PP_3205 (Q88HZ7) encodes FdeI, a soluble cytoplasmic enzyme of the fumarylacetoacetate-hydrolase (FAH) superfamily (hydratase/decarboxylase family; COG0179) that acts in the bacterial catabolic pathway for the plant flavonoid naringenin (and related flavanones/flavones). By orthology and KEGG pathway assignment, its product participates in the lower, "meta-cleavage" portion of the fde (flavonoid-degradation) pathway — the steps that process the ring-fission products generated after dioxygenolytic cleavage of the flavonoid A-ring — funnelling the carbon skeleton toward oxaloacetate and the TCA cycle. It carries out its reaction in the cytoplasm. It is expressed from a flavonoid-inducible operon controlled by the LysR/NodD-like regulator FdeR.
Its precise catalyzed reaction and substrate specificity have not been determined experimentally for PP_3205 itself; the functional assignment is an inference from (i) orthology to the experimentally characterized Herbaspirillum seropedicae SmR1 pathway, (ii) conserved genomic clustering, (iii) KEGG orthology (K26185 / FdeI), and (iv) the diagnostic FAH-fold catalytic architecture in its sequence.
| Attribute | Value |
|---|---|
| Locus tag | PP_3205 (ordered locus name; unambiguous in KT2440) |
| UniProt | Q88HZ7 (330 aa) |
| Organism | Pseudomonas putida KT2440 (ATCC 47054 / DSM 6125 / NCIMB 11950) |
| Genome position | 3,638,005–3,638,997 (+ strand); 993 nt CDS |
| Protein family | Fumarylacetoacetate hydrolase (FAH) superfamily; "hydratase/decarboxylase family" (UniProt SIMILARITY) |
| Domains | FAA_hydro_N (IPR041072 / Pfam PF18288) + FAA_hydrolase C-terminal (IPR011234, fold IPR036663 / Pfam PF01557) |
| Orthology | eggNOG COG0179; KEGG ortholog K26185 (fdeI) |
| KEGG pathway | ppu00946 "Degradation of flavonoids" |
| GenBank protein | AAN68813.1 |
Verification outcome: The locus tag, organism, domain architecture (two-domain FAH fold) and family assignment are mutually consistent. The generic UniProt name ("Fumarylacetoacetate hydrolase family protein") is correct but uninformative; the specific functional identity comes from KEGG orthology (FdeI, naringenin degradation) and the experimentally defined H. seropedicae pathway. No conflicting "same-symbol, different-gene" literature issue arises because PP_3205 is an ordered-locus identifier, not an ambiguous common symbol.
FdeI belongs to the fumarylacetoacetate-hydrolase (FAH) superfamily, whose members are divalent-metal-dependent (typically Ca²⁺/Mg²⁺) enzymes that catalyze C–C bond hydrolysis, hydration, or decarboxylation of oxo-carboxylate intermediates. Canonical examples include fumarylacetoacetase (tyrosine catabolism), fumarylpyruvate hydrolase (gentisate/naphthalene catabolism, e.g., NagK/SgpK; PMID 11133965, PMID 23266498), and 2-keto-4-pentenoate hydratase (aromatic meta-cleavage pathways). The UniProt "hydratase/decarboxylase family" designation reflects this chemistry.
In the experimentally dissected Herbaspirillum seropedicae SmR1 pathway (Marin et al. 2016, PMID 27059806):
1. FdeE (a monooxygenase) performs the initial mono-oxygenation of naringenin to 5,7,8-trihydroxy-2-(4-hydroxyphenyl)-2,3-dihydro-4H-chromen-4-one.
2. FdeC (a ring-cleaving dioxygenase) then meta-cleaves the A-ring.
3. Downstream enzymes (including the FAH-family FdeI) process the ring-opened products; the pathway ultimately generates oxaloacetate, which enters the tricarboxylic acid cycle.
FdeI (PP_3205) is one of these downstream, ring-fission-processing enzymes. KEGG links K26185/FdeI to reactions R13078 (a decarboxylation: 5-hydroxy-5-(4-hydroxyphenyl)-3-oxopentanoate → 4-hydroxy-4-(4-hydroxyphenyl)butan-2-one + CO₂) and R13079 (a hydration/dehydration). These two reactions are flagged "unclear reaction" in KEGG and should be treated as provisional: they capture the type of chemistry expected of a FAH-family enzyme (decarboxylation + hydration of an oxo-carboxylate) but the exact substrate/product for FdeI have not been confirmed enzymologically.
No direct kinetic/substrate data exist for PP_3205. Consistent with the whole operon being induced by apigenin, chrysin, luteolin and naringenin (Wassem et al. 2017, PMID 27878922), the pathway — and therefore FdeI — most plausibly acts on ring-fission intermediates derived from several flavanones/flavones, not naringenin alone.
PP_3205 lies within a contiguous, co-oriented cluster whose members carry catabolic annotations matching a flavonoid meta-cleavage pathway:
| Locus | Annotation | KEGG ortholog |
|---|---|---|
| PP_3201 | BNR-repeat domain protein | — |
| PP_3202 / PP_3203 | transporter / MFS transporter | K07003 / — |
| PP_3204 | conserved protein | K26182 (FdeH) |
| PP_3205 | FAH-family protein | K26185 (FdeI) |
| PP_3206 | conserved protein | K26183 (FdeJ) |
| PP_3207 | putative cyclase | — |
| PP_3208 | Pdr/VanB-family oxidoreductase | — |
| PP_3212 | Rieske 2Fe-2S dioxygenase | — |
The presence of a Rieske dioxygenase, an oxidoreductase, a cyclase, transporters and three Fde orthologs in one locus is the hallmark of an aromatic-compound catabolic island, corroborating the KEGG assignment. The K26185 ortholog group is broadly conserved across Pseudomonas (e.g., PVLB_09785, PSEBR_a2142, T1E_5649).
The gene product functions in the cytoplasm:
- No N-terminal signal peptide (sequence begins MKLATFDDGS…) and no transmembrane segment: whole-protein GRAVY = −0.023 (hydrophilic) and the maximum Kyte–Doolittle hydropathy over a 19-residue window is only 1.04, below the ~1.6 threshold indicative of a membrane helix.
- FAH-superfamily enzymes are characteristically soluble cytoplasmic proteins, and the naringenin ring-fission intermediates they process are generated intracellularly.
- UniProt provides no subcellular-location annotation, consistent with a default cytoplasmic assignment.
HLPLHVHWNGEWFGHPHGG (His217/His221/His223/His231/His233 with adjacent Glu/Asp). In FAH-superfamily structures, an equivalent His/Glu-rich pocket coordinates a divalent metal ion and activates a water nucleophile for C–C bond cleavage/hydration/decarboxylation. The presence of an intact motif argues the protein is a catalytically competent family member rather than a degenerate pseudoenzyme.FdeI contributes to flavonoid catabolism, enabling the bacterium to use plant-derived flavonoids as carbon and energy sources. Because the operon is specifically induced by flavonoids via the NodD-like LysR regulator FdeR (Wassem et al. 2017, PMID 27878922) — a regulator otherwise associated with rhizobial nod-gene control — the pathway is best understood as an adaptation to plant-associated / rhizosphere niches, where flavonoids are abundant plant exudates. The end product (oxaloacetate) is assimilated through central metabolism (Marin et al. 2016, PMID 27059806).
Supported (by convergent evidence):
- H1 — PP_3205 is a FAH-superfamily hydrolase/decarboxylase. (UniProt/Pfam/InterPro/COG0179 + intact catalytic motif.)
- H2 — PP_3205 = FdeI and functions in the naringenin/flavonoid meta-cleavage pathway. (KEGG K26185/ppu00946 + conserved gene cluster + orthology to characterized H. seropedicae pathway, PMID 27059806.)
- H3 — The enzyme is a soluble cytoplasmic protein. (No signal peptide/TM; hydropathy analysis.)
- H4 — The pathway (and thus fdeI) is flavonoid-inducible. (PMID 27878922.)
Refuted / not supported:
- The protein is not a membrane transporter or secreted protein (no TM/signal). The neighbouring transporters (PP_3202/3203, PP_3210/3211) perform substrate import, not FdeI.
- No evidence that PP_3205 acts in tyrosine catabolism (classic fumarylacetoacetase role) in KT2440; that reaction is served by the separate homogentisate pathway. The FAH fold here is repurposed for flavonoid ring-fission chemistry.