Heme oxygenase

Heme oxygenase,

The enzyme has been the subject of extensive investigation into its regulatory signaling, immunomodulatory, and cryoprotective roles.[11] HMOX1 is an essential enzyme. Human HMOX1-deficiency is rare, however several cases have been reported which generally results in death.^[12]

In certain diseases, HMOX is problematic.^[13][14] For example, HMOX1 may counteract certain chemotherapeutic drugs to rescue cancer cells from cytotoxic drugs thereby enabling cancer progression.^[15] HMOX1 inhibitors are in development.^[16]

Heme oxygenase 2

Whereas HO-1 has innumerable inducers, only adrenal glucocorticoids are known to induce HO-2^[12] whereas certain other molecules may increase its catalytic velocity.^[9] Opioids may inhibit HMOX2 activity.^[9] Many drugs that activate and inhibit HO-2 are in development.^[18]

Heme oxygenase 3

A controversial third heme oxygenase (HO-3) is considered to be catalytically inactive and has been speculated to work in heme sensing or detoxification. HO-3 is 33 kDa with greatest presence in the liver, prostate, and kidneys. However, attempts to isolate HO-3 yielded pseudogenes derived from HO-2 transcripts thereby raising questions about the existence of a third isoform.^[9]

Microbial heme oxygenase

Heme oxygenase is conserved across phylogenetic kingdoms.^[19] The human microbiome contains dozens of unique microbial HMOX homologues which use many different abbreviations exemplified by:^[9]

• HMX1 in Saccharomyces cerevisiae
• HmuO in Corynebacterium diphtheriae
• ChuS in commensal strains of Escherichia coli

A critical role of the

Some heme-degrading prokaryotic enzymes produce products such as formaldehyde rather than CO. As an example, certain pathogens such as Escherichia coli O157:H7 can express the non-CO producing ChuW isoform. Many pathogens are susceptible to CO toxicity, therefore expressing non-CO producing heme degradation enzymes evades self-inflicted toxicity while meeting nutritional iron needs. Commensal microbiota generally have CO tolerance as they produce and respond to CO signals; upon excretion from the microbe, the CO either directly benefits the host or applies selection pressure against pathogens thereby serving as a symbiotic currency.^[9]

Plant heme oxygenase

Plants contain HMOX homologues with critical roles in plant physiology.^[21] Although chlorophyll is structurally similar to heme, it is unclear if any HMOX-like enzymes are capable of facilitating metabolism.^[9]

Quasi-enzymatic heme oxidation

As heme oxygenase is an enzymatic catalyst that accelerates the slow natural oxidation of heme, non-enzymatic oxidative degradation of heme, commonly termed 'coupled oxidation', was proposed as early as 1949. Akin to HMOX, coupled oxidation occurs at the alpha-methine bridge and leads to formation of biliverdin although the reaction's stoichiometry is different.^[22] The first attempt to describe HMOX in 1962 by Nakajima turned out to be a non-enzymatic pathway. The complexity of the non-enzymatic pathway has been coined quasi-enzymatic or pseudoenzymatic.^[22] A variety of mechanisms have been proposed.^[22][23]

Reaction

HMOX1 is the rate-limiting step of heme catabolism that is dependent on NADPH-cytochrome P450 reductase and oxygen to cleave heme/porphyrin ring at the alpha-methene bridge to form biliverdin (or verdoglobin if the heme is still intact as hemoglobin). The reaction comprises three steps, which may be:^[24]

Heme b³⁺ + O
₂ + NADPH + H⁺
→ α-meso-hydroxyheme³⁺ + NADP⁺
+ H
₂O

α-meso-hydroxyheme³⁺ + H⁺
+ O
₂ → verdoheme⁴⁺ + CO + H
₂O

verdoheme⁴⁺ + 7/2 NADPH + O
₂+ 3/2 H⁺
 → biliverdin + Fe²⁺ + 7/2 NADP⁺
+ H
₂O

The sum of these reactions is:

Heme b³⁺ + 3O
₂ + 9/2 NADPH + 7/2 H⁺
 → biliverdin + Fe²⁺ + CO + 9/2 NADP⁺
+ 3H
₂O

If the iron is initially in the +2 state, the reaction could be:

Heme b²⁺ + 3O₂ + 4 NADPH + 4 H⁺ → biliverdin + Fe²⁺ + CO + 4 NADP⁺ + 3H₂O

This reaction can occur in virtually every cell; the classic example is the formation of a

HMOX1 is inhibited by certain porphyrins such as zinc protoporphyrin.^[29]

Roles in physiology

HMOX is involved in numerous cellular operations.^[30][31] The cyto-protective benefits of HMOX has stimulated significant research into its therapeutic and pharmaceutical potential.^[32] These effects have not been verified in clinical trials.^[33][8]

Carbon monoxide

HMOX is the main source of endogenous CO production,^[33] though other minor contributors have been identified in recent years. CO is formed at a rate of 16.4 μmol/hour in the human body, ~86% originating from heme via heme oxygenase and ~14% from non-heme sources including: photooxidation, lipid and keto acid peroxidation, microbiome, and xenobiotics.^[9] The average carboxyhemoglobin (CO-Hb) level in a non-smoker is between 0.2% and 0.85% CO-Hb (whereas a smoker may have between 4% and 10% CO-Hb), though genetics, geographic location, occupation, health and behavior are contributing variables.

Erythrocyte recycling in the spleen accounts for ~80% of heme-derived endogenous CO production. The remaining 20% of heme-derived CO production is attributed to hepatic catabolism of

In addition to being a source of carbon monoxide, heme is a critical signal transducer involved in carbon monoxide sensing.[34]^[35] As a signaling agent, carbon monoxide is involved in normal physiology and has therapeutic benefits in many indications such as ameliorating inflammation and hypoxia.^[33][36] It remain under investigation, however, to what extent HMOX is involved in carbon monoxide's protective effect against hypoxia as 3 molar equivalents of oxygen are required to produce carbon monoxide from heme catabolism, along with the question of heme bioavailability,^[37] and slow HMOX1 induction which may take several hours (e.g. the slow healing of a bruise).^[38]

Biliverdin / bilirubin

Ancient documentation for endogenous bilirubin traces back to medical humours written by Hippocrates.^[39]

In most cases, HMOX selectively cleaves heme (iron protoporphyrin IX) at the α-methine bridge. The resulting bilirubin contains the suffix IXα to identify the composition of its structure by indicating its parent molecule was protoporphyrin IX cleaved at the alpha position (see protoporphyrin IX for further information on the Fischer nomenclature system). Drosophila melanogaster contains a unique HMOX that is not alpha specific resulting in formation of biliverdin IXα, IXβ, IXδ.^[5] Non-enzymatic oxidation of heme is likewise non-specific resulting in ring opening at the α, β, γ, or δ positions.^[22]

HMOX1 was first characterized by Tenhunen and Rudi Schmid upon demonstrating it as the enzyme responsible for catalyzing biotransformation of heme to bilirubin.^[12]

Several labs attempted to explain the biotransformation of heme to biliverdin such as Nakajima et al. in 1962 who characterized a soluble "heme α-methenyl oxygenase", however the findings could not be reproduced and alternative non-enzymatic explanations for their observation emerged. The earliest evidence of oxidative enzymatic biotransformation of heme to a bilin was demonstrated by Hans Plieninger and Hans Fischer in 1942.^[45] The discovery of HMOX is a unique case of academic lineage as Fischer was the academic adviser for Cecil Watson, and Watson was an adviser for Rudi Schmid.