General Information
Heme Oxygenase (HO) is a member of the Hemoprotein family and catalyzes the Oxygen-dependent cleavage of the porphyrin ring of heme, using reducing equivalents like NADH to produce biliverdin, iron and CO [1]. HO consists of two main isoforms which are present in mammals, HO-1 and HO-2. The two isoforms are products of different genes, are different molecular sizes (32 kDa and 36 kDa respectively) and contain a different primary structure showing only 58% homology [2]. However studies have shown that the two isoforms share a region with 100% secondary structure homology which is believed to be the catalytic site of the protein[1]. The heme oxygenase isoforms are not free throughout the body but sequestered to certain tissues. The Heme oxygenase -1 is strongly expressed in the spleen and liver whereas Heme Oxygenase-2 is strongly expressed in the brain, testis and vascular systems[3].
Ligand
HO non-covalently binds to a ligand known as a heme group more specifically Heme B. This Heme is the most abundant heme group commonly recognized for its role in oxygen transport and storage within mammalian tissues [4]. This group is contains a large heterocyclic ring known as a porphyrin ring with an iron atom in the center. The center iron atom serves as a source of electrons for the redox reaction to occur [4]. However this group is susceptible to damage from many stressors including physical shock and therefore needs to be broken down or recycled when theses stressors occur [1].
Structure
HO is a 233 residue protein with a secondary structure consisting of which interacts with a [1] at the optimum pH of 7.4; at 37 degrees C [5]. The heme is sandwiched between two helices termed the [6]. The proximal helix provides the His 25 heme ligand along with the various contact residues (Ala 28 and Glu 29), but also Thr 21 which contacts the heme through a water molecule [1]. On the distal side where the ligands binds (the catalytic site) there is a highly conserved sequence of Glycine residues () that provide a required flexibility for the reaction to occur [1]. This results in the backbone atoms of Gly 139 and Gly 143 to directly contact the heme. Inhibition of HO is provided by compounds such as imidazole-dioxolane which disrupt this flexibility, thereby forcing the HO protein to become rigid, stopping its function [7].
Of the four meso edges of the heme only one remains exposed () while the rest remain buried in the protein [2]. The exposed edge is the target of the HO reaction and requires the correct orientation for the hydroxylation reaction to occur[6]. This orientation is aided by the charges associated with the on the heme. The residues of are all near the propionates to anchor it via non convalent bonds so the a-meso carbon is in position for the hydroxylation reaction[8]. The propinates are located on the opposite side as the α-meso edge. The vinyl and methyl heme substituents do not appear to be important in orienting the heme due to the fact that they may be disordered about the y- axis which would change only the location of the methyl and vinyl groups while retaining the position of the propionates and the a-meso edge [8]. .
Function
Figure 1: Heme oxygenase reaction converting Heme to Billiverdin
Heme Oxygenase has two main functions, firstly it recycles iron supplies within the cell to maintain homeostasis and secondly it produces a product (biliverdin) that can be converted to a powerful antioxidant (bilirubin) which can aid in preventing oxidative cell damage [6]. The overall reaction consists of three sequential oxidation steps. In the first step the Oxygen bound to the heme iron is activated to become hydroperoxide. The production of α-hydroxyheme is accomplished by the electrophilic addition of its terminal oxygen to the α-meso carbon [6]. Then Heme Oxygenase converts α-hydroxyheme to verdoheme with the removal of the CO at the α-meso carbon (approximately 85% of CO produced under normal conditions is from this reaction)[6]. Lastly, the oxygen bridge of verdoheme is cleaved to produce biliverdin-iron chelate before the dissociation of the iron to biliverdin. (Figure 1) The electrons required for catalytic turnover of the enzyme are provided, in mammalian systems, by NADPH-cytochrome P450 reductase[6]. Studies suggest that the CO produced by the heme oxygenase reaction also functions to have anti-inflammatory, anti-proliferative and anti-apoptotic effects [7] to prevent cell damaage.
A classic example of this reaction is a bruise. When tissue obtains a hard hit the erythrocytes release the heme creating a Red color. The heme then gets converted to biliverdin via heme oxygenase to produce a green color. Finally the conversion of biliverdin to bilirubin displays a yellow color. Therefore the reaction can be visibly observed [9]..
Medical Significance and Future Implications
Figure 2: Neutralizing oxidant attack with antioxidants produced through the heme oxygenase reaction
The lungs are a major target for various inflammatory, oxidative, carcinogenic and infectious pressures, which have the ability to result in a range of lung diseases like chronic obstructive lung diseases (COLD)[10]. The Induction of HO-1 is a crucial defense mechanism during these acute and chronic lung processes. The defense is obtained from the anti-oxidant, anti-inflammatory and anti-apoptotic properties of the products formed from the HO reaction [10]. (Figure 2) Therefore manipulation of the HO reaction and HO can have immense therapeutic potential against a range of lung diseases, if optimal levels of incorporation can be achieved.
The products of the HO reaction are not just beneficial for lung diseases but also have a cytoprotective effect through the p38-MAPK pathway, and are a potential therapeutic treatment in cancer[1]. Recent studies have also shown that inhibition of the HO-1 reduces Kaposi sarcoma tumor growth [1]. Therefore future implications on regulating and manipulating this protein can have a massive impact on medical treatments.
Additional Resources
For additional information, see:
3D structures of heme oxygenase
Heme oxygenase 3D structures
