pyrC (PP_1086, UniProt Q88NW7) in Pseudomonas putida KT2440: Functional Annotation Report

Summary

The gene pyrC (ordered locus PP_1086; UniProt Q88NW7; PYRC_PSEPK) of Pseudomonas putida KT2440 encodes dihydroorotase (DHOase; EC 3.5.2.3), a cytoplasmic, zinc-dependent metallohydrolase of the amidohydrolase (metallo-dependent hydrolase) superfamily. Its primary and well-defined function is to catalyze the reversible cyclization of N-carbamoyl-L-aspartate (carbamoyl aspartate) to (S)-4,5-dihydro-L-orotate, the third step of the de novo pyrimidine (UMP) biosynthetic pathway. The enzyme is strictly specific: the natively purified P. putida protein hydrolyzed only dihydro-L-orotate and its methyl ester, and the reaction is fully reversible, with a Km for dihydroorotate of ~0.081 mM.

Structurally and mechanistically, this DHOase is a monofunctional ("Class II"/standalone) enzyme that functions as a soluble homodimer with ~41 kDa subunits (native mass ~82 kDa). Each subunit adopts a TIM-barrel ((β/α)₈) fold housing a binuclear zinc active center in which the two Zn²⁺ ions (~3.6 Å apart) are bridged by a post-translationally carbamylated lysine, a catalytic architecture established at atomic resolution for the orthologous Escherichia coli enzyme and conserved across bacterial DHOases. Sequence analysis confirms that Q88NW7 retains the complete catalytic-residue set — His14, His16, the carbamylated Lys100, His137, His175, and Asp248 — mapping one-to-one onto the E. coli scaffold. As a soluble metabolic enzyme with no signal peptide or transmembrane segments, it acts in the cytoplasm.

A crucial disambiguation underlies this report: P. putida carries two DHOase-homologous genes. PP_1086/pyrC (Q88NW7) is the genuinely catalytically active dihydroorotase. It is distinct from a separate vestigial "pyrC'" gene, physically linked to pyrB, which encodes a catalytically dead DHOase homolog that lacks critical histidyl residues and instead serves a purely structural role — stabilizing the dodecameric assembly of the P. putida aspartate transcarbamoylase (ATCase) holoenzyme. This report establishes, on the basis of direct enzymology, structural homology, sequence-level residue conservation, and genome annotation, that Q88NW7 is the active pyrimidine-biosynthetic DHOase and is monocistronic and physically unlinked to the pyr/ATCase locus.

Key Findings

Finding 1 — pyrC (Q88NW7) is a dihydroorotase catalyzing the reversible interconversion of N-carbamoyl-L-aspartate and L-dihydroorotate (step 3 of de novo pyrimidine biosynthesis)

The primary function of the pyrC gene product is enzymatic. It is classified as EC 3.5.2.3 and catalyzes the third reaction of the de novo pyrimidine biosynthetic pathway: the intramolecular cyclization/dehydration of N-carbamoyl-L-aspartate to form (S)-4,5-dihydro-L-orotate plus water, a reaction that operates in both directions.

The most authoritative evidence for the P. putida enzyme comes from direct enzymology on DHOase purified from Pseudomonas putida by Ogawa & Shimizu (1995). They demonstrated strict substrate specificity — "The enzyme only hydrolyzed dihydro-L-orotate and its methyl ester, and the reactions were reversible" (PMID: 8572888) — thereby establishing both the substrate preference and reversibility that define this step in the exact ortholog context. The kinetic parameters were quantified: "The apparent Km and Vmax values for dihydro-L-orotate hydrolysis (at pH 7.4) were 0.081 mM and 18 mumol min-1 mg-1" (PMID: 8572888). For the reverse (cyclization) direction, the Km for N-carbamoyl-DL-aspartate was 2.2 mM with a Vmax of 68 µmol min⁻¹ mg⁻¹ at pH 6.0. The pH optima differing between the two directions (7.4 for hydrolysis, 6.0 for cyclization) are characteristic of this reversible amidohydrolase.

The identity of the reaction is corroborated across bacterial and structural studies. Thoden et al. (2001) state that "Dihydroorotase plays a key role in pyrimidine biosynthesis by catalyzing the reversible interconversion of carbamoyl aspartate to dihydroorotate" (PMID: 11401542), and Lipowska et al. (2019), analyzing pathogen DHOases, confirm that "dihydroorotase (DHO), catalyzes the reversible interconversion of N-carbamoyl-l-aspartate to 4,5-dihydroorotate" (PMID: 31207330). The reaction is thus conserved and well-characterized across the bacterial lineage, with the P. putida enzyme directly measured.

Finding 2 — DHOase is a Zn-dependent metallohydrolase with a binuclear metal center and a carboxylated-lysine bridge in a TIM-barrel fold

The mechanism of catalysis is built on a binuclear zinc center. The gold-standard structural reference is the 1.7 Å crystal structure of E. coli DHOase (Thoden et al. 2001), where "each subunit contains a binuclear zinc center with the metal ions separated by approximately 3.6 A. Lys 102, which is carboxylated, serves as a bridging ligand between the two cations" (PMID: 11401542). In this arrangement, the α-metal is coordinated by His16, His18, the carboxylated Lys102, and Asp250 plus a hydroxide; the β-metal by Lys102, His139, His177, and the bridging hydroxide. The overall subunit is a "'TIM' barrel motif" (PMID: 11401542) — the classic (β/α)₈ scaffold on which amidohydrolase-superfamily active sites are constructed.

The metal dependence is experimentally confirmed for the P. putida enzyme itself: Ogawa & Shimizu found that "The enzyme was inhibited by metal ion chelators and activated by Zn2+" (PMID: 8572888), directly demonstrating that a zinc cofactor is required for activity in the native protein. Independent confirmation of the architecture comes from the human CAD DHOase domain, which likewise contains "two Zn²⁺ ions bridged by a carboxylated lysine" (PMID: 24332717), showing the binuclear-Zn/carbamylated-lysine motif is conserved from bacteria to humans.

This mechanistic picture matches the InterPro/Pfam annotation of Q88NW7: the Amidohydro_1 domain (PF01979), Metal_Hydrolase (IPR032466), and the Dihydroorotase_CS conserved-site signature (IPR002195) — all hallmarks of a metal-dependent hydrolase acting through a bridged bimetal center. The flexibility of a catalytic surface loop is also important mechanistically: kinetic and structural analysis of E. coli mutants showed that residues Thr109 and Thr110 in a flexible loop provide productive substrate binding and stabilize the transition-state intermediate, with loop deletion causing near-complete activity loss despite an intact binuclear zinc center (PMID: 17711307).

Finding 3 — The P. putida enzyme is a cytoplasmic, homodimeric (~41 kDa subunit), monofunctional (Class II) DHOase regulated by pyrimidine-pathway intermediates

Ogawa & Shimizu determined that the native P. putida enzyme is 82 kDa, composed of "two identical subunits with a relative molecular mass of 41 kDa" (PMID: 8572888). This homodimeric organization is the defining feature of the standalone, "Class II" DHOase typified by E. coli (a homodimer; PMID: 11401542), as opposed to the DHOase activity embedded within the large multifunctional eukaryotic CAD protein (PMID: 24332717). Thus, pyrC in P. putida encodes an independent metabolic enzyme, not a fused domain.

The active site's specificity is further illuminated by product- and analog-based inhibition of the P. putida enzyme. It is "competitively inhibited by N-carbamoylamino acids such as N-carbamoylglycine, with a Ki value of 2.7 mM" (PMID: 8572888), and is non-competitively inhibited by dihydrouracil (Ki 3.4 mM) and orotate (Ki 0.75 mM). These inhibition profiles — by structural analogs of the substrate and by downstream pyrimidine-metabolism intermediates — are consistent with a tightly shaped active site and with metabolic feedback sensitivity.

At the pathway/regulatory level, de novo pyrimidine enzymes in P. putida (including DHOase) are regulated by pyrimidine availability and carbon source. Santiago & West-type studies documented that "Regulation at the transcriptional level of de novo pyrimidine biosynthetic enzyme synthesis in P. putida ATCC 17536 was observed" (PMID: 12619820). Consistent with earlier work, repression of these enzymes by pyrimidines in P. putida is comparatively weak — only 1.5-to-2-fold derepression following pyrimidine starvation was detected (PMID: 176312) — indicating modest but real transcriptional control. As a soluble cytoplasmic enzyme lacking any signal peptide or transmembrane feature, the protein performs its catalysis in the cytoplasm.

Finding 4 — PP_1086/pyrC is the ACTIVE standalone DHOase, distinct from the vestigial pyrC' that is a structural subunit of the P. putida ATCase holoenzyme

This is the central identity-verification finding. P. putida encodes two DHOase-homologous genes with fundamentally different roles, and it is essential not to conflate them.

The first is a pyrB-overlapping "pyrC'" gene encoding a 424-residue/44.2 kDa polypeptide that is a catalytically inactive ("vestigial") DHOase. Schurr et al. (1995) showed that "the 44.2-kDa polypeptide lacks specific histidyl residues thought to be critical for DHOase enzymatic function" (PMID: 7896697), and it does not complement E. coli pyrC auxotrophs. Its role is purely structural: "The proposed function for the vestigial DHOase is to maintain ATCase activity by conserving the dodecameric assembly of the native enzyme" (PMID: 7896697). Critically, the authors state that "this 44.2-kDa polypeptide is not considered to be the functional product of the pyrC gene in P. putida, as DHOase activity is distinct from the ATCase complex" (PMID: 7896697).

The second — and the subject of this report — is the functional, catalytically active DHOase, PP_1086/pyrC (Q88NW7): a separate, standalone monofunctional homodimer (82 kDa native; 41 kDa subunits; PMID: 8572888) whose activity is "distinct from the ATCase complex" and which retains the binuclear-Zn His/Lys/Asp active-site ligands defined by the E. coli structure (PMID: 11401542). The evolutionary logic of these two paralogs is captured by the ATCase quaternary-structure classification, in which prokaryotic ATCases fall into classes depending on whether the associated DHOase (PyrC) is active (subclass A1) or inactive (PyrC', subclass A2) (PMID: 14660694).

This finding satisfies the mandatory verification requirement: Q88NW7 is unambiguously the active pyrimidine-biosynthetic DHOase (PyrC), not the vestigial ATCase-scaffolding PyrC'.

Finding 5 — Q88NW7 retains the complete binuclear-Zn catalytic residue set, one-to-one homologous to E. coli DHOase

Direct sequence analysis provides residue-level proof that Q88NW7 is catalytically competent. The 348-amino-acid PYRC_PSEPK sequence was mapped onto the E. coli DHOase scaffold (PMID: 11401542). All six catalytic ligands are present and correctly positioned:

Substrate-contact residues (e.g., the His16-Leu-Arg18 element, Asn42, His137, Leu220, His252, Ala264) are likewise conserved. The retention of the full catalytic set stands in explicit contrast to the vestigial pyrC', which "lacks specific histidyl residues thought to be critical for DHOase enzymatic function" (PMID: 7896697) — reinforcing that PP_1086 is the active enzyme. UniProt curation independently lists the catalytic reaction ((S)-dihydroorotate + H₂O = N-carbamoyl-L-aspartate + H⁺; RHEA:24296), the Zn²⁺ cofactor, the pathway assignment ("UMP biosynthesis via de novo pathway, step 3/3"), and the homodimeric assembly.

Finding 6 — pyrC/PP_1086 is a monocistronic, isolated gene, physically unlinked to other pyr genes

Genome annotation (KEGG, organism ppu) independently confirms PP_1086 = pyrC / dihydroorotase, KO K01465, EC 3.5.2.3, mapped to Pyrimidine metabolism (ppu00240), Metabolic pathways (ppu01100), and Biosynthesis of cofactors (ppu01240); cross-referenced to UniProt Q88NW7 and NCBI protein AAN66711 (348 aa), at genome position complement(1244481..1245527), Pfam Amidohydro_1.

The genomic neighborhood shows the gene is not part of a pyr operon: PP_1084 = tsaA (peroxiredoxin), PP_1085 = rnt (RNase T), PP_1086 = pyrC, PP_1087 = an OmpA-family outer-membrane protein, and PP_1088 = argG (argininosuccinate synthase) — all functionally unrelated to pyrimidine biosynthesis. pyrC is therefore monocistronic and separated from the pyrB/pyrC' (ATCase) locus. This organization is consistent with independent transcriptional regulation of pyrimidine genes in P. putida, as documented by the observation that "Regulation at the transcriptional level of de novo pyrimidine biosynthetic enzyme synthesis in P. putida ATCC 17536 was observed" (PMID: 12619820).

Mechanistic Model / Interpretation

Position in the pathway

pyrC/PP_1086 executes the third of six enzymatic steps in the de novo synthesis of UMP, the precursor of all pyrimidine nucleotides:

Glutamine + HCO3- + 2ATP
│  (carbamoyl-phosphate synthetase, carAB)
▼
   Carbamoyl-phosphate
│  + L-aspartate  (aspartate transcarbamoylase, ATCase, pyrB)
▼
  N-carbamoyl-L-aspartate  ────────────┐
│                              │  ← STEP 3: DIHYDROOROTASE (pyrC / PP_1086 / Q88NW7)
│   reversible cyclization/dehydration (–H2O)
▼                              │
   (S)-4,5-dihydro-L-orotate  ◄────────┘  reversible (hydrolysis adds H2O)
│  (dihydroorotate dehydrogenase, pyrD)
▼
      Orotate
│  (orotate phosphoribosyltransferase, pyrE)
▼
      OMP
│  (OMP decarboxylase, pyrF)
▼
       UMP  →  UDP, UTP, CTP, dTTP, ...

The catalytic machine

Each of the two identical subunits folds into a (β/α)₈ TIM barrel, at the C-terminal end of which sits the active site. Two Zn²⁺ ions (~3.6 Å apart) are held by a constellation of histidines and an aspartate and bridged by a carbamylated lysine (Lys100 in Q88NW7). A metal-bridging hydroxide serves as the nucleophile/leaving group. In the cyclization direction, the bimetal center polarizes the carbamoyl group of N-carbamoyl-L-aspartate and the hydroxide-mediated chemistry closes the six-membered dihydroorotate ring while expelling water; in the hydrolysis direction the same machinery adds water across the ring amide bond. The differing pH optima (7.4 vs 6.0) for the two directions reflect the protonation requirements of the forward vs reverse chemistry.

His14   His16                 His137  His175
   \    /                        \    /
    [Zn_α] ---- OH(bridge) ---- [Zn_β]
    /   \        |                 /
Asp248  (Lys100-CO2- bridges both Zn)
                  |
      substrate: N-carbamoyl-L-aspartate  ⇌  dihydro-L-orotate + H2O

Two paralogs, two fates

The most important interpretive point for correct annotation is the paralog split in P. putida:

This division of labor is an elegant example of subfunctionalization after gene duplication: one copy retained catalysis, the other lost catalysis but was co-opted for a structural role in the ATCase holoenzyme, a scenario recognized in the broader ATCase quaternary-structure/evolution framework (PMID: 14660694).

Localization and regulation

The enzyme is cytoplasmic — it is a soluble metabolic protein with no signal peptide or membrane anchor and it operates on soluble small-molecule substrates in the cytosol. Its expression is under modest transcriptional control by pyrimidine availability and carbon source, and its activity is product/analog-inhibited by pyrimidine-metabolism intermediates (orotate, dihydrouracil, N-carbamoylglycine), providing local feedback tuning.

Evidence Base

Supporting/contextual papers on pathway regulation in related pseudomonads include P. fluorescens (PMID: 16255137) and P. fragi (PMID: 12390485), and the L-hydantoinase structure (PMID: 12093275) illustrating the shared TIM-barrel/binuclear-Zn fold across the amidohydrolase superfamily.

Limitations and Knowledge Gaps

1. No crystal structure of Q88NW7 itself. The atomic mechanism is inferred from the highly homologous E. coli enzyme (PMID: 11401542) and residue mapping. While the conservation is unambiguous (all six catalytic ligands present), a direct P. putida KT2440 DHOase structure has not been reported.

2. Enzymology strain provenance. The definitive kinetic/biochemical characterization (PMID: 8572888) was performed on DHOase purified from P. putida (species-level); it is reasonably but not formally proven that the purified protein corresponds precisely to the KT2440 PP_1086 gene product rather than a very close ortholog. Given the sequence identity and functional match, this is a minor caveat.

3. Regulatory detail is coarse. Transcriptional regulation of pyrimidine genes in P. putida is documented at the pathway level (PMID: 12619820; PMID: 176312), but the specific promoter, regulator(s), and any operon/regulon context for PP_1086 have not been experimentally mapped. Genome context indicates it is monocistronic, but the transcription start site and regulatory elements remain uncharacterized.

4. Metal stoichiometry not directly measured for Q88NW7. Zn dependence is established biochemically for the P. putida enzyme (PMID: 8572888) and structurally for homologs, but the exact in vivo metal occupancy (mono- vs binuclear under physiological conditions) has not been quantified for this specific protein.

5. Physiological flux and essentiality. Whether PP_1086 is strictly essential in KT2440 under standard conditions (versus salvage-pathway rescue) has not been directly tested here.

Proposed Follow-up Experiments / Actions

1. Genetic complementation. Clone PP_1086 and test complementation of an E. coli pyrC auxotroph to formally confirm in vivo catalytic function — the clean, decisive test that distinguishes it from vestigial pyrC'.

2. Recombinant expression + kinetics. Express His-tagged Q88NW7, purify, and re-measure kinetics (Km, kcat, pH profile) and metal content (ICP-MS) to tie the classic P. putida enzymology definitively to the PP_1086 gene product.

3. Structure determination. Solve the KT2440 DHOase crystal or cryo-EM structure (± bound substrate/product/analog) to confirm the binuclear-Zn center, carbamylated Lys100, and the flexible catalytic loop analogous to E. coli Thr109/Thr110 (PMID: 17711307).

4. Active-site mutagenesis. Mutate the mapped ligands (His14, His16, Lys100, His137, His175, Asp248) individually and assay activity loss to experimentally validate the residue assignments made from homology.

5. Transcriptional mapping. Use RNA-seq/5′-RACE and reporter fusions under pyrimidine-replete vs -starved and different carbon sources to define the PP_1086 promoter, transcription start, and regulator(s).

6. Knockout phenotyping. Construct a ΔPP_1086 deletion and test pyrimidine auxotrophy and growth rescue by uracil/uridine to establish essentiality and pathway position in KT2440.

Report prepared for functional annotation of pyrC (PP_1086, UniProt Q88NW7) in Pseudomonas putida KT2440. All claims are attributed to the cited primary literature and database annotations above.

Artifacts

• OpenScientist final report
• OpenScientist final report

Citations

1. PMID:8572888
2. PMID:11401542
3. PMID:31207330
4. PMID:24332717
5. PMID:17711307
6. PMID:12619820
7. PMID:7896697
8. PMID:14660694
9. PMID:16255137
10. PMID:12390485
11. PMID:12093275