Commissioned Module/Pathway/Taxon Review Brief

Question

Commissioned Module/Pathway/Taxon Review Brief

Review Topic

bacterial_branched_chain_amino_acid_catabolism in Pseudomonas putida KT2440

Target Taxon

Target Pathway Or Bucket

Resolved local bucket kegg:ppu00280 with 8 primary genes; module area: amino_acid_metabolism.

Candidate Genes From Local Metadata

Candidate gene count: 35

Generic Module Context

Working Scope

No module YAML was resolved; use the pathway and taxon context.

Provisional Biological Outline

No module YAML outline available.

Known Relationships Among Steps

No module YAML connections available.

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

Commissioned Review: Branched-Chain Amino Acid Catabolism (KEGG ppu00280) in Pseudomonas putida KT2440

Target taxon: Pseudomonas putida KT2440 (PSEPK; NCBI taxon 160488; proteome UP000000556)
Target bucket: kegg:ppu00280 — "Valine, leucine and isoleucine degradation"
Module area: amino acid metabolism
Candidate genes reviewed: 35 local metadata entries
Purpose: support manual module satisfiability and gene-annotation curation

1. Executive summary

Branched-chain amino acid (BCAA) catabolism is present and satisfiable in P. putida KT2440. The pathway is organized as a shared "upper" hub feeding three substrate-specific "lower" arms, and every dedicated biochemical step maps to a credible KT2440 gene. There is no true gap in the core route from Val/Leu/Ile to the terminal C3/C2 products (propionyl-CoA and acetyl-CoA).

The upper hub is the best-characterized part of the pathway in this exact organism: branched-chain aminotransferase (ilvE, PP_3511) generates the three branched 2-oxo acids, which are then oxidatively decarboxylated by the bkd-encoded branched-chain 2-oxo acid dehydrogenase complex (BCKDH: bkdAA/bkdAB/bkdB/lpdV, PP_4401–PP_4404). The bkd operon of P. putida has been cloned, sequenced, and shown to be strictly required for growth on BCAAs, controlled by the Lrp-family activator BkdR and by Crc-mediated catabolite repression — this is direct, target-strain evidence. Downstream, the leucine arm is complete and dedicated (ivd/liuA PP_4064 → mccA/mccB PP_4067/PP_4065 → liuC PP_4066 → HMG-CoA lyase PP_3394 or mvaB PP_3540; equivalent to KEGG module M00036), the valine arm is carried by an mmsAB-like operon (mmsB PP_4666 + mmsA-II PP_4667), and the isoleucine arm is carried largely by promiscuous fatty-acid β-oxidation enzymes.

The dominant curation issues are therefore not missing steps but over-inclusion and paralog ambiguity. Roughly 15 of the 35 ppu00280 members are generic β-oxidation enzymes (thiolases, acyl-CoA dehydrogenases, enoyl-CoA hydratases, fadA/fadB) co-mapped into the pathway purely through shared EC numbers/KOs, not because they are dedicated BCAA enzymes. Several genuine steps have 2–5 paralogs (E3 dihydrolipoamide dehydrogenase, HMG-CoA lyase, methylmalonate-semialdehyde dehydrogenase) that require explicit "which copy" decisions. Finally, the module boundary should be drawn at propionyl-CoA/acetyl-CoA: propionyl-CoA is disposed via the 2-methylcitrate cycle (separate KEGG maps), not the B12 methylmalonyl-CoA-mutase module, and the atu/acyclic-terpene entry present in some pseudomonads is absent in KT2440.

2. Target-organism pathway definition

2.1 Exact process included

ppu00280 covers the oxidative catabolism of the three branched-chain amino acids — L-valine, L-leucine, and L-isoleucine — from the free amino acids to the point where their carbon skeletons enter central metabolism as acetyl-CoA, acetoacetate, and propionyl-CoA. Mechanistically this comprises:

  1. Transamination of Val/Leu/Ile to their 2-oxo (branched-chain keto) acids (BCKAs);
  2. Oxidative decarboxylation of the three BCKAs by a shared multienzyme BCKDH complex, producing the branched acyl-CoA thioesters (isobutyryl-CoA, isovaleryl-CoA, 2-methylbutyryl-CoA);
  3. Substrate-specific "lower" arms that process each branched acyl-CoA by a β-oxidation-like sequence (dehydrogenation, carboxylation/hydration, cleavage) to the terminal products.

2.2 Neighboring pathways to keep separate

2.3 Alternate names / database definitions

3. Expected step model

The pathway in KT2440 can be drawn as a Y-shaped hub-and-arms topology:

   L-Val        L-Leu        L-Ile
     |            |            |
     |  ilvE (PP_3511, EC 2.6.1.42)  [+ ldh/PP_4617 for Leu, alt route]
     v            v            v
 2-oxo-isovalerate  2-oxo-isocaproate  2-oxo-3-methylvalerate
     \            |            /
      \           |           /
       ====  BCKDH complex  ====
   bkdAA/bkdAB/bkdB/lpdV (PP_4401-4404; EC 1.2.4.4, 2.3.1.-, 1.8.1.4)
       /           |           \
      v            v            v
 isobutyryl-CoA  isovaleryl-CoA  2-methylbutyryl-CoA
   (VALINE arm)   (LEUCINE arm)   (ISOLEUCINE arm)
      |            |              |
      |            | ivd/liuA (PP_4064, EC 1.3.8.4)
      |            v              |
      |     3-methylcrotonyl-CoA  |
      |            | mccA/mccB (PP_4067/PP_4065, EC 6.4.1.4)
      |            v              |
      |     3-methylglutaconyl-CoA|
      |            | liuC (PP_4066, EC 4.2.1.18)
      |            v              |
      |        HMG-CoA            |
      |            | HMG-CoA lyase (PP_3394 or mvaB/PP_3540, EC 4.1.3.4)
      |            v              |
      |  acetoacetate + acetyl-CoA|
      |                           |
  [methacrylyl-CoA →         (β-oxidation-like:
   3-OH-isobutyryl-CoA →      acyl-CoA DH, enoyl-CoA
   3-OH-isobutyrate →         hydratase, 3-OH-acyl-CoA
   mmsB/PP_4666 (EC 1.1.1.31)  DH, thiolase)
   → methylmalonate            |
   semialdehyde →              v
   mmsA-II/PP_4667             acetyl-CoA + propionyl-CoA
   (EC 1.2.1.27)]              |
      |                        |
      v                        v
  propionyl-CoA  <-------------+
      |
      v
  2-METHYLCITRATE CYCLE (separate maps) -> central metabolism

Terminal products / module boundary: acetyl-CoA and acetoacetate (leucine), propionyl-CoA (valine and isoleucine). The module should terminate at propionyl-CoA/acetyl-CoA.

4. Candidate genes and evidence

4.1 High-confidence, dedicated BCAA genes

Gene Locus Step EC / KO Evidence type Curation note
ilvE PP_3511 Transamination Val/Leu/Ile → BCKAs 2.6.1.42 / K00826 Homology + strong enzymology in related bacteria Sits at biosynthesis/degradation interface (metadata bucket ppu00290); a single BCAT typically serves all three substrates
bkdAA PP_4401 BCKDH E1α 1.2.4.4 / K00166 Direct, target strain Core hub; part of cloned/sequenced bkd operon
bkdAB PP_4402 BCKDH E1β 1.2.4.4 / K00167 Direct, target strain Core hub
bkdB PP_4403 BCKDH E2 (dihydrolipoamide acyltransferase) 2.3.1.- / K09699 Direct, target strain Core hub
lpdV PP_4404 BCKDH E3 (dihydrolipoamide DH, dedicated) 1.8.1.4 / K00382 Direct, target strain Dedicated E3 co-encoded in bkd operon (see paralog note 5.2)
ivd/liuA PP_4064 Isovaleryl-CoA DH (leucine arm) 1.3.8.4 / K00253 Homology; strong P. aeruginosa enzymology Leucine arm entry; KM for isovaleryl-CoA 2.3 µM in PAO1 LiuA
mccB/liuB PP_4065 3-Methylcrotonyl-CoA carboxylase β 6.4.1.4 / K01969 Homology; P. aeruginosa liu cluster Biotin-dependent carboxylase
mccA/liuD PP_4067 3-Methylcrotonyl-CoA carboxylase α (biotin carboxylase) K01968 Homology; P. aeruginosa liu cluster Forms MCase with mccB
liuC PP_4066 3-Methylglutaconyl-CoA hydratase 4.2.1.18 / K13766 Homology; P. aeruginosa liu cluster Leucine arm
mmsB PP_4666 3-Hydroxyisobutyrate DH (valine arm) 1.1.1.31 / K00020 Homology; P. aeruginosa mms enzymology Adjacent to mmsA-II → mms operon
mmsA-II PP_4667 Methylmalonate-semialdehyde DH (valine arm) 1.2.1.27 / K00140 Homology + genomic-context QC (this review) Correct valine-arm copy (see 5.2)

4.2 Leucine terminal step (HMG-CoA lyase) — displaced paralogs

Gene Locus EC / KO Note
PP_3394 PP_3394 4.1.3.4 / K01640 3-hydroxy-3-methylglutaryl-CoA lyase; leucine terminal step candidate
mvaB PP_3540 4.1.3.4 / K01640 Second HMG-CoA lyase paralog

The KT2440 liu cluster is only PP_4064–4067 and lacks a co-located liuE (HMG-CoA lyase), unlike the P. aeruginosa liuRABCDE cluster. The terminal leucine step is therefore displaced to PP_3394 or mvaB/PP_3540. Curation should mark the HMG-CoA lyase step covered but non-adjacent, and flag the paralog choice for review.

4.3 Alternative entry: leucine dehydrogenase

Gene Locus EC / KO Note
ldh PP_4617 1.4.1.9 / K00263 Leucine dehydrogenase — oxidative deamination of leucine as an alternative to ilvE transamination

This provides a redundant entry to 2-oxo-isocaproate for leucine; it is a legitimate alternative route, not a required step.

4.4 Generic / peripheral β-oxidation enzymes (likely over-mapped)

These are co-mapped into ppu00280 through shared EC numbers/KOs but are generic fatty-acid β-oxidation or tangential enzymes, not dedicated BCAA steps:

These should generally be marked shared/peripheral for the module rather than counted as satisfying dedicated steps — though one or more acyl-CoA DH / enoyl-CoA hydratase / thiolase does carry the isoleucine lower arm (which has no dedicated cluster in KT2440).

5. Gaps, ambiguities, and likely over-annotations

5.1 Over-inclusion (the primary curation problem)

Approximately 15 of 35 ppu00280 members are generic β-oxidation enzymes pulled in by EC/KO sharing (Finding F004). Left uncurated, they inflate apparent pathway membership and obscure which genes are truly dedicated. The dedicated leucine module M00036 is fully covered by a small set of specific genes; the bulk of the remaining membership is peripheral.

5.2 Paralog ambiguity (requires explicit "which-copy" decisions)

5.3 True gaps

None for the dedicated core. The isoleucine lower arm has no dedicated gene cluster and is inferred to run on promiscuous β-oxidation enzymes — this is best marked covered-by-shared-enzymes / candidate_uncertain rather than a gap.

5.4 Boundary / not-expected-in-target-taxon

6. Module and GO-curation recommendations

Module step Recommended status Rationale
Transamination (Val/Leu/Ile → BCKAs) covered (ilvE/PP_3511; ldh/PP_4617 alt for Leu) Homology strong; single BCAT convention
BCKDH oxidative decarboxylation covered (bkdAA/AB/B + lpdV, PP_4401-4404) Direct target-strain evidence
Leucine arm (ivd→mcc→liuC→HMG-CoA lyase) covered (M00036 complete) Dedicated genes; terminal step displaced to PP_3394/mvaB
Valine arm (…→mmsB→mmsA) covered (mmsB/PP_4666 + mmsA-II/PP_4667) Operon structure confirmed by genomic context
Isoleucine lower arm candidate_uncertain Carried by promiscuous β-oxidation enzymes; no dedicated cluster
HMG-CoA lyase paralog choice candidate_uncertain PP_3394 vs mvaB/PP_3540 unresolved
Generic β-oxidation members (≈15) peripheral / module_needs_revision Over-mapped by shared EC/KO; should not count as dedicated steps
atu/terpene entry not_expected_in_target_taxon Cluster absent in KT2440
Propionyl-CoA → central metabolism out_of_scope (2-methylcitrate cycle) Belongs to propanoate maps, not ppu00280

Module boundary recommendation: define ppu00280 for KT2440 to terminate at propionyl-CoA + acetyl-CoA/acetoacetate. Explicitly exclude the propionyl-CoA assimilation steps and the methylmalonyl-CoA-mutase module.

GO / annotation actions: demote the ~15 generic β-oxidation members from "dedicated BCAA degradation" to shared β-oxidation membership; annotate lpdV, mmsA-II, and (tentatively) one HMG-CoA lyase as the dedicated copies; retain ilvE and ldh both as legitimate but redundant entry options.

7. Genes to promote to full fetch-gene review

  1. PP_4401–PP_4404 (bkdAA/bkdAB/bkdB/lpdV) — core hub with direct target-strain evidence; anchor the module and confirm E3 assignment vs lpdG/lpd.
  2. PP_3394 vs PP_3540 (mvaB) — resolve which HMG-CoA lyase paralog carries the displaced leucine terminal step.
  3. PP_4666/PP_4667 (mmsB/mmsA-II) vs PP_0597 (mmsA-I) — confirm the valine-arm operon assignment and clarify the role (if any) of the isolated mmsA-I paralog.
  4. PP_3511 (ilvE) and PP_4617 (ldh) — document the redundant entry routes and substrate ranges.
  5. Isoleucine lower-arm candidates — identify which specific acyl-CoA DH / enoyl-CoA hydratase / 3-hydroxyacyl-CoA DH / thiolase paralogs carry 2-methylbutyryl-CoA processing (currently unassigned).

8. Mechanistic model / interpretation

The KT2440 BCAA catabolic network is best understood as a funnel-then-fan system. All three amino acids funnel through two shared steps — transamination (ilvE, with ldh as a leucine-specific backup) and the BkdR-regulated BCKDH complex — before fanning out into three chemically distinct lower arms. This architecture explains several curation observations:

  1. Why the hub is so well characterized and the arms less so. The bkd operon is a single genetic and regulatory unit (BkdR activation, Crc repression, BCAA induction), making it a natural experimental target; the lower arms are distributed across the genome and overlap general β-oxidation. In P. putida the four structural genes were cloned and sequenced, and the positive regulator BkdR (an Lrp homolog, 36.5% identity to E. coli Lrp) is divergently transcribed and required for growth on BCAAs — bkdR mutants cannot use Val/Leu/Ile (PMID: 8320210). L-BCAAs and D-leucine are the physiological inducers (half-maximal L-Val ≈ 2.8 mM; PMID: 10217783), and the operon is under Crc-mediated catabolite repression (PMID: 10648542, PMID: 10648543). This is finding F001.

  2. Why over-mapping is the main annotation error. Because the branched acyl-CoAs are processed by the same enzyme chemistry as fatty acids (dehydrogenation → hydration → oxidation → thiolytic cleavage), KEGG's EC/KO-based mapping sweeps many generic β-oxidation enzymes into ppu00280. The correct curation stance is a small dedicated core plus a large shared periphery (finding F004/F005).

  3. Why paralog resolution needs genomic context. Where enzymology alone cannot distinguish paralogs (E3, mmsA, HMG-CoA lyase), operon structure is decisive: lpdV sits inside bkd; mmsA-II sits beside mmsB under a shared regulator; the isolated mmsA-I does not (finding F007).

  4. Why the boundary matters. Drawing the module edge at propionyl-CoA/acetyl-CoA keeps ppu00280 coherent and avoids double-counting the 2-methylcitrate cycle and (absent) B12-mutase route. In pseudomonads, propionyl-CoA from branched catabolism is channeled into central metabolism via the 2-methylcitrate cycle (PMID: 19798672), and the shared BCKDH readily acts on the isoleucine-derived 2-oxo acid (PMID: 19910413) — finding F006.

9. Evidence base and key references

Direct target-strain (P. putida KT2440 / P. putida) evidence

Broader / mechanistic context

10. Limitations and knowledge gaps

11. Proposed follow-up experiments / actions

  1. Targeted deletions in KT2440 of ivd/PP_4064, mccAB, liuC, mmsB/mmsA-II, and each HMG-CoA lyase paralog, phenotyped on Val, Leu, and Ile as sole carbon sources — resolves arm assignments and the PP_3394/mvaB question directly in the target strain.
  2. 13C-labeling / metabolomics with labeled Val/Leu/Ile to trace flux to propionyl-CoA vs acetyl-CoA/acetoacetate and confirm the 2-methylcitrate routing.
  3. Comparative genomics / synteny across P. putida strains to confirm the mms and liu operon boundaries and the absence of liuE/atu.
  4. Curation actions: (a) demote the ~15 generic β-oxidation members to shared periphery; (b) fix paralog assignments (lpdV, mmsA-II) using genomic context; (c) set the module terminus at propionyl-CoA/acetyl-CoA; (d) mark atu/terpene entry not_expected_in_target_taxon; (e) promote the genes in Section 7 to full fetch-gene review.

Prepared as a species-aware module/pathway/taxon review to support manual module satisfiability and gene-annotation curation for Pseudomonas putida KT2440.

Artifacts

Citations

  1. PMID:8320210
  2. PMID:10217783
  3. PMID:10648542
  4. PMID:10648543
  5. PMID:19798672
  6. PMID:19910413
  7. PMID:9068646
  8. PMID:8670279
  9. PMID:7836297
  10. PMID:16820476
  11. PMID:18625020
  12. PMID:1339433
  13. PMID:35698914
  14. PMID:32997715