P4HA3 (Prolyl 4-hydroxylase subunit alpha-3) — gene review notes

UniProt: Q7Z4N8 (P4HA3_HUMAN), 544 aa precursor (signal 1-19; chain 20-544). Gene on human chr 11. EC=1.14.11.2.

Core identity and function

P4HA3 is the third catalytic alpha subunit isoform (alpha(III)) of collagen prolyl 4-hydroxylase (C-P4H). The active enzyme is an alpha2-beta2 heterotetramer in which the beta subunit is protein disulfide isomerase (PDI / P4HB). It is an ER-lumenal, Fe(II)- and 2-oxoglutarate-dependent dioxygenase that hydroxylates proline in -X-Pro-Gly- triplets of procollagen to form trans-4-hydroxyproline, which is essential for the thermal stability of the collagen triple helix.

• Founding characterization: PMID:14500733
• Tetramer subunit composition: PMID:14500733
• Demonstrated catalytic activity on collagen: PMID:14500733
• Reaction (UniProt): L-prolyl-[collagen] + 2-oxoglutarate + O2 = trans-4-hydroxy-L-prolyl-[collagen] + succinate + CO2; EC=1.14.11.2 [ECO:0000269|PubMed:14500733].
• Cofactors (UniProt): binds 1 Fe(2+) ion per subunit; requires L-ascorbate. Kinetics (PMID:14500733): KM 0.5 uM Fe(2+), 20 uM 2-oxoglutarate, 24 uM (Pro-Pro-Gly), 370 uM L-ascorbate.
• Catalytic domain: Fe2OG dioxygenase domain (422-529); Fe-binding residues 440, 442, 510; 2-oxoglutarate binding residue 520; TPR repeat (227-260) for peptide-substrate/PDI interaction.

Subcellular location

ER lumen (UniProt: "SUBCELLULAR LOCATION: Endoplasmic reticulum lumen {ECO:0000305}"). Reactome curates ER-lumen localization (R-HSA-1650808, R-HSA-9918779). InterPro/IEA also assigns endoplasmic reticulum (GO:0005783). All consistent with a secreted-pathway, lumenal collagen-modifying enzyme.

Tissue expression / context

Highly expressed in placenta, liver, and fetal skin; weakly in fetal epiphyseal cartilage, fetal liver, fibroblast, lung, skeletal muscle PMID:14500733. Also expressed in the fibrous cap of human atherosclerotic plaque / carotid lesions [PMID:12874193 cloned as the subunit "expressed in the fibrous cap of human atherosclerotic plaque"]. HPA: tissue-enhanced in blood vessel / smooth muscle. P4HA3 is frequently discussed in cancer contexts (proliferation/migration/EMT in various tumors) but those roles are downstream/contextual rather than the enzyme's core biochemical function.

Annotation notes / GOA review reasoning

• GO:0004656 procollagen-proline 4-dioxygenase activity — core MF. EXP/PMID:14500733 (direct), plus IBA and IEA(EC/RHEA) support. ACCEPT.
• GO:0016222 procollagen-proline 4-dioxygenase complex (part_of, IBA) — the alpha2-beta2 (with PDI) tetramer. ACCEPT (the curated CC for the prolyl-4-hydroxylase complex).
• GO:0005506 iron ion binding (IEA, InterPro) — Fe(II) cofactor; correct, supports catalysis. ACCEPT.
• GO:0016705 oxidoreductase activity, acting on paired donors... O2 (IEA, InterPro) — parent dioxygenase MF; correct but generic relative to GO:0004656. ACCEPT (general MF).
• GO:0031418 L-ascorbic acid binding (IEA, keyword/InterPro) — ascorbate cosubstrate; correct. ACCEPT.
• GO:0005783 endoplasmic reticulum (IEA) and GO:0005788 ER lumen (IEA + 2x Reactome TAS) — correct compartment. ACCEPT.
• GO:0005515 protein binding (IPI x4 references) — all four references (PMID:25416956, 26871637, 31515488, 32296183) are large-scale binary interactome / Y2H reference maps. The partner lists are dominated by biologically unrelated proteins (KRTAPs, MAGEs, transcription factors, etc.); these are uninformative bare protein-binding captures, not the functional PDI/collagen interactions. Per curation guidance, keep as non-core. None elevated to core; do not REMOVE (real IntAct captures).
• 25416956 = "A proteome-scale map of the human interactome network" (HuRI/Y2H) PMID:25416956.
• 26871637 = "Widespread Expansion of Protein Interaction Capabilities by Alternative Splicing" (Y2H isoform interactome).
• 31515488 = "Extensive disruption of protein interactions by genetic variants..." (Y2H variant interactome).
• 32296183 = "A reference map of the human binary protein interactome" (HuRI).
• No experimental cancer-context BP annotations are present in this GOA; GO:0030199 collagen fibril organization (IBA) appears in the UniProt DR cross-references but is NOT in the goa.tsv, so it is not part of existing_annotations here.

Cited reference quality summary

• PMID:14500733 — HIGH relevance, VERIFIED (founding biochemical characterization; abstract-only cache but anchored by EC/Rhea reaction and EXP GOA evidence).
• PMID:25416956 / 26871637 / 31515488 / 32296183 — LOW relevance (bare protein binding from HT interactome screens), VERIFIED as correctly-cited interactome papers.
  

Falcon deep-research findings (incorporated 2026-06)

• P4HA3 is overexpressed in clear cell renal cell carcinoma (ccRCC); knockdown inhibits proliferation,
  migration and invasion, and GSEA/Western blot implicate EMT and PI3K/AKT/GSK3beta signaling. This is
  the most P4HA3-specific functional-perturbation evidence in a tumor context.
  PMID:36588128
• Pan-cancer (33 tumor types) analysis with experimental validation: P4HA3 expression tracks tumor-
  microenvironment infiltration, proliferation and EMT markers; P4HA3 depletion inhibits proliferation/
  migration/invasion and promotes anti-tumor PD-1/PD-L1 immunotherapy response (incl. TNBC PDX models).
  PMID:39504328
• Gastric cancer: elevated P4HA3 associates with adverse prognosis and immune-microenvironment features
  (immune infiltration, TMB, MSI, immune checkpoints); proposed as an immunotherapy-response biomarker.
  Correlative/bioinformatic. PMID:39362976
• HNSCC family-level study (all three C-P4HA alpha subunits, including P4HA3): combined higher expression
  is prognostically adverse and antagonized by LLGL2 via occludin; C-P4HA knockdown reduces migration/
  invasion. PMID:39394821
• Shi et al. 2021 review consolidates the three-isoenzyme C-P4H framework (P4HA1/2/3, single P4HB beta),
  regulation by cytokines/TFs/miRNAs, tumor overexpression, and C-P4H inhibitors as anti-tumor strategy.
  PMID:34009234
• Falcon reiterates (review-level) that P4HA3-specific substrate preferences/kinetics remain less defined
  than P4HA1/2 and P4HA3 mRNA is low-abundance, consistent with the existing review's framing and
  suggested_questions. No new primary biochemical-substrate data for P4HA3 in the report.
• All added cancer references are non-core ECM-remodeling/EMT/tumor-context findings; they do not change
  the core catalytic annotation (procollagen-proline 4-dioxygenase activity, EXP PMID:14500733) and were
  not added as supporting_text to core_functions (papers not in publications/ cache).