Commissioned Module/Pathway/Taxon Review Brief

Warnings (2)

Question

Commissioned Module/Pathway/Taxon Review Brief

Review Topic

Bacterial purine nucleoside and nucleobase salvage in Pseudomonas putida KT2440

Target Taxon

Target Pathway Or Bucket

Resolved local bucket kegg:ppu00230 with 36 primary genes; module area: nucleotide_metabolism.

Candidate Genes From Local Metadata

Candidate gene count: 65

Generic Module Context

Working Scope

A reusable bacterial salvage module in which purine nucleosides are phosphorolyzed to free bases and ribose 1-phosphate, after which adenine, hypoxanthine or guanine, and xanthine are returned to the nucleotide pool as AMP, IMP or GMP, and XMP. The three phosphoribosyltransferase branches use PRPP and release diphosphate.

Provisional Biological Outline

Known Relationships Among Steps

Assignment

Write a species-aware review of this module/pathway in the target organism. The
goal is not a generic pathway essay; the goal is to support manual module
satisfiability and gene annotation curation.

Treat the candidate gene list as a starting point, not ground truth. Use the
literature and authoritative resources to decide whether each expected pathway
step is present, absent, ambiguous, replaced by a lineage-specific alternative,
or represented by a likely over-propagated annotation.

Prioritize direct evidence from the target species/strain. When using evidence
from related organisms, state the organism and explain whether transfer to the
target taxon is strong, weak, or uncertain. Distinguish Pseudomonas putida
KT2440 evidence from broader Pseudomonas or generic bacterial evidence.

Questions To Address

  1. Pathway boundaries
  2. What exact biochemical or cellular process is included?
  3. Which neighboring pathways or broad overview maps should be kept separate?

  4. Are there alternate names or database-specific definitions for this pathway?

  5. Satisfiability in the target taxon

  6. Which expected steps are encoded by candidate genes?
  7. Which steps are missing from the metadata but likely present under a
    different gene name, paralog, or enzyme family?

  8. Which steps are probably not expected in this organism?

  9. Candidate gene assessment

  10. For each high-confidence gene, summarize the likely role, evidence type,
    and any curation-relevant caveats.
  11. Identify paralog ambiguity, broad EC/GO mappings, and likely annotation
    over-propagation.

  12. Note genes that should be promoted to full fetch-gene review.

  13. Module implications

  14. What module steps should be marked covered, candidate_uncertain, gap,
    not_expected_in_target_taxon, or module_needs_revision?
  15. Are existing generic module boundaries wrong for this organism?

  16. Are new module documents or GO term requests likely needed?

  17. Evidence and open questions

  18. Which conclusions are supported by direct experiments?
  19. Which are inferred from homology, pathway databases, or neighboring genes?
  20. What experiments or expert questions would resolve the important gaps?

Output Format

Use concise review sections:

  1. Executive summary
  2. Target-organism pathway definition
  3. Expected step model
  4. Candidate genes and evidence
  5. Gaps, ambiguities, and likely over-annotations
  6. Module and GO-curation recommendations
  7. Genes to promote to full review
  8. Key references

Include citations for major claims, preferably PMIDs or DOIs. Be explicit about
uncertainty, species transfer, and whether evidence is direct for the target
organism.

Output

Module Review: Bacterial Purine Nucleoside & Nucleobase Salvage in Pseudomonas putida KT2440

Taxon: Pseudomonas putida KT2440 (PSEPK; NCBI:txid160488; proteome UP000000556)
Target bucket: KEGG ppu00230 "Purine metabolism" (broad overview map)
Module area: nucleotide_metabolism
Purpose: species-aware curation review supporting manual module satisfiability and gene-annotation triage. Curation verdicts use the controlled vocabulary: covered, candidate_uncertain, gap, not_expected_in_target_taxon, module_needs_revision.

1. Executive summary

The commissioned module — phosphorolysis of purine nucleosides followed by re-fixation of the free bases to nucleotides — is a narrow 4-step process, whereas the 65-gene candidate list is a projection of the entire KEGG overview map ppu00230 plus ~15 neighboring buckets. Only 4 of the 65 candidates encode true salvage steps: ppnP (PP_4248), apt (PP_4266), hpt (PP_0747) and xpt (PP_5265). The other 61 belong to de novo IMP biosynthesis, nucleotide interconversion/kinases, purine catabolism/ureide nitrogen assimilation, housekeeping nucleotidases/Nudix hydrolases, and nucleotide-based signalling, and should be scored as neighboring-pathway context, not salvage evidence.

The module is nominally satisfiable. The three phosphoribosyltransferase (PRT) branches are all present:
- Adenine → AMP (apt/PP_4266) — covered, high-quality (SwissProt-reviewed, EC 2.4.2.7).
- Xanthine → XMP (xpt/PP_5265) — covered, reviewed (EC 2.4.2.22).
- Hypoxanthine/Guanine → IMP/GMP (hpt/PP_0747) — covered but candidate_uncertain (unreviewed, "Predicted", no EC assigned).

The one substantive weakness is step 1 (nucleoside phosphorolysis). A proteome-wide UniProt search shows that the only enzyme in KT2440 bearing EC 2.4.2.1 is the accessory Cupin-fold PpnP (PP_4248); there is no DeoD (PNP-I) or XapA (PNP-II) ortholog — the canonical high-flux bacterial purine nucleoside phosphorylases. The best uncharacterised candidate for a classical phosphorylase is PP_3254 (PF01048 / IPR000845 "nucleoside phosphorylase domain"), which is absent from the candidate list. yfiH/PP_0624, listed as "purine nucleoside phosphorylase", is an unreviewed, EC-less, likely over-propagated annotation (YfiH/DUF152 polyphenol-oxidase-like family) and should not be counted.

2. Target-organism pathway definition

Included (this module): the salvage arm only —
1. Phosphorolytic cleavage of purine ribonucleosides (adenosine, inosine, guanosine, xanthosine) → free base + α-D-ribose-1-phosphate;
2. Adenine + PRPP → AMP + PPi (APRT);
3. Hypoxanthine/Guanine + PRPP → IMP/GMP + PPi (HGPRT);
4. Xanthine + PRPP → XMP + PPi (XPRT).

All three PRT branches consume PRPP (supplied by prs/PP_0722 and the second PRPP synthase PP_2744) and release diphosphate; this is a boundary input, not a salvage step.

Explicitly kept separate (neighboring maps / must not be scored as salvage):
- De novo IMP biosynthesis (purF, purD, purN, purT, purL, purM, purK, purE, purC, purB, purH) and the IMP→AMP/GMP branch (purA, purB, guaA, guaB).
- Nucleotide interconversion / kinases (adk, gmk, ndk, nrdAB, prs, PP_2744).
- Purine catabolism → ureide/nitrogen assimilation (xdhAB xanthine dehydrogenase; pucL, pucM, puuE, allA, allE, PP_4310; ureABC). This is a distinct degradative pathway (purine → urate → allantoin → glyoxylate + NH₃/urea) and in P. putida underlies purine/allantoin use as N-source. It should be its own module, not folded into salvage.
- Molybdo-hydroxylase mis-mapping: paoABC (PP_3308–3310), annotated "promiscuous aromatic aldehyde dehydrogenase" (EC 1.2.99.7), is a PaoABC-type aldehyde oxidoreductase erroneously projected onto the purine map — not a purine enzyme.
- Housekeeping nucleotidases / Nudix / NTP-sanitising (yrfG, surE, ushA, PP_2531, nudE, nudF, mazG, apaH, PP_5100, dgt) and nucleotide signalling (relA, spoT, cyaA, pde, ppx).
- Cross-listed non-purine enzymes: cysD/cysNC (ATP sulfurylase), arcC (carbamate kinase), pgm/cpsG/algC (phospho-mutases).

Alternate names / DB definitions: KEGG ppu00230 = "Purine metabolism" (overview, not a module). MetaCyc/BioCyc split the equivalent biology into "purine ribonucleosides degradation" / "purine nucleobases salvage" / "adenine and adenosine salvage" / "guanine and guanosine salvage". The relevant GO umbrella is purine-containing compound salvage (GO:0043101) with base-specific children (see §6).

3. Expected step model

# Step Reaction (EC) Expected enzyme family KT2440 candidate
1 Nucleoside phosphorolysis nucleoside + Pi → base + R1P (2.4.2.1/2.4.2.2) PNP-I (DeoD)/PNP-II (XapA)/PpnP-Cupin ppnP PP_4248 (+ candidate PP_3254)
2 Adenine salvage adenine + PRPP → AMP (2.4.2.7) APRT (PF00156) apt PP_4266
3 Hypoxanthine/guanine salvage Hx/Gua + PRPP → IMP/GMP (2.4.2.8) HGPRT (PF00156) hpt PP_0747
4 Xanthine salvage xanthine + PRPP → XMP (2.4.2.22) XPRT (PF00156-clan) xpt PP_5265

Accessory funnels that determine which branch carries flux: adenine deaminase (PP_0591, EC 3.5.4.2, adenine→hypoxanthine) and guanine deaminase (guaD/PP_4281, EC 3.5.4.3, guanine→xanthine). Their presence means salvage does not require a distinct enzyme per base — deaminated bases converge on the HGPRT/XPRT branches.

4. Candidate genes and evidence

Evidence tiers derived from UniProt UP000000556 (reviewed status, EC assignment, protein-existence).

Gene Locus / Acc Step Evidence tier Verdict Curation notes
apt PP_4266 / Q88F33 2 (Ade→AMP) Reviewed, EC 2.4.2.7, PF00156 covered Single-copy APRT; strong. Homology-inferred but unambiguous family.
xpt PP_5265 / Q88CB6 4 (Xan→XMP) Reviewed, EC 2.4.2.22 covered XPRT. (No Pfam mapped in UniProt — minor annotation quirk, not a concern.)
ppnP PP_4248 / Q88F51 1 (phosphorolysis) Reviewed, EC 2.4.2.1/2.4.2.2, PF06865 (Cupin) covered (accessory) Genuine PpnP class, broad pyrimidine+purine specificity (PMID 35094440). In E. coli PpnP is accessory to DeoD, so may be low-capacity here.
hpt PP_0747 / Q88PV1 3 (Hx/Gua→IMP/GMP) Unreviewed, "Predicted", NO EC, PF00156 covered / candidate_uncertain Name-only HGPRT; broad PRT clan. Promote to full review.
PP_3254 Q88HU9 1 (phosphorolysis) Unreviewed, "Predicted", PF01048 / IPR000845 new candidate for step 1 Not in candidate list. Nucleoside-phosphorylase-domain (DeoD/MTAP/UP superfamily); best hit for a classical high-flux PNP. Promote.
yfiH PP_0624 / Q88Q72 (claimed 1) Unreviewed, no EC, PF02578 (DUF152/YfiH) likely over-annotation "Purine nucleoside phosphorylase" name unsupported; do not score.
PP_3230 Q88HX3 (possible 3/4) Unreviewed, PF00156 "PRT-domain protein" candidate_uncertain Possible additional/second PRT (e.g. a gpt-type). Check substrate.
PP_0591 (Ade deaminase) Q88QA3 funnel Reviewed, EC 3.5.4.2 context Routes adenine→hypoxanthine; species-relevant flux nuance.
guaD PP_4281 / Q88F18 funnel Unreviewed, EC 3.5.4.3 context Guanine→xanthine.
amn, PP_3662 PP_4779, PP_3662 (not salvage) EC 3.2.2.4 out-of-scope AMP nucleosidase = AMP→adenine+R5P (degradation, not salvage). amn shares PF01048 but is functionally distinct.

Paralog ambiguity: The type-I PRT clan PF00156 in KT2440 contains apt, hpt/PP_0747, purF, pyrE (OPRT), upp/pyrR, comF, plus uncharacterised PP_3230 and PP_0361. Pfam membership alone cannot assign a substrate; substrate specificity for hpt/PP_0747 and PP_3230 rests on best-hit orthology, not experiment.

5. Gaps, ambiguities, and likely over-annotations

  1. Step-1 phosphorylase identity — the principal gap. No DeoD/XapA ortholog exists in the proteome (only ppnP carries EC 2.4.2.1). Either (a) PpnP is the physiological purine-nucleoside phosphorylase in KT2440, or (b) PP_3254 (uncharacterised PF01048/IPR000845) is the real workhorse. This is the single most important item to resolve. Verdict: candidate_uncertain for step 1.
  2. yfiH/PP_0624 over-propagation. YfiH/DUF152 is a polyphenol-oxidase/laccase-like family; the "purine nucleoside phosphorylase" label is EC-less and unreviewed. Recommend down-weighting / re-annotation review; do not treat as salvage evidence.
  3. hpt/PP_0747 under-characterised. Unreviewed + "Predicted" + no EC. Functionally likely correct (conserved HGPRT synteny across Pseudomonas), but evidence is homology-only.
  4. ppnP broad EC. EC 2.4.2.1 and 2.4.2.2 with 8 substrate synonyms (adenosine…xanthosine, plus pyrimidines). This breadth is a family-level over-listing; in vivo purine contribution is uncertain.
  5. Catabolism mislabelled as core purine map. paoABC (aldehyde oxidoreductase) and the ureide genes (pucL/pucM/puuE/allA/allE, ureABC, xdhAB) are catabolic/N-assimilatory, not salvage. Their inclusion inflates apparent module coverage — a module_needs_revision boundary issue.
  6. arcC, cysD/cysNC, pgm/cpsG/algC, cyaA, pde, relA/spoT, ppx, Nudix set — clearly out of scope; over-inclusive projection from the overview map.

6. Module and GO-curation recommendations

Per-step module verdicts:
- Step 1 (nucleoside phosphorolysis): candidate_uncertain — PpnP present (EC-supported) but likely accessory; classical PNP unrepresented; investigate PP_3254.
- Step 2 (adenine→AMP, apt): covered.
- Step 3 (Hx/Gua→IMP/GMP, hpt): covered with a candidate_uncertain evidence flag (predicted, no EC).
- Step 4 (xanthine→XMP, xpt): covered.

Boundary / module-document recommendations:
- module_needs_revision at the source: the KEGG-ppu00230→module projection is far too broad. The salvage module document should enumerate only the 4 steps and explicitly exclude de novo synthesis, interconversion, catabolism, signalling and housekeeping hydrolases.
- Create a separate "purine catabolism / ureide N-assimilation" module for xdhAB, pucL/pucM/puuE/allA/allE, PP_4310, ureABC (and possibly paoABC after re-annotation). This is where the bulk of the mis-attributed genes belong and is biologically important in P. putida (purines/allantoin as N sources).
- GO annotation targets: apt → GO:0006168 (adenine salvage); hpt → GO:0032264 (IMP salvage) + GO:0032263 (GMP salvage); xpt → GO:0032265 (XMP salvage, if present) under GO:0043101 (purine-containing compound salvage); ppnP/PP_3254 → GO:0006152/GO:0006157-type purine ribonucleoside catabolic/phosphorylase activity (GO:0004731 purine-nucleoside phosphorylase activity). No new GO term requests appear necessary; existing salvage terms suffice.

7. Genes to promote to full fetch-gene review

  1. PP_3254 (Q88HU9) — highest priority. Test whether this IPR000845 nucleoside-phosphorylase-domain protein is the classical (DeoD/MTAP-type) purine nucleoside phosphorylase; would close the step-1 gap.
  2. hpt / PP_0747 (Q88PV1) — confirm HGPRT substrate range and assign EC 2.4.2.8; currently predicted/no-EC.
  3. yfiH / PP_0624 (Q88Q72) — adjudicate the "purine nucleoside phosphorylase" name vs. the YfiH/DUF152 polyphenol-oxidase assignment; likely re-annotate.
  4. PP_3230 (Q88HX3) — resolve whether this extra PF00156 PRT is a second hypoxanthine/xanthine-guanine PRT (gpt) or unrelated.
  5. ppnP / PP_4248 (Q88F51) — confirm physiological purine (vs pyrimidine) role and whether it is the sole phosphorylase.

8. Evidence status and open questions

Key references

Artifacts