Heme Oxygenase: Figures

Figure 1. Primary structures of human heme oxygenase

Crystal structure of human heme oxygenase 1 (HO-1) in complex with heme. The cartoon shows one copy of HO-1 complexed to one copy of protoporphyrin IX containing Fe (Heme) and one copy of sulfate ion (PDB 1n45) ⁶⁰.

 

Figure 2. Primary structures of human heme oxygenase

Overall structure of homodimeric HO-1. The cartoon shows two copies of HO-1 complexed to two copies of protoporphyrin IX containing Fe (Heme) and one copy of sulfate ion (PDB 1n45) ⁶⁰.

 

Figure 3. Primary structures of human heme oxygenases

The cartoon shows the domain architecture and critical amino acids in human HO-1 and HO-2. The conserved histidine in the catalytic domain (His25 in HO-1 and His45 in HO-2) serves as the proximal heme ligand. Within the catalytic core, the conserved His132 in HO-1 as well as His151 in HO-2 are required for heme oxidation. Black boxes in HO-2 indicate the heme regulatory motifs (HRMs). The C-terminal transmembrane segment (TMS) is highlighted in grey. Image was reproduced, with permission, from Muñoz-Sánchez and Chanez-Cardenas (2014) ¹⁵⁰.

 

Figure 4. Oxidation of heme to biliverdin.

Heme oxygenases (HO-1, HO-2) catalyze the NADPH- and O₂-dependent degradation of heme to biliverdin IXα, carbon monoxide (CO), and ferrous iron (Fe²⁺). The reaction requires the co-enzyme cytochrome p450 reductase (CPR). Afterwards, biliverdin is enzymatically converted to bilirubin by biliverdin reductase.  Both molecules serve as important anti-oxidants able to scavenge reactive oxygen species (ROS). CO functions as crucial second messenger implicated in several signal transduction pathways leading to the production of anti-inflammatory cytokines, up-regulation of anti-apoptotic effectors, and inhibition of thrombosis. Activation of HO-1 also up-regulates Fe²⁺-mediated ferritin expression which competitively inhibits Fe²⁺, thus detoxifying its pro-oxidant activity. Image was reproduced, with permission, from Chiang et al. (2018) ³¹¹.

 

Figure 5. Canonical and non-canonical functions of heme oxygenases (HOs) and their metabolites.

Image was reproduced from Vanella et al. (2016) ¹⁷. License at https://creativecommons.org/licenses/by/3.0/

 

Figure 6. The Nrf-1/Keap-1 pathway.

Under non-stress conditions, Nrf-2 is held in an inactive state in the cytoplasm in complex with the Kelch-like ECH-associated protein 1 (Keap-1) which blocks the nuclear translocation and determines the targeted ubiquitination of Nrf-2 by the cullin 3-based E3 ubiquitin ligase complex, thus identifying Nrf-2 for proteasomal degradation. Stress-induced phosphorylation and activation of Keap-1 facilitates the release and translocation of Nrf-2 to the nucleus, where it forms heterodimers with small Maf proteins followed by its binding to StRE/ARE sequences and the induction of gene transcription. The heme binding protein BTB and CNC homolog 1 (Bach-1) was recently identified as a transcriptional repressor of HMOX1. Under non-stress conditions, Bach-1 associates with small Maf proteins to form heterodimers that bind to the StRE/ARE sequences and thus competes with Nrf-2 for binding to StRE/AREs (not shown). Oxidative stress or increasing heme concentrations induce conformational re-arrangements of Bach-1 that displace Bach-1 from StRE/ARE sequences and enable its proteasomal degradation so that Nrf-2 can associate with Maf and bind to StRE/AREs within the regulatory sequences of cell defense genes. Image was reproduced from Jaramillo et al. (2013) ⁶³⁴. License at https://creativecommons.org/licenses/by-nc/3.0/

 

Figure 7. Proposed sequence of the heme oxygenase (HO) reaction.

HO catalyzes the cleavage of heme at the α-methene bridge carbon leading to the generation of biliverdin IXα, CO, and ferrous (Fe²⁺) iron. The HO reaction involves three serial mono-oxygenation cycles, each of which requiring one molecule of O₂ . The overall reaction necessitates seven electrons, provided most likely by  NADPH:cytochrome p450 reductase, for the reduction of the heme iron. Biliverdin IXα, released from the HO reaction, is enzymatically reduced by NAD(P)H:biliverdin reductase to form bilirubin IXα. For details, see text.

Figure 8. Role of HO-1 in ferroptosis.

For details, see text. Image was reproduced from Chiang et al. (2018) ³¹¹. License at https://creativecommons.org/licenses/by/4.0/

 

Fig. 9. Signaling pathways mediated by carbon monoxide (CO).

The scheme illustrates the pleiotropic effects of CO and the putative signaling networks involved. For details, see text.