Heme Oxygenase: Family Members

Heme Oxygenase Family

HO-1 and its mammalian isoform HO-2 are representatives of the heme oxygenase (HO) family of enzymes, also known as HSP32 family. HO-2 is constitutively expressed under basic requirements in the majority of human tissues, while HO-1 represents the inducible HO variant which is commonly expressed at low levels in most tissues but whose expression is highly up-regulated upon exposure to different kinds of stress 2-4. Recently, a novel splice variant of HO-1, harboring a 164-aa deletion (aa49 – 212 of 32 kDa HO-1) and a molecular mass of 14 kDa, was identified in human malignant cells 6. HO family members are encoded by two different functional protein-coding genes in humans (HMOX1, HMOX2) 82 clustered on chromosome 22q12.3 and 16p13.3, respectively. HO-3 (HMOX3) is supposed to be a pseudogene resulting from alternative splicing of the HMOX2 mRNA 40 and referred to as Hmox2-ps1 and Hmox3, respectively. Expression of Hmox3 could not be detected at the mRNA level in a genomic DNA-free liver library, neither at the protein level in kidney from rats 40. Hmox3 represents a single-exon gene in rats which is absent in other mammals.


Several organisms have been found to express HOs. However, there is divergence with respect to structural and functional properties reflecting a diversity of responsibility. Based on these characteristics, two groups of HOs have been classified. The first discovered group of HOs is the HO-1 subfamily comprising canonical HOs that are present in both, prokaryotes and eukaryotes including, e.g., mammalian HO-1/-2 as well as microbial HmuO and PigA/HemO 1, 2, 41, 57, 67, 84. Members of this group belong to the alpha-only protein class and share a common secondary structure bearing nine to ten α-helices and a highly conserved catalytic site in the cytoplasmic domain 76. They degrade heme to biliverdin IXα, carbon monoxide (CO), and ferrous iron (Fe2+) 1. The second group of HOs which can predominantly be found in a limited number of bacterial species contains non-canonical and IsdG-like HOs (e.g., IsdG/l from S. aureus and MuhD from M. tuberculosis) converting heme to metabolites different from biliverdin and CO, such as staphylobilin, formaldehyde and mycobilin 48, 87, 88. Several members of the IsdG subfamily are selectively expressed under low-iron conditions. The founding members of this group, IsdG/I, and also MuhD belong to the class of α + β proteins as part of the dimeric α + β-barrel superfamily that dimerize across their β-sheets 89. In E. coli, the IsdG subfamily member ChuW has been reported to anaerobically degrade heme to the small molecule anaerobilin and free iron 90. It is worth mentioning that in a most recent study an HO-like enzyme, termed Lfo-1, was identified in the unicellular green alga Chlamydomonas reinhardtii bearing an antibiotic biosynthesis mono-oxygenase (ABM) domain typically found amongst IsdG subfamily members 11. This HO-like protein aerobically degrades heme to a distinct unidentified heme metabolite, its predicted secondary structure resembles those of the IsdG subfamily members, and it harbors the functionally conserved catalytic residues found in all HOs of the IsdG subfamily.


Heat Shock Protein Superfamily

Heme oxygenase 1 (HO-1) is an ubiquitiously expressed stress response molecule and a member of the huge heat shock protein (HSP) superfamily. HSPs are classified into various subgroups according to their molecular weights. The main HSP families described so far are HSP40, HSP60, HSP70, HSP90, HSP110, and small HSP 91.


The HSP40/DNAJ family represents a heterogeneous group of co-chaperones characterized by the presence of the remarkably conserved J-domain, responsible for the regulation of the ATPase activity of HSP70s 91-95. HSP40s/DNAJs are homodimeric proteins residing in the cytosol of prokaryotes and various subcellular compartments of eukaryotes as well as in the extracellular milieu 96-101. Members of the DNAJ/HSP40 family have been preserved during evolution and play essential roles in gene expression and translational initiation, folding and unfolding as well as translocation and degradation of proteins 102, 103 (see also HSP40 Scientific Resource Guide).


The HSP60/CPN60 (HSPD) family represents a group of well-conserved proteins that consists of molecular chaperones of approximately 60 kDa in size comprising stress inducible and constitutively expressed members 104. HSP60 chaperones can be found not only in the cytosol, chloroplasts, hydrogenosomes and mitochondria 105 but also on the cell surface and in the extracellular milieu 106 where they serve as danger signals of stressed or damaged cells and act as potent stimulators of immune responses 107, 108. HSP60s interact with many polypeptides in an ATP-dependent manner and possess a peptide-dependent ATPase activity 109. Intracellular HSP60s not only play an essential role in protein folding and trafficking 110, 111, but also in peptide-hormone signaling 112 as well as in pro-apoptotic and pro-survival pathways 113. Hsp60 acts as an etiologic and pathogenic factor in both, inherited and acquired pathological conditions. With respect to cancer, Hsp60 plays an ambiguous role in tumorigenesis as Hsp60 elicits both, pro-survival and pro-apoptotic functions in tumors. As the level of Hsp60 in biological fluids is currently being associated with a plethora of clinical conditions, the chaperone has potential utility as biomarker for diagnosis and assessing prognosis or response to therapeutic intervention. For more information, please visit: HSP60 Scientific Resource Guide.


Members of the HSP70 (HSPA) family of chaperones represent one of the most ubiquitous classes of chaperones and can be found not only in eukaryotic cytosol, membranes, chloroplasts, ER and mitochondria but also in the extracellular milieu as well as in bacteria and certain archaea where they fulfill specific organellar or tissue-specific functions 98, 114-116. Multiple Hsp70 isoforms are frequently co-expressed in the same cytosol encoded by distinct genes. All of the cellular activities of HSP70s depend on their ATP-regulated ability to interact with exposed hydrophobic surfaces of proteins. Depending on their cellular localization HSP70s mediate different functions. Intracellular residing HSP70s protect cells against lethal damage induced by stress, and support folding and transport of newly synthesized polypeptides and aberrant proteins as well as the assembly of multi-protein complexes 117. Extracellular HSPs are considered as molecules with immunomodulatory functions 107, 108, either as cross-presenters of immunogenic peptides via MHC antigens 118, 119 or in a peptide-free version as chaperokines 120 or stimulators of immune responses 121. For more detailed information see also Radons (2016) 122 and Zuiderweg et al. (2017) 123 as well as HSP70 Scientific Resource Guide.


HSP90 proteins define a widespread family of molecular chaperones (HSPC family) found in bacteria and all eukaryotes playing a fundamental role in protein homoeostasis and viability 124. HSP90s primarily exist as homodimers whose activities are regulated by ATP. HSP90 chaperones can be found not only in the cytosol, ER, chloroplasts, mitochondria, and the nucleus 125-127 but also in the extracellular milieu 107, 108. While intracellular HSP90s play a key role in proteomic homeostasis, membrane-bound and extracellular HSP90s have been shown to act as potent stimulators of immune responses 107, 108. For a review see Radons (2016) 124, Schopf et al. (2017) 128, and Hoter et al. (2018) 129 as well as HSP90 Scientific Resource Guide.


The family of small HSPs (HSPB family) consists of 11 members characterized by the presence of a conserved a-crystallin signature domain enclosed by variable N- and C-termini 91. Small HSPs often form oligomeric complexes involving one or more family members, thereby providing the cell with a large diversity of chaperone activities. Whereas single members (HSPB1, HSPB5, HSPB6, HSPB8) are ubiquitously expressed in nearly any cell type, others (HSPB3, HSPB4, HSPB7, HSPB9, and HSPB10) are less frequently expressed with high expression levels exclusively found in certain tissues 130-133. Expression of sHSPs is inducible by most of the stressors that activate the so-called heat shock response, and also in the absence of stress. However, all members of the HSPB family are essential for cell physiology under normal and stressful conditions (see also HSP27 Scientific Resource Guide).


The HSP110 (HSPH) family encompasses four gene products with high homology to HSP70 family members 91. Hsp110 (HspH1), also known as Hsp105, is one of the central eukaryotic HSPs. Two isoforms of Hsp110 have been characterized: (i) the constitutively expressed cytosolic Hsp105α, and (ii) the strictly heat-inducible nuclear Hsp105β representing an alternatively spliced isoform of Hsp105α 134, 135. Grp170 (HspH4) is the ER-resident representative of this family of large HSPs and was initially characterized as being inducible by glucose starvation 136. Together with HspH2 (HspA4, Apg-2) and HspH3 (HspA4L, Apg-1), Hsp110 and Grp170 serve the function of nucleotide exchange factors for the HSP70 (HSPA) family 137, 138. For a review, see Bracher and Verghese (2015) 139 and Zuo et al. (2016) 140.


Table 2: Heme oxygenases of various pro- and eukaryotic organisms

Protein UniProt ID Aliases Length (aa) Chromosomal location Gene ID Gene names
HO-1 P09601 Heme oxygenase 1; heat shock protein 32 kDa (Hsp32); p32; heme oxygenase (decycling) 1; NP_002124.1 288, 124 22q12.3 3162 HO1; HSP32; HMOX1D; HO-1; bK286B10; HMOX1
HO-2 P30519 Heme oxygenase 2; Hmox-2; heme oxygenase (decycling) 2; NP_001120676.1; XP_011520775.1 316, 287 16p13.3 3163 HO2; HMOX2;
Rattus norwegicus
HO-1 P06762 Heme oxygenase 1; Hsp32; HSP32; heme oxygenase (decycling) 1; NP_036712.1 289 19p11 24451 Hmox1; Ho1; Heox; Hmox; Ho-1; HEOXG; hsp32
HO-2 P23711 Heme oxygenase 2; heme oxygenase (decycling) 2; heme oxygenase-2 non-reducing isoform; NP_001264002.1; XP_006245889.1 315 10q12 79239 Hmox2; Ho-2; Ho2
HO-3 O70453 Heme oxygenase 3; putative heme oxygenase 3; Hmox-3; heme oxygenase (decycling) 3; heme oxygenase 2, pseudogene 1; Hmox2-ps1; heme oxygenase (decycling) 2, pseudogene 1


290 2q41 100359946 heme oxygenase (decycling) 2, pseudogene 1; HO3; Hmox3; Hmox2-ps1
Arabidopsis thaliana
AtHO-1 O48782 Heme oxygenase 1, chloroplastic; AtHO1; protein genomes uncoupled 2; protein REVERSAL OF THE DET PHENOTYPE 4; HY1; 282, 234 Chr2; NC_003071.7


817208 HO1; GUN2; HY1; HY6; TED; hy1 At2g26670; F18A8.4
AtHO-2 O48722 Heme oxygenase 2, chloroplastic; AtHO2; HY2; probable inactive heme oxygenase 2, chloroplastic 299, 354, 314, 284 Chr2; NC_003071.7 817196 HO2; hy2; At2g26550; T9J22.22
AtHO-3 Q9C9L4 Heme oxygenase 3, chloroplastic; AtHO3; HY3; NP_001117574.1; NP_177130.1 285, 227 Chr1; NC_003070.9


843308 HO3; hy3; At1g69720; T6C23.8
AtHO-4 Q9LQC0 Heme oxygenase 4, chloroplastic; AtHO4; NP_176126.1; HY4 283 Chr1; NC_003070.9


842199 HO4; hy4; At1g58300; F19C14.8
Saccharomyces cerevisiae
Hmx1p P32339 Heme oxygenase; heme-binding protein HMX1; NP_013306.2 317 Chr12; NC_001144.5 850902 HMX1
Candida albicans  
Hmx1p Q5AB97 Heme oxygenase; heme-binding protein HMX1; XP_719035.1 291 Chr1; NC_032089.1 3639294 HMX1
Rhodella violacea (Red alga)
PbsA O19998 Heme oxygenase 235 unspecified ? pbsA
Chlamydomonas rheinhardtii
Hmox-1 A8ICF4 Heme oxygenase; HO-1; HMOX1; HMOX-1; XP_001702583.1 308 unspecified 5728227 HMOX1; CHLREDRAFT_195947
Hmox-2 A8JBZ0 Heme oxygenase; HO-2; HMOX2; HMOX-2; XP_001699515.1 271 unspecified 5725028 HMOX2; CHLREDRAFT_152591
Lfo-1 Putative heme oxygenase LFO1 171 unspecified ? LFO1; Cre07.g312300


HemO Q9RGD9 Heme utilization protein 230 NC_003112.2 903441 hemO; NMA510612_2169
Corynebacterium diphtheriae  
HmuO P71119 Heme oxygenase; WP_014319205.1; biliverdin-producing heme oxygenase 215 NZ_LN831026.1 29421545 hmuO; AT687_RS08085; ERS451417_01679
Corynebacterium jeikeium  
HmuO Q4JXI5 Heme oxygenase 224 NC_007164.1 3433186 hmuO; JK_RS01625
Staphylococcus aureus
IsdG Q5HGU8 Heme oxygenase 1; heme-degrading monooxygenase 1; iron-regulated surface determinant 1; YP_499634.1 107 NC_007795.1 3919250 isdG; SAOUHSC_00130
IsdI Q5HJK5 Heme oxygenase 2; heme-degrading monooxygenase 2; iron-regulated surface determinant 2; YP_498730.1 108 NC_007795.1 3919839 isdI; SACOL0152
Pseudomonas aeruginosa  
PigA O69002 Biliverdin-producing heme oxygenase; heme oxygenase; iron starvation protein PigA; NP_249363.1; HemO 198 NC_002516.2 880820 pigA; hemO; C0046_0440; C8257_2632; DI492_28485; PAMH19_3887; RW109_RW109_05557
BphO Q9HWR4 Heme oxygenase; NP_252805.1 195 NC_002516.2 879778 bphO
PhuS A0A0H2ZI03 Putative hemin degrading factor 354 5,559,904-5,560,968


PA14_62330 phuS
E. coli  
ChuS A0A0H3MLZ2 Heme oxygenase; heme/hemoglobin transporter; hemin transporter; hemin-degrading factor; putative hemin-degrading protein; YP_002409893.1 342 NC_011750.1 7154115 chuS
ChuW A0A384LP51 Anaerobilin synthase 445 unspecified 2827929 NEWENTRY; chuW
Euryarchaeota archaeon  
Ho A0A2E9PUL4 Heme oxygenase; HO 225 unspecified ? CMA59_00105; ho