Heme Oxygenase-1 (HO-1), also known as heme oxygenase (decycling) 1 (HMOX1), is a 32 kDa inducible enzyme that catalyzes the rate-limiting step in heme degradation. HO-1 degrades heme into biliverdin (subsequently converted to bilirubin), carbon monoxide (CO) — a signaling molecule, and free iron (Fe²⁺) sequestered by ferritin.
HO-1 is a critical cytoprotective enzyme induced by oxidative stress, inflammation, hypoxia, and heme accumulation. It plays a vital role in neuroprotection and is implicated in Alzheimer's, Parkinson's, ALS, and other neurodegenerative disorders.
| Heme Oxygenase-1 (HO-1) | |
|---|---|
| Gene | [HMOX1](/genes/hmox1) |
| UniProt ID | [P09601](https://www.uniprot.org/uniprot/P09601) |
| PDB Structure | 1T40, 1T41, 1T42 |
| Molecular Weight | 32,800 Da |
| Length | 288 amino acids |
| Subcellular Localization | Endoplasmic reticulum (membrane) |
| Protein Family | Heme oxygenase family |
The HMOX1 gene is located on chromosome 5q33.1 (5q31.2-p33.1) and encodes a 288-amino acid protein. Key structural features include:
| Feature | Description |
|---|---|
| N-terminal transmembrane domain | ~23 amino acids, ER membrane anchor |
| Conserved heme-binding pocket | His-25, His-82 are critical for heme binding |
| Substrate access channel | Regulates heme access to active site |
| C-terminal catalytic domain | Contains the heme degradation activity |
HO-1 catalyzes heme degradation through a 3-step process consuming O₂ and NADPH:
HO-1 is expressed in multiple cell types in the central nervous system:
HO-1 expression is induced by:
HO-1 provides neuroprotection through multiple mechanisms:
The free iron released by heme degradation is highly reactive through Fenton chemistry:
Fe²⁺ + H₂O₂ → Fe³⁺ + •OH + OH⁻
However, HO-1 simultaneously induces ferritin (FTL, FTH1), which sequesters this iron safely. This coupling of heme degradation with iron sequestration is essential for neuroprotection.
Carbon monoxide acts as a signaling molecule through:
In Alzheimer's disease, HO-1 exhibits complex regulation:
Elevated HO-1 is observed in:
Mechanisms:
HO-1 is strongly induced in Parkinson's disease:
The LRRK2-HO-1 pathway is implicated in PD pathogenesis:
Therapeutic potential:
In ALS:
HO-1 regulation:
├── [Nrf2](/entities/nrf2) → ARE-mediated transcriptional activation
├── [NF-κB](/entities/nf-kappa-b) → Pro-inflammatory suppression
├── [MAPK pathways](/proteins/mapk1-protein) → Stress response kinase cascades
└── [p38 signaling](/proteins/mapk14-protein) → Cell survival signaling
HO-1 products:
├── Biliverdin → Bilirubin (antioxidant)
├── CO → cGMP signaling (anti-apoptotic, anti-inflammatory)
└── Fe²⁺ → Ferritin sequestration (prevents Fenton chemistry)
| HO-1 Product | Primary Effect | Neuroprotective Mechanism |
|---|---|---|
| Biliverdin | Antioxidant | Scavenges peroxyl radicals |
| Bilirubin | Antioxidant | Neutralizes peroxynitrite |
| CO | Signaling | Anti-apoptotic, anti-inflammatory |
| Fe²⁺ | Ferritin induction | Prevents iron-mediated ROS |
| Compound | Mechanism | Status | Reference |
|---|---|---|---|
| Hemin | Nrf2 activation | Clinical | [5] |
| Curcumin | Nrf2 activation | Preclinical | — |
| Statins | MEK/ERK dependent | Clinical | — |
| CBD (cannabidiol) | VDR/Nrf2 dependent | Phase 2 | — |
| Sulforaphane | KEAP1-Nrf2 | Phase 1 | — |
| Compound | Mechanism | Status |
|---|---|---|
| Tin protoporphyrin IX | Competitive inhibitor | Research |
| Zinc protoporphyrin IX | Competitive inhibitor | Research |
| Pegylated Zinc PP-IX | Improved delivery | Preclinical |
| Trial | Compound | Phase | Status | Disease |
|---|---|---|---|---|
| NCT01716594 | Hemin | Phase 2 | Completed | Acute brain injury |
| NCT03729768 | Hemin | Phase 1 | Recruiting | PD |
| NCT04561072 | CDDO-Im | Phase 2 | Active | AD |
| SNP | Effect | Disease Association |
|---|---|---|
| rs2071746 (promoter) | Altered HO-1 expression | PD risk |
| rs2071747 | Modified oxidative stress response | AD risk |
| rs5995178 | Altered promoter activity | ALS risk |
| Model | Application | Reference |
|---|---|---|
| Hmox1-/- mice | HO-1 knockout | Lethal (embryonic) |
| Hmox1+/- mice | Haploinsufficiency | Oxidative stress sensitive |
| TG-HMOX1 | Neuron-specific overexpression | Protected in PD/AD models |
Cutler RG, et al. HO-1 and tau pathology in AD. 2014. ↩︎
Song SY, et al. Nrf2-HO-1 pathway in Parkinson's disease. 2017. ↩︎
Espey MG, et al. Hemin as neuroprotective in PD. 2018. ↩︎
Drew W, et al. HO-1 in ALS models. 2018. ↩︎
Barone E, et al. HO-1 induction in stroke and neuroprotection. 2011. ↩︎
Yang Y, et al. HO-1 gene therapy approaches. 2023. ↩︎
Chen PL, et al. HO-1 modulation in clinical trials. 2024. ↩︎