| Claim/Aspect | Key finding (with quantitative values where available) | Organism/strain | Evidence type | Source (author year journal) | URL | Citation ID |
|---|---|---|---|---|---|---|
| Reaction / primary function | **hpd / PP_3433** encodes **4-hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27)**, catalyzing **4-hydroxyphenylpyruvate → homogentisate** in tyrosine catabolism; HPPD is a **non-heme Fe(II)-dependent oxygenase** | *Pseudomonas putida* KT2440; broader HPPD family | Pathway genetics; biochemical review | Arias-Barrau et al. 2004 *J Bacteriol*; Santucci et al. 2017 *J Med Chem* | https://doi.org/10.1128/jb.186.15.5062-5077.2004 ; https://doi.org/10.1021/acs.jmedchem.6b01395 | (pqac-00000007, pqac-00000012) |
| Pathway role | Hpd performs the **peripheral step feeding the homogentisate central pathway**; downstream HmgA/HmgC/HmgB convert homogentisate to **fumarate + acetoacetate** | *P. putida* KT2440 / *P. putida* U | Genetics; pathway reconstruction | Arias-Barrau et al. 2004 *J Bacteriol* | https://doi.org/10.1128/jb.186.15.5062-5077.2004 | (pqac-00000007, pqac-00000023) |
| Genomic context / identity verification | Early KT2440 genome annotation identifies **ORF03099 / hpd**, a **358-aa Hpd** with **88% aa identity** to *P. fluorescens* HPPD; located in a chromosomal region containing **hmgA, mai, fah** and related aromatic-amino-acid catabolic genes | *P. putida* KT2440 | Genomics / comparative annotation | Jiménez et al. 2002 *Environ Microbiol* | https://doi.org/10.1046/j.1462-2920.2002.00370.x | (pqac-00000002) |
| Regulation | The **Crc global regulator represses/catabolically controls** expression of **hpd** and **hmgA** in the homogentisate pathway during growth in rich medium | *P. putida* | Proteomics; regulatory genetics | Morales et al. 2004 *J Bacteriol* | https://doi.org/10.1128/jb.186.5.1337-1344.2004 | (pqac-00000005) |
| Omics evidence in KT2440 | Proteomics showed **Hpd and HmgA are induced by phenylalanine**, supporting assignment of Hpd to the phenylalanine/tyrosine → homogentisate route | *P. putida* KT2440 | Proteomics | Kim et al. 2006 *PROTEOMICS* | https://doi.org/10.1002/pmic.200500329 | (pqac-00000006) |
| Mutant phenotype / carbon utilization | **hpd mutants were unable to grow on minimal medium with phenylalanine or tyrosine as sole carbon/energy source**; mutants accumulated a colored oxidation product consistent with pathway blockage upstream of homogentisate utilization | *P. putida* derivatives; pathway validated against KT2440 homologous locus | Genetics / growth phenotype | Arias-Barrau et al. 2004 *J Bacteriol* | https://doi.org/10.1128/jb.186.15.5062-5077.2004 | (pqac-00000001) |
| Engineering phenotype | Targeted **hpd disruption** increased p-hydroxybenzoate production from **1.8 to 2.3 mM**; **Yps 10.5 → 13.4 C-mol%** (**22% improvement**); **qp 1.6 → 2.3 μmol gCDW⁻¹ min⁻¹**; **qp,max 3.8 → 4.4 μmol gCDW⁻¹ min⁻¹** | *P. putida* S12palB1 → S12palB2 (Δhpd) | Metabolic engineering; phenotype | Verhoef et al. 2010 *Appl Microbiol Biotechnol* | https://doi.org/10.1007/s00253-010-2626-z | (pqac-00000003, pqac-00000017) |
| Transcriptomic signal during production strain engineering | In a p-hydroxybenzoate-producing strain, **hpd (PP3433)** was upregulated with reported table values **1.5 (below threshold)** and **2.8**, consistent with tyrosine degradation competing with product formation | *P. putida* S12palB1 | Transcriptomics / proteomics | Verhoef et al. 2010 *Appl Microbiol Biotechnol* | https://doi.org/10.1007/s00253-010-2626-z | (pqac-00000000, pqac-00000020) |
| Related production phenotype | In phenol-production engineering, **hpd** was upregulated **2.8-fold**; an **hpd knockout** shifted phenol production post-growth, with **62%** of phenol formed after growth versus **28%** in the parent strain | *P. putida* S12-derived strains | Transcriptomics; knockout phenotype | Wierckx et al. 2008 *J Bacteriol* | https://doi.org/10.1128/jb.01379-07 | (pqac-00000018, pqac-00000022) |
| Recent 2024 RB-TnSeq findings | Machine-learning analysis of **RB-TnSeq fitness data (179 conditions)** placed **hpd** in a module for **L-phenylalanine/tyrosine catabolism**; **hpd transposon knockouts showed substantial growth defects on L-Phe**, supporting a direct role in aromatic amino-acid assimilation | *P. putida* KT2440 | RB-TnSeq fitness genomics / ML analysis | Borchert et al. 2024 *mSystems* | https://doi.org/10.1128/msystems.00942-23 | (pqac-00000008) |
| Mechanistic annotation support | HPPD enzymes are typically **tetrameric in bacteria**, use a conserved **His/His/Glu metal-binding triad**, and catalysis requires **substrate-bound Fe(II) plus O2**; these features support inference for PP_3433 as a canonical bacterial HPPD-family enzyme | Bacterial HPPDs broadly | Structural / mechanistic review | Santucci et al. 2017 *J Med Chem*; Trezza et al. 2024 *Biomedicines* | https://doi.org/10.1021/acs.jmedchem.6b01395 ; https://doi.org/10.3390/biomedicines12061196 | (pqac-00000010, pqac-00000013) |


*Table: This table summarizes the main evidence supporting functional annotation of Pseudomonas putida KT2440 hpd (PP_3433; UniProt Q88HC7), including its enzymatic reaction, pathway placement, regulation, phenotypes, and recent 2024 functional-genomics support.*