| Aspect | Key points | Evidence/notes |
|---|---|---|
| Identity / verification | **pcaG = PP_4655 = α-subunit of protocatechuate 3,4-dioxygenase (PcaGH)** in *Pseudomonas putida* KT2440; enzyme is a heterodimeric/oligomeric intradiol dioxygenase with β-subunit **pcaH**. | KT2440 engineering papers explicitly refer to **pcaGH (PP_4655-4656)** as protocatechuate 3,4-dioxygenase; biosensor work identifies **pcaH/pcaG** as the two-subunit PCA 3,4-dioxygenase; modeling paper states α = **pcaG**, β = **pcaH**. (pqac-00000001, pqac-00000008, pqac-00000007) |
| Reaction | Catalyzes **intradiol 3,4-cleavage of protocatechuate (PCA)** to **3-carboxy-cis,cis-muconate** (also written β-carboxy-cis,cis-muconate). This is the ring-opening step of the PCA branch. | Bacterial PCA pathway summary names the product as 3-carboxy-cis,cis-muconic acid; modeling paper assigns reversible conversion PCA ↔ β-carboxy-cis,cis-muconate. (pqac-00000010, pqac-00000007) |
| Enzyme class / mechanism | Member of the **intradiol ring-cleaving dioxygenase** family; mechanism proceeds through O2 addition and peroxo/anhydride intermediates characteristic of Fe-dependent intradiol cleavage chemistry. | Structural/mechanistic studies on 3,4-PCD captured alkylperoxo and anhydride intermediates and define it as an intradiol dioxygenase. (pqac-00000012) |
| Cofactor / active site | Active site contains **Fe3+** coordinated by a **2-His/2-Tyr** ligand set; tyrosines contribute ligand-to-metal charge transfer features. | High-confidence mechanistic evidence from crystal structures of 3,4-PCD. (pqac-00000012) |
| Substrates / specificity | Primary physiological substrate is **protocatechuate**. In KT2440-focused biochemical work, **PcaHG also cleaves gallate**, with expected lower specificity than for protocatechuate; products were investigated as **(Z)-OMAe and PDC** in engineering contexts. | KT2440 thesis work specifically tested protocatechuate and gallate, hypothesizing lower specificity for gallate and linking activity to PDC production. (pqac-00000003, pqac-00000000) |
| Quantitative mechanistic data | For 3,4-PCD with alternative substrate 4-fluorocatechol: observable stopped-flow steps had **RRTs 0.92, 0.50, and 0.16 s−1**; apparent **Kd ≈ 7.5 mM** for the initial weak complex; overall substrate **Kd predicted ≈ 88 μM**, direct titration **≈ 75 μM**. | These values come from a mechanistic 3,4-PCD study and are informative for enzyme behavior, though not KT2440-specific in vivo physiology. (pqac-00000012) |
| Pathway role | Central enzyme of the **β-ketoadipate / protocatechuate branch**, funnelling diverse aromatics after biological funneling to PCA toward central metabolism / TCA-cycle entry. | Reviews and KT2440 engineering papers place pcaGH at the PCA ring-cleavage step connecting lignin-derived aromatic catabolism to central carbon metabolism. (pqac-00000000, pqac-00000002, pqac-00000006, pqac-00000015) |
| Gene organization | pca catabolic genes in *Pseudomonas* are classically named **pcaGH, pcaB, pcaC, pcaD**; one systems model represents structural genes on a **polycistronic pcaBKCHG mRNA**. | Useful for functional annotation, though organization can vary across taxa and publications. (pqac-00000010, pqac-00000007) |
| Regulation | **PcaU** is the local regulator: an **IclR-family** transcription factor that acts as **activator in the presence of protocatechuate** and **repressor in its absence**; PcaU-based regulatory parts were portable enough to engineer a PCA biosensor in KT2440. | Modeling and biosensor papers support PCA-responsive regulation through PcaU. (pqac-00000007, pqac-00000011, pqac-00000014) |
| Transport context | **PcaK** can transport protocatechuate into the cell, coupling uptake to pca pathway function. | Relevant to interpreting intracellular PCA availability and pcaGH knockout phenotypes. (pqac-00000007) |
| Cellular localization | Evidence supports an **intracellular/cytosolic** role in aromatic catabolism rather than secretion or membrane localization. | Biosensor/pathway studies describe intracellular PCA accumulation/catabolism; ring-cleavage enzymes in these studies are treated as intracellular pathway enzymes. Direct localization experiment for KT2440 PcaG was not identified in retrieved sources. (pqac-00000007, pqac-00000008, pqac-00000013) |
| KT2440 knockout phenotype | **ΔpcaGH** blocks PCA ring cleavage, allowing PCA accumulation and preventing further catabolism through the native β-ketoadipate pathway. In glucose-containing media, knockout reportedly had **no discernible impact on growth** in one study. | Seen in KT2440 strains used for PCA accumulation and sensor characterization. (pqac-00000002, pqac-00000005, pqac-00000011, pqac-00000013) |
| KT2440 engineering: PCA accumulation from model aromatics | In engineered KT2440 **KT1 (ΔpcaGH)**, PCA yields from 1 g/L substrates were **97.7% from p-coumarate**, **98.5% from 4-hydroxybenzaldehyde**, and **93.1% from 4-hydroxybenzoate**. | Demonstrates that blocking pcaGH efficiently diverts flux to PCA accumulation. (pqac-00000002, pqac-00000013) |
| KT2440 engineering: vanAB overexpression with ΔpcaGH | In **KT2**, endogenous **vanAB** overexpression on top of **ΔpcaGH** increased PCA yield from **2.61% to 75.63%** for ferulic acid; strain also converted **10 g/L p-coumarate to 6.11 g/L PCA in 72 h**; with ferulate, **2.5 g/L consumed** and **1.9 g/L PCA** after 72 h. | Shows pcaGH deletion is a key chassis modification for lignin-monomer valorization. (pqac-00000005) |
| KT2440 engineering: hydrolysate valorization | In 2024 hydrolysate work, engineered KT2440 produced **253.88 mg/L PCA** at **70.85% yield** from one corncob hydrolysate and **433.72 mg/L PCA** from another without added nutrients. | Figure/pathway summary explicitly shows **pcaGH knockout** as the enabling design feature. (pqac-00000001, pqac-00000015) |
| KT2440 engineering: gallate/PDC route | Chromosomal overexpression of **pcaHG** (e.g., **Ptac:pcaHG**) in KT2440 was used to enhance conversion of syringate/gallate-derived intermediates toward **2-pyrone-4,6-dicarboxylate (PDC)**. | Highlights that pcaG can be used both as a **knockout target** (to accumulate PCA) and an **overexpression target** (to drive ring-cleavage chemistry on noncanonical substrates). (pqac-00000000, pqac-00000003) |
| Biosensor application | A PcaU-based PCA biosensor evolved in KT2440 detected PCA at **<0.003 mM** with **>12-fold contrast ratio**; FACS selections used inductions from **0.01–10 mM PCA** and selected the **top 1%** induced cells after pre-clearing the bottom **40%** dark population. | While not measuring PcaG directly, this is a practical KT2440 application exploiting native PCA/pca regulation and ΔpcaGH backgrounds. (pqac-00000009, pqac-00000008) |
| Interpretation for annotation | Best-supported annotation for **Q88E13 / PP_4655**: intracellular α-subunit of the Fe(III)-dependent **protocatechuate 3,4-dioxygenase** that catalyzes intradiol ring opening of PCA in the β-ketoadipate pathway; highly relevant to lignin-derived aromatic catabolism and metabolic engineering in KT2440. | Consolidated from organism-specific and mechanistic sources. (pqac-00000001, pqac-00000002, pqac-00000007, pqac-00000012, pqac-00000015) |


*Table: This table summarizes the verified identity, biochemical function, pathway context, regulation, localization, and engineering relevance of *Pseudomonas putida* KT2440 pcaG (UniProt Q88E13/PP_4655). It also captures key numeric results from recent KT2440 metabolic-engineering studies and supporting mechanistic work.*