independent lines of evidence converge on refutation. (1) Fold incompatibility: every signature database maps Q88HH8 to a metal-dependent (β/α)₈ xylose-isomerase-like TIM barrel (IPR050312 IolE/XylA/MocC family; IPR036237 superfamily; Gene3D 3.20.20.150; SUPFAM SSF51658; COG1082 "sugar-phosphate isomerases/epimerases"). EC 1.1.1.43 is catalyzed by an NAD(P)-dependent Rossmann-fold oxidoreductase — a structurally unrelated fold. Q88HH8 has no Pfam NAD-binding/Rossmann domain (no PF03446/PF00393) listed at all. (2) High-confidence structure: the AlphaFold model AF-Q88HH8-F1 spans the full 259 aa at mean pLDDT 94.88, a single compact TIM-barrel domain — a genuine metal-dependent isomerase/epimerase, not a fragment or pseudogene, and with no room for a separate cofactor-binding Rossmann lobe. (3) Family consensus: a survey of all 75 reviewed (Swiss-Prot) members of KguE's InterPro family IPR050312 found zero oxidoreductases — the family is exclusively EC class 4 (lyases/dehydratases) and class 5 (isomerases/epimerases). EC 1.1.1.43 is biochemically foreign to this family.
Finally, the pathway context identifies where the real EC 1.1.1.43 activity resides. kguE sits in the PtxS-regulated 2-ketogluconate (kgu) operon alongside kguK (2-ketogluconokinase) and kguT (transporter). The characterized cytoplasmic 2-ketogluconate branch converts 2-ketogluconate → (kinase) → 2-keto-6-phosphogluconate → (NAD(P)-dependent reductase) → 6-phosphogluconate. That reductase/dehydrogenase step (the genuine EC 1.1.1.43 chemistry) belongs to a distinct Rossmann-fold enzyme (KguD-type), not to the KguE epimerase. The most likely explanation is that the EC number was mis-attached to the epimerase gene during submission. Curation lead: remove GO:0008114; assign a carbohydrate epimerase/isomerase molecular-function term consistent with the "kguE epimerase" identity and the TIM-barrel fold, pending experimental confirmation in PMID: 39770733.
Most important caveat: No direct enzymatic assay of purified KguE exists in the literature reviewed. The refutation rests on strong structural/evolutionary incompatibility and pathway logic rather than a negative activity assay. The precise substrate/product of KguE (its true epimerase/isomerase reaction) remains experimentally undetermined.
UniProt Q88HH8 (kguE, locus PP_3379) carries no recommended name, no EC number in its name field, and no catalytic-activity or function comment. Its submission name is literally "Epimerase," with EC 1.1.1.43 attached to that submission record. The single molecular-function GO annotation on the entry is GO:0008114 (phosphogluconate 2-dehydrogenase activity), evidence code IEA, source tag UniProtKB-EC — i.e., the GO_REF:0000003 ec2go mapping that mechanically translates the submitted EC 1.1.1.43 into its corresponding GO term. This is a database pipeline artifact, not an experimental or curator-reviewed assignment.
The domain architecture flatly contradicts a dehydrogenase assignment. Q88HH8's signatures are all consistent with a metal-dependent (β/α)₈ TIM-barrel isomerase/epimerase: InterPro IPR050312 (IolE/XylA/MocC family), IPR036237 (xylose-isomerase-like superfamily), Gene3D 3.20.20.150 (divalent-metal-dependent TIM barrel), SUPFAM SSF51658, eggNOG COG1082 ("sugar-phosphate isomerases/epimerases"), and PANTHER PTHR12110 (hydroxypyruvate isomerase). By contrast, EC 1.1.1.43 describes a 6-phospho-D-gluconate:NAD(P)⁺ 2-oxidoreductase — an NAD(P)-dependent Rossmann-fold enzyme. The TIM barrel and the Rossmann fold are structurally distinct protein architectures with no catalytic overlap; IPR050312 members catalyze inosose dehydration and sugar isomerization/epimerization, reactions that require no NAD(P) cofactor. A TIM-barrel metalloenzyme cannot perform NAD(P)-dependent hydride transfer.
UniProt locus mapping places kguE (PP_3379) adjacent to kguK (PP_3378, 2-ketogluconokinase, EC 2.7.1.13, Q88HH9), kguT (PP_3377, 2-ketogluconate transporter, Q88HI0), and ptxS (PP_3380, PtxS regulator, Q88HH7). This is the characterized 2-ketogluconate (kgu) utilization operon.
Daddaoua et al. 2010 (PMID: 20581202) describe the cytoplasmic 2-ketogluconate route: the metabolite "is subsequently transported to the cytoplasm, where two reactions are carried out, giving rise to 6-phosphogluconate, which enters the Entner-Doudoroff pathway." Those two reactions are a kinase (KguK, producing 2-keto-6-phosphogluconate) plus an NAD(P)-dependent 2-keto-6-phosphogluconate reductase (the true EC 1.1.1.43 activity, i.e. KguD). del Castillo et al. 2008 (PMID: 18245293) assign the 2-ketogluconate utilization genes to the PtxS regulon: "PtxS … controls the enzymes for oxidation of gluconate to 2-ketogluconate, its transport and metabolism."
Critically, in UniProt the only KT2440 protein carrying EC 1.1.1.43 is kguE itself — and it carries it solely by virtue of its "Epimerase" submission record, not any reductase annotation. This strongly indicates the EC number was mis-attached to the epimerase gene rather than to the genuine reductase during genome submission. The true reductase (KguD) is a separate gene product with a Rossmann fold.
Q88HH8's signature databases are exclusively TIM-barrel/isomerase: Gene3D 3.20.20.150, SUPFAM SSF51658 (xylose-isomerase-like), PANTHER PTHR12110 (hydroxypyruvate isomerase), InterPro IPR050312/IPR036237. No Pfam domain is even listed, and — most tellingly — no NAD-binding/Rossmann signature (e.g., PF03446 6-phosphogluconate dehydrogenase C-terminal, or PF00393 6-phosphogluconate dehydrogenase NAD-binding) appears anywhere on the entry. Without a cofactor-binding module, EC 1.1.1.43 chemistry is mechanistically impossible.
The AlphaFold model AF-Q88HH8-F1 covers the full 259 aa at mean pLDDT 94.88 (very high confidence), depicting a single compact domain consistent with a real metal-dependent (β/α)₈ TIM-barrel enzyme — not a truncated fragment or degraded pseudogene. There is no second domain into which a Rossmann NAD(P)-binding lobe could fit.
In KT2440, the genuine 6-phosphogluconate-dehydrogenase / β-hydroxyacid-dehydrogenase family (Pfam PF03446) comprises distinct genes — mmsB/PP_4666, glxR/PP_4299, gntZ/PP_4043 (EC 1.1.1.44), PP_2798, PP_1143 — and kguE is not among them. del Castillo et al. 2007 (PMID: 17483213) confirm the step is a reductase: periplasmic glucose-derived 2-ketogluconate "is transported to the cytoplasm, and subsequently phosphorylated and reduced to 6-phosphogluconate," locating the EC 1.1.1.43 chemistry in an NAD(P)-dependent reductase distinct from any epimerase.
A systematic survey of all 75 reviewed (Swiss-Prot) UniProt entries in KguE's InterPro family IPR050312 (IolE/XylA/MocC) reveals a strong functional consensus that excludes oxidoreductase activity:
| Recommended-name class | Count |
|---|---|
| Inosose dehydratase (EC 4.2.1.44) | 36 |
| 3-dehydroshikimate dehydratase | 4 |
| Inosose isomerase | 4 |
| Ketose/sugar 3-epimerases (fructoselysine/D-psicose/D-tagatose/L-ribulose) | multiple |
The EC numbers present across the family are exclusively class 4 (lyases) — EC 4.2.1.44 (×38), 4.2.1.118 (×4), 4.1.99.-, 4.1.1.120 — and class 5 (isomerases/epimerases) — EC 5.3.99.11 (×4), 5.1.3.41, 5.1.3.30, 5.1.3.31, 5.1.3.35, 5.3.1.36, 5.1.3.-. EC 1.1.1.43 — and indeed any EC class 1 oxidoreductase — is entirely ABSENT from the reviewed family (0/75). This is exactly what the metal-dependent (β/α)₈ TIM-barrel mechanism predicts: these enzymes perform acid/base-catalyzed isomerization, epimerization, and dehydration using a divalent metal, never NAD(P)-dependent hydride transfer. The gene symbol kguE ("E" for epimerase) and the submission name "Epimerase" are consistent with the family consensus — and inconsistent with the dehydrogenase annotation.
The 2-ketogluconate (kgu) branch of peripheral glucose catabolism in P. putida KT2440 funnels oxidized glucose derivatives into the Entner-Doudoroff pathway via 6-phosphogluconate. The genomic neighborhood and biochemical logic are:
Periplasm Cytoplasm (kgu operon, PtxS-regulated)
--------- --------------------------------------
glucose --(Gcd)--> gluconate
gluconate --(Gad)--> 2-ketogluconate
|
kguT (PP_3377, transporter)
v
2-ketogluconate ---- kguK (PP_3378, kinase, EC 2.7.1.13) ---->
2-keto-6-phosphogluconate
|
KguD-type reductase <== GENUINE EC 1.1.1.43
(Rossmann fold, NAD(P)-dependent)
v
6-phosphogluconate ---> Entner-Doudoroff pathway
kguE (PP_3379) = "Epimerase", TIM-barrel (COG1082/IPR050312)
NOT a dehydrogenase; true reaction = sugar
epimerization/isomerization (substrate TBD)
ptxS (PP_3380) = LacI-family repressor controlling the operon
The annotation error arises from a single point of failure: during EMBL submission, the EC number 1.1.1.43 (which properly belongs to the pathway's reductase, KguD) was attached to the "Epimerase" (kguE) record. GO_REF:0000003 then mechanically propagated this via ec2go into GO:0008114. Every downstream line of evidence — fold family, absence of a Rossmann/NAD-binding domain, the high-confidence TIM-barrel AlphaFold model, and the unanimous isomerase/epimerase/lyase character of 75 reviewed family members — shows the enzyme cannot catalyze NAD(P)-dependent 2-dehydrogenation. KguE's true molecular function is almost certainly a carbohydrate epimerase or isomerase (consistent with its name and fold), with the specific substrate still to be determined experimentally.
| Property | EC 1.1.1.43 (annotated) | KguE (observed signatures) |
|---|---|---|
| Fold | Rossmann (NAD(P)-binding) | (β/α)₈ TIM barrel |
| Superfamily / InterPro | 6-PGDH-like (PF03446/PF00393) | Xylose-isomerase-like (IPR036237) |
| Cofactor | NAD(P)⁺/NAD(P)H | Divalent metal (no NAD(P)) |
| Reaction class | Oxidoreductase (EC 1) | Isomerase/epimerase/lyase (EC 5/4) |
| Present in Q88HH8? | No domain signature | All signatures |
| AlphaFold | — | Single domain, pLDDT 94.88 |
| Citation | Evidence type | Supports/Refutes/Qualifies | Claim tested | Key finding | Context | Confidence & limitations |
|---|---|---|---|---|---|---|
| UniProt Q88HH8 record | Database/computational | Refutes | Origin of GO:0008114 | Submission name "Epimerase"; EC 1.1.1.43 attached; only MF GO is IEA/ec2go; signatures all TIM-barrel; no Rossmann domain | P. putida KT2440, PP_3379 | High for provenance; database-level |
| AlphaFold AF-Q88HH8-F1 | Structural/computational | Refutes | Does KguE have a cofactor-binding Rossmann lobe? | Single compact TIM barrel, 259 aa, mean pLDDT 94.88; no second domain | In silico model | High-confidence model; not an experimental structure |
| InterPro IPR050312 family survey (75 Swiss-Prot entries) | Structural/evolutionary | Refutes | Is oxidoreductase activity found in this family? | 0/75 are EC class 1; all are EC 4 (lyase) or EC 5 (isomerase/epimerase) | Cross-organism reviewed set | High; family-wide consensus |
| PMID: 20581202 Daddaoua 2010 | Primary pathway/mutant | Qualifies (competing) | Where is the 6-PG-forming activity? | Two cytoplasmic reactions (kinase + reductase) form 6-phosphogluconate | P. putida KT2440 | High; locates EC 1.1.1.43 in reductase, not epimerase |
| PMID: 18245293 del Castillo 2008 | Primary regulatory | Qualifies | Operon/regulatory context | PtxS controls gluconate→2-ketogluconate oxidation, transport, metabolism | P. putida KT2440 | High; establishes kgu operon context |
| PMID: 17483213 del Castillo 2007 | Primary genomic/flux | Qualifies (competing) | Nature of the 6-PG-forming step | 2-ketogluconate is "phosphorylated and reduced to 6-phosphogluconate" (reductase) | P. putida KT2440 | High; confirms reductase, not epimerase |
| PMID: 39770733 Sun 2024 | Primary (operon characterization) | Pending | kgu operon function & regulation | Characterizes 2-ketogluconate utilization operon (no abstract retrieved) | P. putida | Primary source to verify KguE's true role |
Supporting literature orientation (gluconate/2-ketogluconate physiology): PMID: 39261865, PMID: 31931111, PMID: 28921555, PMID: 23967821, PMID: 21513516 — these establish the physiological importance of the 2-ketogluconate loop but do not assign dehydrogenase activity to KguE.
Likely curation action (lead requiring curator verification):
Remove / do not retain GO:0008114 (phosphogluconate 2-dehydrogenase activity, MF) on kguE/Q88HH8. The annotation is an IEA ec2go artifact (GO_REF:0000003) propagated from a self-contradictory submission and is contradicted by fold family, absence of a Rossmann/NAD-binding domain, the high-confidence TIM-barrel AlphaFold model, and the complete absence of oxidoreductases among 75 reviewed family homologs.
Reassign a more defensible molecular-function term. The evidence supports a carbohydrate epimerase or isomerase activity (MF), consistent with the "Epimerase" submission name, the kguE symbol, and the IPR050312/COG1082 TIM-barrel family. A conservative lead is a racemase/epimerase or intramolecular isomerase acting on carbohydrates term; the exact child term should await identification of the substrate (see Knowledge Gaps). Do not default to "protein binding."
Biological process context (BP): retain/associate kguE with 2-ketogluconate / carbohydrate catabolic process given its operon membership, but flag that its specific enzymatic step within that process is unconfirmed.
The genuine EC 1.1.1.43 / GO:0008114 activity should be associated with the KguD-type Rossmann-fold reductase, not kguE — a cross-reference worth noting for the curator.
The immediate molecular function being tested is a direct catalytic activity: whether the KguE polypeptide catalyzes the NAD(P)-dependent oxidation of 6-phospho-D-gluconate (EC 1.1.1.43). This is a gene-product–intrinsic claim, not a downstream phenotype. The refutation is therefore at the most direct level — the protein's fold and domain composition make the stated chemistry mechanistically impossible — rather than relying on loss-of-function phenotypes or pathway inferences. The pathway/operon evidence (Findings 2–3) is supporting context that correctly relocates the EC 1.1.1.43 reaction to a separate reductase, but the core refutation stands on the structural/evolutionary incompatibility of KguE itself.
No disease, developmental, or pleiotropic manifestations are relevant here; this is a bacterial metabolic enzyme annotation.
No direct enzymatic assay of purified KguE. The refutation is structural/evolutionary and contextual, not a negative activity measurement. Checked: literature search returned no biochemical characterization of purified Q88HH8. Why it matters: a positive identification of KguE's true substrate would let a curator assign a precise MF term. Resolution: purify KguE and test candidate epimerase/isomerase substrates.
KguE's exact substrate and reaction are unknown. Checked: family survey narrows it to epimerase/isomerase/dehydratase chemistry on a sugar or sugar-phosphate. Why it matters: the replacement GO term's specificity depends on this. Resolution: metabolomics of a ΔkguE mutant; in vitro substrate screening.
PMID:39770733 (Sun 2024) abstract not retrieved. Checked: no abstract available via the search. Why it matters: this is the most recent, directly relevant primary characterization of the kgu operon and may explicitly state KguE's role. Resolution: obtain and read the full text; it is the single highest-value document for this curation.
AlphaFold is a model, not an experimental structure. Checked: high pLDDT (94.88) gives strong confidence, but no crystal/cryo-EM structure confirms the active-site metal or substrate. Resolution: experimental structure or at least docking of candidate substrates.
All leads below require curator verification.
The hypothesis that kguE directly has phosphogluconate 2-dehydrogenase activity (GO:0008114) is refuted / over-annotated. The annotation is an electronic ec2go artifact from a self-contradictory submission. KguE is a high-confidence metal-dependent TIM-barrel enzyme of the xylose-isomerase-like family (IPR050312/COG1082) with no NAD(P)-binding domain; its entire characterized family (0/75 reviewed members are oxidoreductases) performs isomerase/epimerase/lyase chemistry, and the genuine EC 1.1.1.43 reductase step in the 2-ketogluconate pathway belongs to a separate Rossmann-fold KguD enzyme. The recommended curation action is to remove GO:0008114 and assign a carbohydrate epimerase/isomerase molecular-function term, pending experimental substrate identification.