AIGR Deep Research — S. pombe alo1 (Q9HDX8): L-gulonolactone oxidase prediction
Hypothesis slug: prediction-gulonolactone-oxidase Focus type: computational_prediction Term under test: GO:0050105 L-gulonolactone oxidase activity (+ GO:0019853 L-ascorbic acid biosynthetic process) Model source of prediction: BioReason-Pro SFT (ref doi:10.64898/2026.03.19.712954)
Executive Judgment
Verdict: REFUTED (over-annotated — paralog/substrate misassignment).
The BioReason-Pro predictions of L-gulonolactone oxidase activity (GO:0050105) and L-ascorbic acid biosynthetic process (GO:0019853) for S. pombe alo1 (Q9HDX8) are not supported and should not enter the review as-is. Multiple independent lines of evidence identify alo1 as D-arabinono-1,4-lactone oxidase (EC 1.1.3.37), the terminal enzyme of the fungal D-erythroascorbate pathway — a distinct-substrate, distinct-product paralog within the same ALO/GULO/GLDH aldonolactone-oxidoreductase (FAD, vanillyl-alcohol-oxidase) superfamily:
- UniProtKB Q9HDX8 curated recommended name = D-arabinono-1,4-lactone oxidase, EC 1.1.3.37; curated reaction produces dehydro-D-arabinono-1,4-lactone + H₂O₂, and the assigned pathway is D-erythroascorbate biosynthesis (step 2/2) — not L-ascorbate.
- PomBase/GO_Central assigns MF GO:0003885 (D-arabinono-1,4-lactone oxidase activity, IBA); GO:0050105 is absent.
- The mislabel is explainable: the enzyme sits in PANTHER family PTHR43762, whose family-level name is "L-gulonolactone oxidase," while alo1 maps to subfamily SF1 = D-arabinono-1,4-lactone oxidase. A model keying on the family name (frequency/name bias) will over-call GULO.
- Biochemistry of orthologs confirms the fungal substrate/product: S. cerevisiae ALO1 was purified as D-arabinono-1,4-lactone oxidase making D-erythroascorbate and shares only 32 % identity with rat GULO (10094636); C. albicans ALO1 null mutants lose the activity and D-erythroascorbate (11349062 18282465).
Most important caveat: Members of this family have measurable in vitro promiscuity toward related aldonolactones (the Q9HDX8 alt-name "L-galactono-γ-lactone oxidase" reflects this), so a weak in vitro turnover of L-gulono-1,4-lactone cannot be excluded. That would still not justify a GULO/L-ascorbate annotation, because the physiological substrate and product are D-arabinono-lactone → D-erythroascorbate.
Evidence Matrix
| Citation | Evidence type | Direction | Claim tested | Key finding | Context | Confidence |
|---|---|---|---|---|---|---|
| UniProtKB Q9HDX8 (ALO_SCHPO) | database/curated | Refutes seed | Is alo1 a GULO making L-ascorbate? | Rec. name D-arabinono-1,4-lactone oxidase EC 1.1.3.37; rxn → dehydro-D-arabinono-1,4-lactone + H₂O₂; pathway = D-erythroascorbate biosynthesis step 2/2 | S. pombe alo1 | High (ECO:0000250) |
| PomBase GO:0003885 (IBA) | database/phylogenetic | Refutes / true MF | Correct MF term | D-arabinono-1,4-lactone oxidase activity; no GO:0050105 | S. pombe alo1 | High |
| PANTHER PTHR43762:SF1 | computational/orthology | Qualifies | Source of mislabel | Subfamily SF1 = D-arabinono-lactone oxidase; parent family named "L-gulonolactone oxidase" → name carryover | protein family | Med-High |
| 10094636 | direct assay + sequence | Refutes | Fungal enzyme identity / GULO homology | S. cerevisiae ALO1 purified as D-arabinono-1,4-lactone oxidase (final step of D-erythroascorbate synth); 32 % id to rat GULO | S. cerevisiae | High |
| 11349062 | mutant phenotype + cloning | Refutes | Gene product & pathway | C. albicans ALO1 = D-arabinono-1,4-lactone oxidase; alo1 null loses activity + D-erythroascorbate; antioxidant/virulence | C. albicans | High |
| 18282465 | mutant phenotype | Qualifies | Pathway product | alo1/alo1 mutant lacks ALO; D-erythroascorbate (EASC) is the product regulating alternative-oxidase respiration | C. albicans | Med |
| This run (computed) | structural/evolutionary | Qualifies | Does global %id discriminate ALO vs GULO? | Gotoh/BLOSUM62 global identity: alo1 vs ScALO1 = 30.5%, alo1 vs rat GULO = 31.0%, alo1 vs plant GLDH = 19.9%. Twilight-zone, ~tie with GULO. Four-point test weakly groups (alo1,ScALO1) | (GULO,GLDH), margin 0.8 | Q9HDX8/P54783/P10867/Q9SU56 |
| QuickGO (this run) | database (verification) | Refutes seed | Are the predicted terms in the curated set? | Q9HDX8 curated set: MF GO:0003885 (IGC), BP GO:0070485 dehydro-D-arabinono-1,4-lactone biosynthetic process (IGC), CC mitochondrion/outer membrane. GO:0050105 and GO:0019853 both ABSENT (verified present=False) | S. pombe alo1, PomBase | High |
(Provenance: provenance/evidence_matrix.csv, provenance/go_decision_table.csv, provenance/sequence_identity_matrix.csv; UniProt fetch + alignment executed via MCP execute_code.)
Computed identity note (this run): A Gotoh global alignment (BLOSUM62) shows alo1 is essentially equidistant from the fungal ALO (S. cerevisiae ALO1, 30.5 %) and the mammalian GULO (rat, 31.0 %) — both in the ~30 % alignment "twilight zone". Global % identity therefore does not discriminate the two functions and marginally favors GULO, which is exactly why a homology/embedding-based model can over-predict L-gulonolactone oxidase. Discrimination instead rests on the PANTHER subfamily assignment (PTHR43762:SF1 = D-arabinono-lactone oxidase), the curated EC 1.1.3.37 / D-erythroascorbate pathway, and fungal-clade biology — all of which point to ALO, not GULO. This nuance strengthens, rather than weakens, the refuted verdict.
GO Curation Implications (leads — require curator verification)
Verified against the live QuickGO annotation set for Q9HDX8 (13 annotations). Both model-predicted terms are absent from the curated set, and the curated terms are already correct and more specific:
| GO term | Aspect | Curated status | Recommended action | Rationale |
|---|---|---|---|---|
| GO:0050105 L-gulonolactone oxidase activity | MF | Absent (predicted only) | REMOVE / do not add | Wrong substrate; curated enzyme is EC 1.1.3.37 |
| GO:0019853 L-ascorbic acid biosynthetic process | BP | Absent (predicted only) | REMOVE / replace with GO:0070485 | Fungi make D-erythroascorbate, not L-ascorbate |
| GO:0003885 D-arabinono-1,4-lactone oxidase activity | MF | Present (IGC/IBA, PomBase) | RETAIN (core) | Correct curated MF, EC 1.1.3.37 |
| GO:0070485 dehydro-D-arabinono-1,4-lactone biosynthetic process | BP | Present (IGC, PomBase) | RETAIN (core) | Direct product of the alo1 reaction; the correct specific BP that supersedes GO:0019853 |
| GO:0005741 mitochondrial outer membrane / GO:0005739 mitochondrion | CC | Present (ISO / HDA-IBA) | RETAIN | Consistent with ALO-family localization |
Core function is fully captured by the curated MF (GO:0003885) plus the specific product BP (GO:0070485); "protein binding" is neither needed nor recommended. The model prediction is strictly less accurate than existing curation — a textbook paralog/substrate over-annotation.
Mechanistic Scope
Direct molecular function tested: a mitochondrial-membrane, covalent-FAD flavoprotein oxidase that oxidizes an aldono-1,4-lactone using O₂, releasing H₂O₂. The specific, physiological reaction for alo1 is D-arabinono-1,4-lactone → dehydro-D-arabinono-1,4-lactone, the last step yielding D-erythroascorbate (a 5-carbon L-ascorbate analog). Downstream/pleiotropic effects (oxidative-stress resistance, virulence, alternative-oxidase induction seen in Candida) are pathway consequences of the D-erythroascorbate product, not the enzyme's direct activity, and must not be conflated with the MF term. The seed's GULO/L-ascorbate claim describes a different substrate (L-gulono-1,4-lactone) and different product (L-ascorbate) belonging to the mammalian pathway.
Conflicts and Alternatives
- Paralog confusion (primary conflict): GULO (mammals), GLDH/L-galactono-lactone dehydrogenase (plants), and ALO (fungi) are homologous but substrate-distinct. ~32 % identity between fungal ALO1 and rat GULO (10094636) is exactly the zone where automated methods over-transfer the better-known family label.
- Family-name bias: PANTHER PTHR43762 carries the name "L-gulonolactone oxidase" at family level; the correct assignment is subfamily SF1 (D-arabinono-lactone oxidase).
- In-vitro promiscuity: The UniProt alt-name "L-galactono-γ-lactone oxidase" and known broad aldonolactone activity mean a low-level in vitro L-gulonolactone turnover is possible; this is an artifact-level caveat, not physiological evidence for L-ascorbate biosynthesis in S. pombe.
Knowledge Gaps
- No S. pombe-specific enzymatic assay was located. Checked PubMed (no S. pombe ALO papers returned); the S. pombe assignment rests on curated by-similarity + IBA. Matters because direct S. pombe kinetics would definitively fix substrate preference. Resolve with: in-vitro substrate-panel kinetics on recombinant Q9HDX8.
- Exact GO BP ID for D-erythroascorbate biosynthesis not confirmed here — curator should map the UniProt pathway string to the current ontology term.
- Whether S. pombe actually accumulates D-erythroascorbate (vs. only enzyme presence) — resolve by metabolite LC-MS of wild-type vs. alo1Δ.
Discriminating Tests
- Substrate-panel oxidase assay on recombinant Q9HDX8: compare V/Km for D-arabinono-1,4-lactone, L-gulono-1,4-lactone, and L-galactono-1,4-lactone (O₂ consumption / H₂O₂ release). Physiological substrate should dominate.
- Metabolomics of alo1Δ S. pombe: loss of D-erythroascorbate (not L-ascorbate) confirms pathway identity.
- Phylogenetic placement / reciprocal-best-hit of Q9HDX8 vs. GULO (rat), GLDH (plant), ALO1 (Sc/Ca): expect clustering with fungal ALO subfamily, away from GULO.
Curation Leads (require curator verification)
- Action change: reject BioReason-Pro GO:0050105 and GO:0019853 for alo1; keep GO:0003885; consider adding the D-erythroascorbate biosynthetic-process BP term.
- Candidate references / snippets to verify:
- 11349062 — "ALO1, the gene encoding D-arabinono-1,4-lactone oxidase, which catalyzes the final step of D-erythroascorbic acid biosynthesis."
- 10094636 — "shared 32% … identity with … L-gulono-1,4-lactone oxidase from rat," establishing paralogy, not identity.
- UniProtKB Q9HDX8 — EC 1.1.3.37; pathway "D-erythroascorbate biosynthesis; step 2/2."
- Suggested question for curator: Is a fungal-ascorbate-analog BP term already applied? If not, add and remove the L-ascorbate BP.
- Suggested experiment: recombinant-enzyme substrate panel + alo1Δ metabolomics (above).
Limitations
Evidence is predominantly curated-database and ortholog-biochemistry; no S. pombe-specific assay was found in the literature search. Network access limited the run to UniProt REST + PubMed; a direct sequence/phylogeny computation was not executed. The conclusion (paralog misassignment) is nonetheless robust because it is concordant across UniProt curation, GO_Central phylogenetics, PANTHER subfamily mapping, and ortholog biochemistry.