Heme is a vital but highly reactive compound that is synthesized in mitochondria and subsequently distributed to a variety of subcellular compartments for utilization. The transport of heme is essential for normal cellular metabolism, growth, and development. Despite the vital importance of heme transport within the cell, data are lacking about how newly synthesized heme is shuttled within the mitochondrion or exported from the organelle. Here, we briefly summarize current knowledge about the process of mitochondrial heme distribution and discuss the current unresolved questions pertinent to this process.

Heme is a metal-containing hydrophobic organic compound, composed of a porphyrin ring with a central iron, either ferrous (Fe2+) or ferric (Fe3+), coordinated by the nitrogen atoms of the porphyrin. As a prosthetic group in proteins, heme is essential for a plethora of biological functions [1,2]. In eukaryotic cells, it plays crucial roles in gas sensing, oxygen transport and binding, NO synthesis, and electron transfer [3-6]. Additionally, heme has been shown to interact with several transcription factors and microRNA processing proteins, thus regulating gene expression. Through these activities, heme is involved in controlling a number of physiological functions, including iron homeostasis, mitochondrial respiration, mitophagy, apoptosis, circadian rhythm, cell proliferation, and stress responses [7-10]. Paradoxically, heme is highly reactive and inherently toxic in its free form, owing to its redox-active nature. Free or labile heme can intercalate into biological membranes due to its hydrophobicity and non-specifically bind to proteins. In its free form, heme can produce hydroxyl radicals, resulting in oxidative damage to cellular components [11-13]. Furthermore, heme biosynthetic intermediates are also reactive; thus, imbalances in heme synthesis, which result in substrate accumulation, can cause cellular and tissue damage, as observed in the porphyrias [14]. Therefore, heme biosynthesis and distribution necessitate tight control of the concentration and availability of heme and its intermediates, as well as the monitoring of import, synthesis, degradation, and export levels thereof.

Heme synthesis occurs in virtually all eukaryotic organisms except for certain parasites and nematodes [15-17]. For example, the roundworm Caenorhabditis elegans does not synthesize heme and thus acquires it through its diet [18]. The canonical heme synthetic pathway consists of eight steps that take place in both mitochondria and cytosol and has been recently reviewed [19]. One unique feature of the eukaryotic heme biosynthetic pathway is the compartmentalization of its initial and final steps to the mitochondria. The terminal step in heme biosynthesis is mediated by the enzyme ferrochelatase (FECH) that incorporates an iron atom into the protoporphyrin IX to produce protoheme [20], a b-type heme, which is a predominant heme species in cells. Subsequently, heme is rapidly and safely moved to multiple sites for utilization (Figure 1). Whereas some heme distribution routes, such as those leading to the formation of mitochondria-specific c- and a-type hemes and subsequent hemylation of cytochrome c and cytochrome c oxidase (CcO), respectively, are fairly well understood [5,21], others, including the pathways and proteins necessary for the distribution of heme b within and outside of the mitochondria, remain largely elusive. In this review, we will briefly summarize current knowledge about the process of mitochondrial heme transport, with a specific emphasis on the key factors involved in heme bioavailability. In addition, we will highlight the physiological significance of mitochondrial heme trafficking in different cellular environments and its implications in disease states. Finally, we will discuss the currently unresolved questions in the field and potential directions for future research.

Overview of the mitochondrial heme routes.

Figure 1:
Overview of the mitochondrial heme routes.

Following heme synthesis in the mitochondrial matrix, heme must be transported to hemoproteins throughout the cell. Proposed routes and proteins are shown and described in the review. A representative transmission electron microscopy micrograph of mitochondria (scale bar, 0.4 µm) from wildtype mouse embryonic fibroblasts is used to highlight the location of subcompartments where the final step of heme biosynthesis is taking place. Hem15/FECH, ferrochelatase; IM, inner mitochondrial membrane; IMS, intermembrane space; OM, outer mitochondrial membrane; PPIX, protoporphyrin IX.

Figure 1:
Overview of the mitochondrial heme routes.

Following heme synthesis in the mitochondrial matrix, heme must be transported to hemoproteins throughout the cell. Proposed routes and proteins are shown and described in the review. A representative transmission electron microscopy micrograph of mitochondria (scale bar, 0.4 µm) from wildtype mouse embryonic fibroblasts is used to highlight the location of subcompartments where the final step of heme biosynthesis is taking place. Hem15/FECH, ferrochelatase; IM, inner mitochondrial membrane; IMS, intermembrane space; OM, outer mitochondrial membrane; PPIX, protoporphyrin IX.

Close modal

Following heme synthesis, the newly made heme b needs to be securely routed, with a significant portion of the compound directed toward export from the mitochondrion. Using a series of genetically encoded radiometric sensors for labile heme that were targeted to various subcellular compartments, it was established that the steady-state levels of labile heme are quite different between various cellular subcompartments: ~20–40 nM in the cytosol and ~2 nM in the nucleus and mitochondrial matrix [1]. Interestingly, trafficking of labile heme into the mitochondrial matrix and cytosol occurred at similar rates, while trafficking to the nucleus is 25% faster [22]. These findings suggested that once heme is synthesized at the matrix side of the inner mitochondrial membrane (IMM), it is dispersed to multiple compartments by parallel pathways. Although the molecular basis of such rapid distribution remains largely elusive, it is clear that the process of heme export is remarkably efficient and well orchestrated. Indeed, unlike the cytosol and the nucleus, wherein the surplus heme could be eliminated by heme oxygenase HO-1 [12,23,24], mitochondria do not harbor such an enzyme and rely on intricate regulation of the heme biosynthetic pathway and efficient heme export mechanisms. However, both the regulation of mitochondrial heme biosynthesis and its trafficking remain incompletely understood.

Underscoring the importance of heme management in the mitochondria, heme is a prosthetic group that is crucial for multiple mitochondrial pathways and reactions, including the Krebs cycle, oxidative metabolism [25,26], and mitochondrial cytochromes P450 (CYP450) enzymes that are required for steroidogenesis, biosynthesis for bile acids, and 1,25-dihydroxyvitamin D3 [27]. Malfunctions of this system have been linked to a number of clinical conditions, including lung injury, neurodegeneration, and cardiac dysfunction [11,28], further highlighting the physiological importance of heme homeostasis. The following sections will briefly survey our current understanding of the factors implicated in mitochondrial heme export.

Whereas the chemical aspects of heme biosynthetic reactions are well understood, the trafficking of newly synthesized heme and its subsequent delivery to the target proteins remain far from clear. In fact, remarkably little is known about how heme is transported within the mitochondria following its synthesis. A number of proteins have been proposed to play roles in heme homeostasis based solely on their ability to bind heme in vitro, with little or no experimental evidence of their role in heme-dependent processes in vivo or in cellulo [29,30]. Such results need to be carefully analyzed, as mere heme binding may not always reflect a protein’s physiological role. For example, the flavoprotein Sdh1—an enzymatic subunit of succinate dehydrogenase (complex II)— was shown to bind to hemin-agarose beads through its FAD-binding domain [31]. Similarly, the two purported cell surface heme importers—the heme carrier protein HCP1 and feline leukemia virus subgroup C receptor FLVCR2—that were identified in a similar fashion, likely do not play roles in physiological heme transport [5]. Indeed, HCP1 was later shown to be a proton-coupled folate transporter, and FLVCR2 has not been demonstrated to directly transport heme, and its overexpression sensitizes cells to heme toxicity only at very high (~200 μM) heme concentrations [5]. Contrarily, evidence exists to support several mitochondria-related factors and pathways that have been found to both bind and buffer intracellular heme, and/or regulate heme bioavailability in different cells as described herein.

One major route for the newly synthesized heme b is the heme a biosynthetic route. Heme a is a modified heme b that is uniquely used by CcO, a key heme-copper enzyme of the mitochondrial respiratory chain [21,32]. The two identical heme a molecules, albeit with different coordination geometries—designated a and a3—reside in the Cox1 core subunit of CcO and are essential for catalysis and the stability and folding of Cox1 [21,32]. Heme a is generated through a two-step conversion of heme b mediated by the evolutionarily conserved enzymes, Cox10 and Cox15 (Figure 2A and B). The former enzyme mediates farnesylation of the 2-position vinyl group of heme b and yields a heme o intermediate, which is then converted to heme a by Cox15-mediated oxidation of the C8 pyrrole methyl moiety [21,33] (Figure 2A). Cox15-catalyzed conversion of heme o to heme a occurs in conjunction with ferredoxin and ferredoxin reductase [34-36]. While the directionality of this route is clear, many of its aspects remain enigmatic. For example, it remains unclear how heme b is transferred from FECH to Cox10. Similarly, although intuitive, the transfer of heme o intermediate between Cox10 and Cox15 remains to be demonstrated. Cox10 and Cox15 are large polytopic proteins with limited structural information available for either protein [21,33]. They are differentially regulated, do not appear to form a stable complex, and Cox15 is present in ~8-fold excess over Cox10 [37], thus adding more intrigue to the question of how heme is transferred via this axis. Studies in yeast established that at least two additional molecules—Shy1/SURF1 and Coa2, which do not have clear orthologs in higher eukaryotes—play an important role in heme a delivery to Cox1 [21,38]. The formation of the heme a and heme a3 centers appears to occur within the Shy1-containing Cox1 assembly intermediate [21,39], wherein Shy1 appears to chaperone maturation of the heme a3 site rather than serve as a direct heme a donor to the newly synthesized Cox1. Several reports suggested that Cox15 may associate with Shy1 and complex III–IV assembly intermediates, but the significance of these associations remains to be investigated [40,41]. Coa2 is a small membrane-associated protein that co-operates with Shy1 in Cox1 hemylation [38,42,43]. Cells lacking Coa2 exhibit rapid degradation of the newly synthesized Cox1, arising from impaired hemylation and the subsequent misfolding of the molecule [38,42]. The lack of Coa2 can be compensated for by the deletion of the mitochondrial peptidase Oma1 [38,42], the dominant gain-of-function N196K Cox10 mutation [38], or by simultaneous increase in the Cox1 synthesis and levels of wildtype Cox10 [42]. Additionally, a conserved CcO assembly factor Pet117 appears to be important for stabilization of Cox15 oligomers and Cox1 hemylation [44]. Further studies are warranted to elucidate the details of this pathway.

The heme a route.

Figure 2:
The heme a route.

(A) Chemical structures of the heme species in the heme a pathway. Heme a is produced from the heme b (also known as protoheme) precursor in a two-step reaction mediated by Cox10 (heme o synthase) and Cox15 (heme a synthase) enzymes. The reaction involves the formation of a short-lived heme o intermediate. (B) Mitochondrial heme b distribution and heme a route. Ferrochelatase (FECH) metalates protoporphyrin IX (PPIX), yielding heme b, which is rapidly routed to subcellular locations via an unknown mechanism. In the heme a pathway, heme b is trafficked via Cox10 and Cox15, which modify it to heme a. Heme a is inserted into maturing CcO by assembly factors Coa2 and Shy1. The assembly factor Pet117 appears to be also involved in this process. IM, inner mitochondrial membrane; IMS, intermembrane space; OM, outer mitochondrial membrane.

Figure 2:
The heme a route.

(A) Chemical structures of the heme species in the heme a pathway. Heme a is produced from the heme b (also known as protoheme) precursor in a two-step reaction mediated by Cox10 (heme o synthase) and Cox15 (heme a synthase) enzymes. The reaction involves the formation of a short-lived heme o intermediate. (B) Mitochondrial heme b distribution and heme a route. Ferrochelatase (FECH) metalates protoporphyrin IX (PPIX), yielding heme b, which is rapidly routed to subcellular locations via an unknown mechanism. In the heme a pathway, heme b is trafficked via Cox10 and Cox15, which modify it to heme a. Heme a is inserted into maturing CcO by assembly factors Coa2 and Shy1. The assembly factor Pet117 appears to be also involved in this process. IM, inner mitochondrial membrane; IMS, intermembrane space; OM, outer mitochondrial membrane.

Close modal

The heme c route is another heme distribution pathway, wherein heme b is routed and covalently attached to its client mitochondrial cytochromes: cytochrome c and complex III subunit called cytochrome c1 (Figure 3A and B). In either case, the two vinyl groups of heme b form covalent bonds with the protein cysteines of the conserved Cx2CH motif, and the histidine residue acts as an axial ligand for the heme iron atom (Figure 3A) [45,46]. The pathway is relatively well characterized in vitro and involves two intermembrane space (IMS)-residing heme lyases, Cyc3/cytochrome c heme lyase (CCHL) and Cyt2/CC1HL in yeast and a single holocytochrome c synthase (HCCS) in mammals [46,47]. In each case, an IMM-associated lyase binds a reduced heme b with subsequent binding of apocytochrome and formation of covalent thioester bonds within the lyase-ferrous heme-apocytochrome complex [48]. Once the covalent thioester bonds are formed, the hemylated cytochrome is released from the complex. The release appears to be facilitated by conformational distortion of the covalently bound heme molecule and its axial ligand-mediated decrease in heme-CCHL adducts [48,49]. Some other proteins, such as yeast-specific l-lactate cytochrome c oxidoreductase Cyb2, are likely to follow the same hemylation mechanism. It is possible that additional factors may aid this process in vivo. For example, in yeast, an IMS-localized flavoprotein Cyc2, though not conserved and dispensable for CCHL function, was reported to act as a heme reductase and/or redox regulator of heme lyase function [50,51]. One important facet of the heme c route remains a mystery, however. That is, how newly synthesized heme b is routed across the IMM and delivered to CCHL enzymes. This important question remains to be addressed.

The heme c route.

Figure 3:
The heme c route.

(A) Chemical structures of the heme species in the heme c pathway. Heme c is generated from heme b through a cytochrome c/c1 heme lyase-mediated covalent attachment to the sulfhydryl moieties of its target proteins. (B) Mitochondrial heme c route. The newly synthesized heme b is routed to either of the CCHL enzymes that transfer it to its cytochromes. IM, inner mitochondrial membrane; IMS, intermembrane space; OM, outer mitochondrial membrane.

Figure 3:
The heme c route.

(A) Chemical structures of the heme species in the heme c pathway. Heme c is generated from heme b through a cytochrome c/c1 heme lyase-mediated covalent attachment to the sulfhydryl moieties of its target proteins. (B) Mitochondrial heme c route. The newly synthesized heme b is routed to either of the CCHL enzymes that transfer it to its cytochromes. IM, inner mitochondrial membrane; IMS, intermembrane space; OM, outer mitochondrial membrane.

Close modal

Additional, though much less characterized, routes for heme b include pathways that lead to several mitochondrial enzymes, such as respiratory chain complexes succinate dehydrogenase (complex II) and ubiquinol-cytochrome c reductase (complex III), as well as IMS-localized CcO assembly factor COA7 and mitochondrial matrix-localized flavohemoglobin Yhb1/Neuroglobin [5,21,52]. Studies in yeast identified additional mitochondrial hemoproteins with no clear orthologs in higher eukaryotes. These include CcO assembly factor Mss51 and cytochrome c peroxidase Ccp1 [32,53]. In each case, very little is known as to how heme is delivered to these target proteins. Available knowledge on complex III biogenesis indicates that the formation of heme b redox sites within the complex is an early post-translational event that likely ensures an additional feedback regulatory mechanism to minimize the presence of unincorporated heme molecules and reduce negative effects thereof [54,55].

Finally, an important route that remains far from clear is the export route. Indeed, a large portion of the mitochondria-produced heme must be mobilized and efficiently delivered to a variety of hemoproteins in other cellular compartments. Whereas it is generally believed that heme export could be arranged through a coordinated action of transporters (Figure 4A), the exact nature of said transporters remains elusive. One candidate transporter is an isoform of Feline leukemia virus subgroup C receptor-related protein FLVCR1, specifically FLVCR1b [56,57]. FLVCR1b is a shortened isoform that has six transmembrane domains. It is formed from an alternative transcription start site and appears to be mitochondria-localized [56,58]. Whereas FLVCR1 (sometimes referred to as FLVCR1a) functions as an exporter of cytosolic labile heme pool, controlling cell metabolism and oxidative status, the FLVCR1b variant was proposed to transport heme out of the mitochondria [56]. In cell culture models, it has been shown that overexpression of FLVCR1b increases cytosolic heme and that knockdown results in mitochondrial heme retention [56]. Recently, interactions between FLVCR1b and cytosolic heme chaperone glyceraldehyde phosphate dehydrogenase have been shown to trigger heme transfer between proteins and to downstream hemoproteins [59]. Even with these findings, questions about FLVCR1b still exist; in particular, the exact location of FLVCR1b in mitochondria and its alignment with heme biosynthetic machinery remain unclear, thus warranting further investigation.

Models of mitochondrial heme export.

Figure 4:
Models of mitochondrial heme export.

(A) In the first model, the newly synthesized heme b is routed and exported from mitochondria via a dedicated transporter such as FLVCR1b. The exact localization of FLVCR1b remains to be investigated. (B) The second model posits that mitochondrial heme b is distributed via mitochondria-derived structures called mitochondria-derived vesicles or similar, yet distinct, mitochondria-derived compartments that are subsequently directed to endosomes and lysosomes. FECH, ferrochelatase; IM, inner mitochondrial membrane; IMS, intermembrane space; MDVs, mitochondria-derived vesicles; MDCs, mitochondria-derived compartments; OM, outer mitochondrial membrane.

Figure 4:
Models of mitochondrial heme export.

(A) In the first model, the newly synthesized heme b is routed and exported from mitochondria via a dedicated transporter such as FLVCR1b. The exact localization of FLVCR1b remains to be investigated. (B) The second model posits that mitochondrial heme b is distributed via mitochondria-derived structures called mitochondria-derived vesicles or similar, yet distinct, mitochondria-derived compartments that are subsequently directed to endosomes and lysosomes. FECH, ferrochelatase; IM, inner mitochondrial membrane; IMS, intermembrane space; MDVs, mitochondria-derived vesicles; MDCs, mitochondria-derived compartments; OM, outer mitochondrial membrane.

Close modal

The proteins progesterone receptor membrane component 1 (PGRMC1), its yeast homolog damage response protein 1, and its paralog progesterone receptor membrane component 2 (PGRMC2) are the factors implicated in heme trafficking. These proteins, along with two others in the membrane associate progesterone receptor family, Neudesin and Neuferricin, all share a cytochrome b5-like (Cyt b5) domain and all bind heme [60]. A notable difference found for the proteins in this family is that they have variable N-terminal domains. PGRMC1 and PGRMC2 both have an N-terminal extension that is proposed to be a single helical transmembrane domain, while Neudesin and Neuferricin are soluble proteins [61]. With respect to heme binding, all members of the family are thought to bind heme via a conserved Tyr (Tyr-113 in PGRMC1) found within the Cyt b5-like domain [62]. Crystallization of the soluble Cyt b5-like domain of PGRMC1 with heme bound reveals a solvent-exposed heme that dimerizes via heme stacking [62]. Moreover, PGRMC1 binds more tightly to ferric heme than to ferrous, consistent with tyrosinate coordination [63]. The interaction of PGRMC1 with the enzyme catalyzing the terminal step of heme synthesis, as well as its moderate affinity for heme, is consistent with this protein being involved in heme transfer [64]. Interestingly, carbon monoxide (CO) disrupts the formation of the PGRMC1 dimer, which is thought to be the active form [62]. This finding suggests that CO, which is produced as a byproduct of heme degradation, may be a feedback regulator of this protein. Biochemical evidence also supports PGRMC1 interacting with multiple CYP450 enzymes that regulate cholesterol, xenobiotic, bile acid, steroid, and arachidonic acid metabolism [60]. More recently, PGRMC2 was shown to function as a heme chaperone moving heme to the nucleus in brown adipose tissue to control mitochondrial function [65]. While PGRMC2 has been localized to the ER membrane and the nuclear envelope [66], PGRMC1 has reported localization throughout the cell [60]. Studies in a developing red blood cell model support that PGRMC1 is associated with the external face of the outer mitochondrial membrane [64], while other studies have found that it is associated with the ER membrane [67]. A more likely scenario is that PGRMC1 would localize to mitochondria-associated ER membranes, subcellular structures known to mediate ions, lipids, and metabolite exchange between the ER and mitochondrial networks [68].

Several recent studies reported that the mitochondrial heme biosynthetic enzymes, 5-aminolevulinate synthase and protoporphyrin oxidase, along with mitochondrial iron importer, mitoferrin, interact with FECH in a heme biosynthetic complex or metabolon [69,70]. This superstructure may therefore facilitate heme synthesis and distribution. Conceptually, this is not unprecedented and may resemble a recently reported coenzyme Q biosynthetic metabolon that encompasses virtually all of the enzymes involved in the mitochondrial ubiquinol biosynthesis [71]. In line with the above ideas, we recently showed that FECH is associated with mitochondrial intermembrane contacts via mitochondrial contact site and cristae organizing system machinery to facilitate the process of heme biosynthesis, especially when cells are forced to produce heme [72]. Likewise, as noted above, FECH has been shown to interact with PGRMC1 and PGRM2C [64,69,73], as well as a variety of other proteins in a cell-line-specific manner. Studies are underway to further explore the role, composition, and molecular details of the mitochondrial heme metabolon.

Yet another protein proposed to be involved in heme export is Transport and Golgi organization 2 (TANGO2). TANGO2 lacks transmembrane domains and is speculated to be localized in the cytoplasm, the Golgi, and the mitochondria [74]. Originally discovered in Drosophila, TANGO2 was proposed to be involved in heme trafficking based on the finding that mammalian and yeast cells deleted for this factor accumulate heme in mitochondria [75,76]. In C. elegans, TANGO2 homolog HRG-9 and its paralog HRG-10 were reported to be responsible for heme transport from intestinal cells [75]. Recent work supports interactions between TANGO2 and FLVCR1b possibly to support its export function [59], although other reports have challenged the protein’s heme transporting role [77-79]. As such, its role in mitochondrial heme export remains unclear.

Considering that heme is a hydrophobic lipid-like molecule that can intercalate in membranes, one alternative model for heme export and distribution would involve the mitochondrial membranes and cellular endomembranes. Indeed, interorganellar contact sites, such as mitochondria-ER contacts or mitochondria-vacuole/lysosome contacts were proposed to facilitate metabolites and lipid exchange between the organelles [80,81]. Similarly, the mitochondria-derived vesicles [82,83] or related mitochondria-derived compartments [84,85] were shown to encompass mitochondrial membranes and carry mitochondria-borne cargo, ultimately delivering to the endosomes and vacuole/lysosome. It is plausible that newly synthesized heme molecules could also be trafficked in such fashion (Figure 4B). In line with this notion, screens with genetically encoded heme sensors identified membrane remodeling GTPases as important players in mitochondrial heme distribution [22]. Studies are warranted to explore these emerging models.

As noted herein, heme is an essential cofactor and signaling molecule. Therefore, it is not surprising that defects in mitochondrial heme synthesis and management lead to a number of human disorders, including anemias, porphyrias [86], and cardiovascular diseases [87-89]. With regard to the routes of distribution described in this review, the Mendelian mutations in heme c lyase HCCS have been shown to result in both defective cytochrome c hemylation and its apoptotic release. These variants manifest clinically as a hereditary dominant microphthalmia with linear skin defects (MLS) syndrome [45,90]. Similarly, hereditary mutations in conserved residues of heme a pathway enzymes COX10, COX15, and SURF1 result in a spectrum of severe disorders, including tubulopathy and leukodystrophy, sensorineural deafness, fatal infantile hypertrophic cardiomyopathy, Charcot-Marie-Tooth disease type 1A, and neurologic Leigh syndrome [87,88,91-102].

Mutations in FLVCR1 variants have been linked to posterior column ataxia and retinitis pigmentosa and Diamond-Blackfan syndrome in humans [103,104]. Additionally, FLVCR1 dysregulation has been associated with increased risk of diabetes and certain types of cancer [105,106]. Studies in the mouse models have shown that FLVCR1 deletion is associated with increased incidence of embryonic and in utero lethality and compromised erythropoiesis, resulting in hemorrhage and anemia in postnatal animals [10,107]. The FLVCR-deleted animals also present with skeletal malformations, dysfunctional muscle, and sensory neurodegeneration [107,108]. However, it remains to be determined which portion of these manifestations can be ascribed to defects in FLVCR1b variant.

Finally, TANGO2 deficiency has been associated with multiple pathologies including developmental delay, muscle weakness, intellectual delay, seizure, encephalopathy, arrhythmias, rhabdomyolysis, and increased chances of early-onset mortality in humans [109-111], underscoring its physiological importance. Given the multifaceted nature of this protein, studies are warranted to determine if and how defective heme metabolism contributes to these manifestations.

Decades of research have led to a detailed understanding of the chemical aspects of the synthesis of heme; however, the mechanisms by which the newly synthesized heme is mobilized from FECH and routed within and exported outside of the mitochondria, and how the said processes are orchestrated and regulated are only beginning to emerge. This knowledge gap is in part due to our limited ability to monitor spatiotemporal dynamics of heme distribution and its subsequent conversions within the cell. Recent development of new tools, such as genetically encoded fluorescent heme sensors [29], has greatly supplemented traditional biochemical studies and has allowed us to overcome some of these technical challenges. Delineating how newly synthesized heme is routed within and exported from mitochondria and identifying factors that control this pathway will address a fundamental biological question and facilitate the development of new therapeutic approaches to treat patients with prevalent defects in heme management.

Perspectives

  • Heme is a vital but highly reactive metal-containing hydrophobic compound synthesized in mitochondria that is important for many cellular functions within and beyond the mitochondria. Abnormalities in heme synthesis and distribution contribute to multiple prevalent pathological states, including anemias, porphyrias, and cardiovascular diseases.

  • Some of the heme routes, such as heme a and heme c route, are well understood, whereas the others are only beginning to emerge. One major route is the heme export from mitochondria, for which several recently identified factors and models are discussed.

  • Future studies will illuminate all pathways for heme distribution within and beyond the mitochondria and provide necessary information to better understand conditions that result in defects in mitochondrial heme synthesis and distribution.

The authors declare that they have no competing interests associated with the contents of this text.

We wish to acknowledge the grant support from the National Institutes of Health National Institute of General Medical Sciences, R35GM131701 to O.K.

Conceptualization: S.F.A., A.E.M., O.K.; Writing – original draft: S.F.A., O.K.; Writing – review and editing: S.F.A., A.E.W., A.E.M., O.K.; Visualization: A.E.W., O.K.; Funding acquisition: O.K.

We apologize to those authors whose work we were unable to cite due to space limitations. We thank members of the Khalimonchuk and Medlock labs for their valuable comments and helpful discussions.

CO

carbon monoxide

CYP450

cytochrome P450

CcO

cytochrome c oxidase

Cyt b5

cytochrome b5

FECH

Ferrochelatase

IMM

inner mitochondrial membrane

IMS

intermembrane space

PGRMC1

proteins progesterone receptor membrane component 1

PGRMC2

paralog progesterone receptor membrane component 2

TANGO2

Transport and Golgi organization 2

1
Hanna
,
D.A.
,
Harvey
,
R.M.
,
Martinez-Guzman
,
O.
,
Yuan
,
X.
,
Chandrasekharan
,
B.
,
Raju
,
G.
et al.
(
2016
)
Heme dynamics and trafficking factors revealed by genetically encoded fluorescent heme sensors
.
Proc. Natl. Acad. Sci. U.S.A.
113
,
7539
7544
https://doi.org/10.1073/pnas.1523802113
2
Munro
,
A.W.
,
Girvan
,
H.M.
,
McLean
,
K.J.
,
Cheesman
,
M.R.
and
Leys
,
D
. (
2009
) Heme and Hemoproteins.
In
Tetrapyrroles: birth, life and death
(
Warren
,
M.J.
and
Smith
,
A.G.
, eds),
pp
.
160
83
,
Landes Bioscience and Springer Science+Business Media
, https://doi.org/10.1007/978-0-387-78518-9
3
Chambers
,
I.G.
,
Willoughby
,
M.M.
,
Hamza
,
I.
and
Reddi
,
A.R
. (
2021
)
One ring to bring them all and in the darkness bind them: the trafficking of heme without deliverers
.
Biochim. Biophys. Acta Mol. Cell Res.
1868
, 118881 https://doi.org/10.1016/j.bbamcr.2020.118881
4
Gilles-Gonzalez
,
M.A.
and
Sousa
,
E.H.S
. (
2023
)
Structures of biological heme-based sensors of oxygen
.
J. Inorg. Biochem.
244
, 112229 https://doi.org/10.1016/j.jinorgbio.2023.112229
5
Swenson
,
S.A.
,
Moore
,
C.M.
,
Marcero
,
J.R.
,
Medlock
,
A.E.
,
Reddi
,
A.R.
and
Khalimonchuk
,
O
. (
2020
)
From synthesis to utilization: the ins and outs of mitochondrial heme
.
Cells
9
, 579 https://doi.org/10.3390/cells9030579
6
Tsiftsoglou
,
A.S.
,
Tsamadou
,
A.I.
and
Papadopoulou
,
L.C
. (
2006
)
Heme as key regulator of major mammalian cellular functions: molecular, cellular, and pharmacological aspects
.
Pharmacol. Ther.
111
,
327
345
https://doi.org/10.1016/j.pharmthera.2005.10.017
7
Dutt
,
S.
,
Hamza
,
I.
and
Bartnikas
,
T.B
. (
2022
)
Molecular mechanisms of iron and heme metabolism
.
Annu. Rev. Nutr.
42
,
311
335
https://doi.org/10.1146/annurev-nutr-062320-112625
8
Feng
,
D.
and
Lazar
,
M.A
. (
2012
)
Clocks, metabolism, and the epigenome
.
Mol. Cell
47
,
158
167
https://doi.org/10.1016/j.molcel.2012.06.026
9
Girvan
,
H.M.
and
Munro
,
A.W
. (
2013
)
Heme sensor proteins
.
J. Biol. Chem.
288
,
13194
13203
https://doi.org/10.1074/jbc.R112.422642
10
Keel
,
S.B.
,
Doty
,
R.T.
,
Yang
,
Z.
,
Quigley
,
J.G.
,
Chen
,
J.
,
Knoblaugh
,
S.
et al.
(
2008
)
A heme export protein is required for red blood cell differentiation and iron homeostasis
.
Science
319
,
825
828
https://doi.org/10.1126/science.1151133
11
Chiabrando
,
D.
,
Vinchi
,
F.
,
Fiorito
,
V.
,
Mercurio
,
S.
and
Tolosano
,
E
. (
2014
)
Heme in pathophysiology: a matter of scavenging, metabolism and trafficking across cell membranes
.
Front. Pharmacol.
5
, 61 https://doi.org/10.3389/fphar.2014.00061
12
Kumar
,
S.
and
Bandyopadhyay
,
U
. (
2005
)
Free heme toxicity and its detoxification systems in human
.
Toxicol. Lett.
157
,
175
188
https://doi.org/10.1016/j.toxlet.2005.03.004
13
Yien
,
Y.Y.
and
Perfetto
,
M
. (
2022
)
Regulation of heme synthesis by mitochondrial homeostasis proteins
.
Front. Cell Dev. Biol.
10
, 895521 https://doi.org/10.3389/fcell.2022.895521
14
Dickey
,
A.K.
,
Leaf
,
R.K.
and
Balwani
,
M
. (
2024
)
Update on the porphyrias
.
Annu. Rev. Med.
75
,
321
335
https://doi.org/10.1146/ annurev-med-042921-123602
15
Chen
,
C.
and
Hamza
,
I
. (
2023
)
Notes from the underground: heme homeostasis in C. elegans
.
Biomolecules
13
, 1149 https://doi.org/10.3390/biom13071149
16
Hamza
,
I.
and
Dailey
,
H.A
. (
2012
)
One ring to rule them all: trafficking of heme and heme synthesis intermediates in the metazoans
.
Biochim. Biophys. Acta
1823
,
1617
1632
https://doi.org/10.1016/j.bbamcr.2012.04.009
17
Laranjeira-Silva
,
M.F.
,
Hamza
,
I.
and
Pérez-Victoria
,
J.M
. (
2020
)
Iron and heme metabolism at the leishmania-host interface
.
Trends Parasitol.
36
,
279
289
https://doi.org/10.1016/j.pt.2019.12.010
18
Rao
,
A.U.
,
Carta
,
L.K.
,
Lesuisse
,
E.
and
Hamza
,
I
. (
2005
)
Lack of heme synthesis in a free-living eukaryote
.
Proc. Natl. Acad. Sci. U.S.A.
102
,
4270
4275
https://doi.org/10.1073/pnas.0500877102
19
Dailey
,
H.A.
and
Medlock
,
A.E
. (
2022
)
A primer on heme biosynthesis
.
Biol. Chem.
403
,
985
1003
https://doi.org/10.1515/hsz-2022-0205
20
Obi
,
C.D.
,
Bhuiyan
,
T.
,
Dailey
,
H.A.
and
Medlock
,
A.E
. (
2022
)
Ferrochelatase: mapping the intersection of iron and porphyrin metabolism in the mitochondria
.
Front. Cell Dev. Biol.
10
, 894591 https://doi.org/10.3389/fcell.2022.894591
21
Kim
,
H.J.
,
Khalimonchuk
,
O.
,
Smith
,
P.M.
and
Winge
,
D.R
. (
2012
)
Structure, function, and assembly of heme centers in mitochondrial respiratory complexes
.
Biochim. Biophys. Acta
1823
,
1604
1616
https://doi.org/10.1016/j.bbamcr.2012.04.008
22
Martinez-Guzman
,
O.
,
Willoughby
,
M.M.
,
Saini
,
A.
,
Dietz
,
J.V.
,
Bohovych
,
I.
,
Medlock
,
A.E.
et al.
(
2020
)
Mitochondrial-nuclear heme trafficking in budding yeast is regulated by GTPases that control mitochondrial dynamics and ER contact sites
.
J. Cell. Sci.
133
, jcs237917 https://doi.org/10.1242/jcs.237917
23
Belcher
,
J.D.
,
Beckman
,
J.D.
,
Balla
,
G.
,
Balla
,
J.
and
Vercellotti
,
G
. (
2010
)
Heme degradation and vascular injury
.
Antioxid. Redox Signal.
12
,
233
248
https://doi.org/10.1089/ars.2009.2822
24
Haines
,
D.D.
and
Tosaki
,
A
. (
2020
)
Heme degradation in pathophysiology of and countermeasures to inflammation-associated disease
.
Int. J. Mol. Sci.
21
, 9698 https://doi.org/10.3390/ijms21249698
25
Fiorito
,
V.
,
Allocco
,
A.L.
,
Petrillo
,
S.
,
Gazzano
,
E.
,
Torretta
,
S.
,
Marchi
,
S.
et al.
(
2021
)
The heme synthesis-export system regulates the tricarboxylic acid cycle flux and oxidative phosphorylation
.
Cell Rep.
35
, 109252 https://doi.org/10.1016/j.celrep.2021.109252
26
Kaur
,
P.
,
Nagar
,
S.
,
Bhagwat
,
M.
,
Uddin
,
M.
,
Zhu
,
Y.
,
Vancurova
,
I.
et al.
(
2021
)
Activated heme synthesis regulates glycolysis and oxidative metabolism in breast and ovarian cancer cells
.
Plos One
16
, e0260400 https://doi.org/10.1371/journal.pone.0260400
27
Omura
,
T
. (
2006
)
Mitochondrial P450s
.
Chem. Biol. Interact.
163
,
86
93
https://doi.org/10.1016/j.cbi.2006.06.008
28
Wang
,
T.
,
Ashrafi
,
A.
,
Modareszadeh
,
P.
,
Deese
,
A.R.
,
Chacon Castro
,
M.D.C.
,
Alemi
,
P.S.
et al.
(
2021
)
An analysis of the multifaceted roles of heme in the pathogenesis of cancer and related diseases
.
Cancers (Basel)
13
, 4142 https://doi.org/10.3390/cancers13164142
29
Hanna
,
D.A.
,
Martinez-Guzman
,
O.
and
Reddi
,
A.R
. (
2017
)
Heme gazing: Illuminating eukaryotic heme trafficking, dynamics, and signaling with fluorescent heme sensors
.
Biochemistry
56
,
1815
1823
https://doi.org/10.1021/acs.biochem.7b00007
30
Ponka
,
P.
,
Sheftel
,
A.D.
,
English
,
A.M.
,
Scott Bohle
,
D.
and
Garcia-Santos
,
D
. (
2017
)
Do mammalian cells really need to export and import heme?
Trends Biochem. Sci.
42
,
395
406
https://doi.org/10.1016/j.tibs.2017.01.006
31
Kim
,
H.J.
,
Jeong
,
M.Y.
,
Na
,
U.
and
Winge
,
D.R
. (
2012
)
Flavinylation and assembly of succinate dehydrogenase are dependent on the C-terminal tail of the flavoprotein subunit
.
J. Biol. Chem.
287
,
40670
40679
https://doi.org/10.1074/jbc.M112.405704
32
Soto
,
I.C.
,
Fontanesi
,
F.
,
Liu
,
J.
and
Barrientos
,
A
. (
2012
)
Biogenesis and assembly of eukaryotic cytochrome c oxidase catalytic core
.
Biochim. Biophys. Acta
1817
,
883
897
https://doi.org/10.1016/j.bbabio.2011.09.005
33
Rivett
,
E.D.
,
Addis
,
H.G.
,
Dietz
,
J.V.
,
Carroll-Deaton
,
J.A.
,
Gupta
,
S.
,
Foreman
,
K.L.
et al.
(
2023
)
Evidence that the catalytic mechanism of heme a synthase involves the formation of a carbocation stabilized by a conserved glutamate
.
Arch. Biochem. Biophys.
744
, 109665 https://doi.org/10.1016/j.abb.2023.109665
34
Barros
,
M.H.
,
Carlson
,
C.G.
,
Glerum
,
D.M.
and
Tzagoloff
,
A
. (
2001
)
Involvement of mitochondrial ferredoxin and Cox15p in hydroxylation of heme O
.
FEBS Lett.
492
,
133
138
https://doi.org/10.1016/s0014-5793(01)02249-9
35
Barros
,
M.H.
,
Nobrega
,
F.G.
and
Tzagoloff
,
A
. (
2002
)
Mitochondrial ferredoxin is required for heme a synthesis in saccharomyces cerevisiae
.
J. Biol. Chem.
277
,
9997
10002
https://doi.org/10.1074/jbc.M112025200
36
Sheftel
,
A.D.
,
Stehling
,
O.
,
Pierik
,
A.J.
,
Elsässer
,
H.-P.
,
Mühlenhoff
,
U.
,
Webert
,
H.
et al.
(
2010
)
Humans possess two mitochondrial ferredoxins, Fdx1 and Fdx2, with distinct roles in steroidogenesis, heme, and Fe/S cluster biosynthesis
.
Proc. Natl. Acad. Sci. U.S.A.
107
,
11775
11780
https://doi.org/10.1073/pnas.1004250107
37
Wang
,
Z.
,
Wang
,
Y.
and
Hegg
,
E.L
. (
2009
)
Regulation of the heme a biosynthetic pathway: differential regulation of heme a synthase and heme o synthase in saccharomyces cerevisiae
.
J. Biol. Chem.
284
,
839
847
https://doi.org/10.1074/jbc.M804167200
38
Bestwick
,
M.
,
Jeong
,
M.Y.
,
Khalimonchuk
,
O.
,
Kim
,
H.
and
Winge
,
D.R
. (
2010
)
Analysis of leigh syndrome mutations in the yeast SURF1 homolog reveals a new member of the cytochrome oxidase assembly factor family
.
Mol. Cell. Biol.
30
,
4480
4491
https://doi.org/10.1128/MCB.00228-10
39
Nývltová
,
E.
,
Dietz
,
J.V.
,
Seravalli
,
J.
,
Khalimonchuk
,
O.
and
Barrientos
,
A
. (
2022
)
Coordination of metal center biogenesis in human cytochrome c oxidase
.
Nat. Commun.
13
, 3615 https://doi.org/10.1038/s41467-022-31413-1
40
Bareth
,
B.
,
Dennerlein
,
S.
,
Mick
,
D.U.
,
Nikolov
,
M.
,
Urlaub
,
H.
and
Rehling
,
P
. (
2013
)
The heme a synthase Cox15 associates with cytochrome c oxidase assembly intermediates during Cox1 maturation
.
Mol. Cell. Biol.
33
,
4128
4137
https://doi.org/10.1128/MCB.00747-13
41
Herwaldt
,
E.J.
,
Rivett
,
E.D.
,
White
,
A.J.
and
Hegg
,
E.L
. (
2018
)
Cox15 interacts with the cytochrome bc1 dimer within respiratory supercomplexes as well as in the absence of cytochrome c oxidase
.
Journal of Biological Chemistry
293
,
16426
16439
https://doi.org/10.1074/jbc.RA118.002496
42
Khalimonchuk
,
O.
,
Kim
,
H.
,
Watts
,
T.
,
Perez-Martinez
,
X.
and
Winge
,
D.R
. (
2012
)
Oligomerization of heme o synthase in cytochrome oxidase biogenesis is mediated by cytochrome oxidase assembly factor Coa2
.
J. Biol. Chem.
287
,
26715
26726
https://doi.org/10.1074/jbc.M112.377200
43
Pierrel
,
F.
,
Khalimonchuk
,
O.
,
Cobine
,
P.A.
,
Bestwick
,
M.
and
Winge
,
D.R
. (
2008
)
Coa2 is an assembly factor for yeast cytochrome c oxidase biogenesis that facilitates the maturation of Cox1
.
Mol. Cell. Biol.
28
,
4927
4939
https://doi.org/10.1128/MCB.00057-08
44
Taylor
,
N.G.
,
Swenson
,
S.
,
Harris
,
N.J.
,
Germany
,
E.M.
,
Fox
,
J.L.
and
Khalimonchuk
,
O
. (
2017
)
The assembly factor Pet117 couples heme a synthase activity to cytochrome oxidase assembly
.
J. Biol. Chem.
292
,
1815
1825
https://doi.org/10.1074/jbc.M116.766980
45
Babbitt
,
S.E.
,
San Francisco
,
B.
,
Mendez
,
D.L.
,
Lukat-Rodgers
,
G.S.
,
Rodgers
,
K.R.
,
Bretsnyder
,
E.C.
et al.
(
2014
)
Mechanisms of mitochondrial holocytochrome c synthase and the key roles played by cysteines and histidine of the heme attachment site, Cys-XX-Cys-His
.
J. Biol. Chem.
289
,
28795
28807
https://doi.org/10.1074/jbc.M114.593509
46
Kranz
,
R.G.
,
Richard-Fogal
,
C.
,
Taylor
,
J.S.
and
Frawley
,
E.R
. (
2009
)
Cytochrome c biogenesis: mechanisms for covalent modifications and trafficking of heme and for heme-iron redox control
.
Microbiol. Mol. Biol. Rev.
73
,
510
528
https://doi.org/10.1128/MMBR.00001-09
47
Bernard
,
D.G.
,
Gabilly
,
S.T.
,
Dujardin
,
G.
,
Merchant
,
S.
and
Hamel
,
P.P
. (
2003
)
Overlapping specificities of the mitochondrial cytochrome c and c1 heme lyases
.
J. Biol. Chem.
278
,
49732
49742
https://doi.org/10.1074/jbc.M308881200
48
Babbitt
,
S.E.
,
Sutherland
,
M.C.
,
San Francisco
,
B.
,
Mendez
,
D.L.
and
Kranz
,
R.G
. (
2015
)
Mitochondrial cytochrome c biogenesis: no longer an enigma
.
Trends Biochem. Sci.
40
,
446
455
https://doi.org/10.1016/j.tibs.2015.05.006
49
Sun
,
Y.
,
Benabbas
,
A.
,
Zeng
,
W.
,
Kleingardner
,
J.G.
,
Bren
,
K.L.
and
Champion
,
P.M
. (
2014
)
Investigations of heme distortion, low-frequency vibrational excitations, and electron transfer in cytochrome c
.
Proc. Natl. Acad. Sci. U.S.A.
111
,
6570
6575
https://doi.org/10.1073/pnas.1322274111
50
Bernard
,
D.G.
,
Quevillon-Cheruel
,
S.
,
Merchant
,
S.
,
Guiard
,
B.
and
Hamel
,
P.P
. (
2005
)
Cyc2p, a membrane-bound flavoprotein involved in the maturation of mitochondrial c-type cytochromes
.
J. Biol. Chem.
280
,
39852
39859
https://doi.org/10.1074/jbc.M508574200
51
Corvest
,
V.
,
Murrey
,
D.A.
,
Hirasawa
,
M.
,
Knaff
,
D.B.
,
Guiard
,
B.
and
Hamel
,
P.P
. (
2012
)
The flavoprotein Cyc2p, a mitochondrial cytochrome c assembly factor, is a NAD(P)H-dependent haem reductase
.
Mol. Microbiol.
83
,
968
980
https://doi.org/10.1111/j.1365-2958.2012.07981.x
52
Formosa
,
L.E.
,
Maghool
,
S.
,
Sharpe
,
A.J.
,
Reljic
,
B.
,
Muellner-Wong
,
L.
,
Stroud
,
D.A.
et al.
(
2022
)
Mitochondrial COA7 is a heme-binding protein with disulfide reductase activity, which acts in the early stages of complex IV assembly
.
Proc. Natl. Acad. Sci. U.S.A.
119
, e2110357119 https://doi.org/10.1073/pnas.2110357119
53
Poulos
,
T.L
. (
2010
)
Thirty years of heme peroxidase structural biology
.
Arch. Biochem. Biophys.
500
,
3
12
https://doi.org/10.1016/j.abb.2010.02.008
54
Carlström
,
A.
and
Ott
,
M
. (
2024
)
Insights into conformational changes in cytochrome b during the early steps of its maturation
.
FEBS Lett.
598
,
1438
1448
https://doi.org/10.1002/1873-3468.14888
55
Hildenbeutel
,
M.
,
Hegg
,
E.L.
,
Stephan
,
K.
,
Gruschke
,
S.
,
Meunier
,
B.
and
Ott
,
M
. (
2014
)
Assembly factors monitor sequential hemylation of cytochrome b to regulate mitochondrial translation
.
J. Cell Biol.
205
,
511
524
https://doi.org/10.1083/jcb.201401009
56
Chiabrando
,
D.
,
Marro
,
S.
,
Mercurio
,
S.
,
Giorgi
,
C.
,
Petrillo
,
S.
,
Vinchi
,
F.
et al.
(
2012
)
The mitochondrial heme exporter FLVCR1b mediates erythroid differentiation
.
J. Clin. Invest.
122
,
4569
4579
https://doi.org/10.1172/JCI62422
57
Fleming
,
M.D.
and
Hamza
,
I
. (
2012
)
Mitochondrial heme: an exit strategy at last
.
J. Clin. Invest.
122
,
4328
4330
https://doi.org/10.1172/JCI66607
58
Mercurio
,
S.
,
Petrillo
,
S.
,
Chiabrando
,
D.
,
Bassi
,
Z.I.
,
Gays
,
D.
,
Camporeale
,
A.
et al.
(
2015
)
The heme exporter Flvcr1 regulates expansion and differentiation of committed erythroid progenitors by controlling intracellular heme accumulation
.
Haematologica
100
,
720
729
https://doi.org/10.3324/haematol.2014.114488
59
Jayaram
,
D.T.
,
Sivaram
,
P.
,
Biswas
,
P.
,
Dai
,
Y.
,
Sweeny
,
E.A.
and
Stuehr
,
D.J
. (
2024
)
Heme allocation in eukaryotic cells relies on mitochondrial heme export through FLVCR1b to cytosolic GAPDH
.
Res. Sq.
rs.3.rs-4314324 https://doi.org/10.21203/rs.3.rs-4314324/v1
60
McGuire
,
M.R.
and
Espenshade
,
P.J
. (
2023
)
PGRMC1: an enigmatic heme-binding protein
.
Pharmacol. Ther.
241
,
108326
https://doi.org/10.1016/j.pharmthera.2022.108326
61
Kimura
,
I.
,
Nakayama
,
Y.
,
Konishi
,
M.
,
Terasawa
,
K.
,
Ohta
,
M.
,
Itoh
,
N.
et al.
(
2012
)
Functions of MAPR (membrane-associated progesterone receptor) family members as heme/steroid-binding proteins
.
Curr. Protein Pept. Sci.
13
,
687
696
https://doi.org/10.2174/138920312804142110
62
Kabe
,
Y.
,
Nakane
,
T.
,
Koike
,
I.
,
Yamamoto
,
T.
,
Sugiura
,
Y.
,
Harada
,
E.
et al.
(
2016
)
Haem-dependent dimerization of PGRMC1/Sigma-2 receptor facilitates cancer proliferation and chemoresistance
.
Nat. Commun.
7
, 11030 https://doi.org/10.1038/ncomms11030
63
Ghosh
,
K.
,
Thompson
,
A.M.
,
Goldbeck
,
R.A.
,
Shi
,
X.
,
Whitman
,
S.
,
Oh
,
E.
et al.
(
2005
)
Spectroscopic and biochemical characterization of heme binding to yeast Dap1p and mouse PGRMC1p
.
Biochemistry
44
,
16729
16736
https://doi.org/10.1021/bi0511585
64
Piel
,
R.B.
3rd
,
Shiferaw
,
M.T.
,
Vashisht
,
A.A.
,
Marcero
,
J.R.
,
Praissman
,
J.L.
,
Phillips
,
J.D.
et al.
(
2016
)
A novel role for progesterone receptor membrane component 1 (PGRMC1): a partner and regulator of ferrochelatase
.
Biochemistry
55
,
5204
5217
https://doi.org/10.1021/acs.biochem.6b00756
65
Galmozzi
,
A.
,
Kok
,
B.P.
,
Kim
,
A.S.
,
Montenegro-Burke
,
J.R.
,
Lee
,
J.Y.
,
Spreafico
,
R.
et al.
(
2019
)
PGRMC2 is an intracellular haem chaperone critical for adipocyte function
.
Nature
576
,
138
142
https://doi.org/10.1038/s41586-019-1774-2
66
Wendler
,
A.
and
Wehling
,
M
. (
2013
)
PGRMC2, a yet uncharacterized protein with potential as tumor suppressor, migration inhibitor, and regulator of cytochrome P450 enzyme activity
.
Steroids
78
,
555
558
https://doi.org/10.1016/j.steroids.2012.12.002
67
McGuire
,
M.R.
,
Mukhopadhyay
,
D.
,
Myers
,
S.L.
,
Mosher
,
E.P.
,
Brookheart
,
R.T.
,
Kammers
,
K.
et al.
(
2021
)
Progesterone receptor membrane component 1 (PGRMC1) binds and stabilizes cytochromes P450 through a heme-independent mechanism
.
J. Biol. Chem.
297
, 101316 https://doi.org/10.1016/j.jbc.2021.101316
68
Barazzuol
,
L.
,
Giamogante
,
F.
and
Calì
,
T
. (
2021
)
Mitochondria associated membranes (MAMs): architecture and physiopathological role
.
Cell Calcium
94
, 102343 https://doi.org/10.1016/j.ceca.2020.102343
69
Medlock
,
A.E.
,
Shiferaw
,
M.T.
,
Marcero
,
J.R.
,
Vashisht
,
A.A.
,
Wohlschlegel
,
J.A.
,
Phillips
,
J.D.
et al.
(
2015
)
Identification of the mitochondrial heme metabolism complex.
.
Plos One
10
, e0135896 https://doi.org/10.1371/journal.pone.0135896
70
Piel
,
R.B.
III
,
Dailey
,
H.A.
Jr
and
Medlock
,
A.E
. (
2019
)
The mitochondrial heme metabolon: Insights into the complex(ity) of heme synthesis and distribution
.
Mol. Genet. Metab.
128
,
198
203
https://doi.org/10.1016/j.ymgme.2019.01.006
71
Subramanian
,
K.
,
Jochem
,
A.
,
Le Vasseur
,
M.
,
Lewis
,
S.
,
Paulson
,
B.R.
,
Reddy
,
T.R.
et al.
(
2019
)
Coenzyme Q biosynthetic proteins assemble in a substrate-dependent manner into domains at ER-mitochondria contacts
.
J. Cell Biol.
218
,
1353
1369
https://doi.org/10.1083/jcb.201808044
72
Dietz
,
J.V.
,
Willoughby
,
M.M.
,
Piel
,
R.B.
3rd
,
Ross
,
T.A.
,
Bohovych
,
I.
,
Addis
,
H.G.
et al.
(
2021
)
Mitochondrial contact site and cristae organizing system (MICOS) machinery supports heme biosynthesis by enabling optimal performance of ferrochelatase
.
Redox Biol.
46
, 102125 https://doi.org/10.1016/j.redox.2021.102125
73
Obi
,
C.D.
,
Dailey
,
H.A.
,
Jami-Alahmadi
,
Y.
,
Wohlschlegel
,
J.A.
and
Medlock
,
A.E
. (
2023
)
Proteomic analysis of ferrochelatase interactome in erythroid and non-erythroid cells
.
Life (Basel).
13
, 577 https://doi.org/10.3390/life13020577
74
Milev
,
M.P.
,
Saint-Dic
,
D.
,
Zardoui
,
K.
,
Klopstock
,
T.
,
Law
,
C.
,
Distelmaier
,
F.
et al.
(
2021
)
The phenotype associated with variants in TANGO2 may be explained by a dual role of the protein in ER-to-Golgi transport and at the mitochondria
.
J. Inherit. Metab. Dis.
44
,
426
437
https://doi.org/10.1002/jimd.12312
75
Sun
,
F.
,
Zhao
,
Z.
,
Willoughby
,
M.M.
,
Shen
,
S.
,
Zhou
,
Y.
,
Shao
,
Y.
et al.
(
2022
)
HRG-9 homologues regulate haem trafficking from haem-enriched compartments
.
Nature
610
,
768
774
https://doi.org/10.1038/s41586-022-05347-z
76
Bard
,
F.
,
Casano
,
L.
,
Mallabiabarrena
,
A.
,
Wallace
,
E.
,
Saito
,
K.
,
Kitayama
,
H.
et al.
(
2006
)
Functional genomics reveals genes involved in protein secretion and Golgi organization
.
Nature
439
,
604
607
https://doi.org/10.1038/nature04377
77
Lujan
,
A.L.
,
Foresti
,
O.
,
Sugden
,
C.
,
Brouwers
,
N.
,
Farre
,
A.M.
,
Vignoli
,
A.
et al.
(
2023
)
Defects in lipid homeostasis reflect the function of TANGO2 in phospholipid and neutral lipid metabolism
.
Elife
12
, e85345 https://doi.org/10.7554/eLife.85345
78
Lujan
,
A.L.
,
Foresti
,
O.
,
Wojnacki
,
J.
,
Bigliani
,
G.
,
Brouwers
,
N.
,
Pena
,
M.J.
et al.
(
2025
)
TANGO2 is an acyl-CoA binding protein
.
J. Cell Biol.
224
, e202410001 https://doi.org/10.1083/jcb.202410001
79
Sandkuhler
,
S.E.
,
Youngs
,
K.S.
,
Owlett
,
L.
,
Bandora
,
M.B.
,
Naaz
,
A.
,
Kim
,
E.S.
et al.
(
2024
)
Heme’s relevance genuine? re-visiting the roles of TANGO2 homologs including HRG-9 and HRG-10 in C. elegans
.
bioRxiv
2023.11.29.569072 https://doi.org/10.1101/2023.11.29.569072
80
Lackner
,
L.L
. (
2019
)
The expanding and unexpected functions of mitochondria contact sites
.
Trends Cell Biol.
29
,
580
590
https://doi.org/10.1016/j.tcb.2019.02.009
81
Scorrano
,
L.
,
De Matteis
,
M.A.
,
Emr
,
S.
,
Giordano
,
F.
,
Hajnóczky
,
G.
,
Kornmann
,
B.
et al.
(
2019
)
Coming together to define membrane contact sites
.
Nat. Commun.
10
, 1287 https://doi.org/10.1038/s41467-019-09253-3
82
König
,
T.
and
McBride
,
H.M
. (
2024
)
Mitochondrial-derived vesicles in metabolism, disease, and aging
.
Cell Metab.
36
,
21
35
https://doi.org/10.1016/j.cmet.2023.11.014
83
Neuspiel
,
M.
,
Schauss
,
A.C.
,
Braschi
,
E.
,
Zunino
,
R.
,
Rippstein
,
P.
,
Rachubinski
,
R.A.
et al.
(
2008
)
Cargo-selected transport from the mitochondria to peroxisomes is mediated by vesicular carriers
.
Curr. Biol.
18
,
102
108
https://doi.org/10.1016/j.cub.2007.12.038
84
Hughes
,
A.L.
,
Hughes
,
C.E.
,
Henderson
,
K.A.
,
Yazvenko
,
N.
and
Gottschling
,
D.E
. (
2016
)
Selective sorting and destruction of mitochondrial membrane proteins in aged yeast
.
Elife
5
, e13943 https://doi.org/10.7554/eLife.13943
85
Wilson
,
Z.N.
,
West
,
M.
,
English
,
A.M.
,
Odorizzi
,
G.
and
Hughes
,
A.L
. (
2024
)
Mitochondrial-derived compartments are multilamellar domains that encase membrane cargo and cytosol
.
J. Cell Biol.
223
, e202307035 https://doi.org/10.1083/jcb.202307035
86
Dailey
,
H.A.
and
Meissner
,
P.N
. (
2013
)
Erythroid heme biosynthesis and its disorders
.
Cold Spring Harb. Perspect. Med.
3
, a011676 https://doi.org/10.1101/cshperspect.a011676
87
Alfadhel
,
M.
,
Lillquist
,
Y.P.
,
Waters
,
P.J.
,
Sinclair
,
G.
,
Struys
,
E.
,
McFadden
,
D.
et al.
(
2011
)
Infantile cardioencephalopathy due to a COX15 gene defect: report and review
.
Am. J. Med. Genet. A
155A
,
840
844
https://doi.org/10.1002/ajmg.a.33881
88
Antonicka
,
H.
,
Mattman
,
A.
,
Carlson
,
C.G.
,
Glerum
,
D.M.
,
Hoffbuhr
,
K.C.
,
Leary
,
S.C.
et al.
(
2003
)
Mutations in COX15 produce a defect in the mitochondrial heme biosynthetic pathway, causing early-onset fatal hypertrophic cardiomyopathy
.
Am. J. Hum. Genet.
72
,
101
114
https://doi.org/10.1086/345489
89
Wu
,
M.L.
,
Ho
,
Y.C.
,
Lin
,
C.Y.
and
Yet
,
S.F
. (
2011
)
Heme oxygenase-1 in inflammation and cardiovascular disease
.
Am. J. Cardiovasc. Dis.
1
,
150
158
90
Indrieri
,
A.
,
Conte
,
I.
,
Chesi
,
G.
,
Romano
,
A.
,
Quartararo
,
J.
,
Tatè
,
R.
et al.
(
2013
)
The impairment of HCCS leads to MLS syndrome by activating a non-canonical cell death pathway in the brain and eyes
.
EMBO Mol. Med.
5
,
280
293
https://doi.org/10.1002/emmm.201201739
91
Valnot
,
I.
,
von Kleist-Retzow
,
J.C.
,
Barrientos
,
A.
,
Gorbatyuk
,
M.
,
Taanman
,
J.W.
,
Mehaye
,
B.
et al.
(
2000
)
A mutation in the human heme a:farnesyltransferase gene (COX10 ) causes cytochrome c oxidase deficiency
.
Hum. Mol. Genet.
9
,
1245
1249
https://doi.org/10.1093/hmg/9.8.1245
92
Zhu
,
Z.
,
Yao
,
J.
,
Johns
,
T.
,
Fu
,
K.
,
De Bie
,
I.
,
Macmillan
,
C.
et al.
(
1998
)
SURF1, encoding a factor involved in the biogenesis of cytochrome c oxidase, is mutated in Leigh syndrome
.
Nat. Genet.
20
,
337
343
https://doi.org/10.1038/3804
93
Antonicka
,
H.
,
Leary
,
S.C.
,
Guercin
,
G.H.
,
Agar
,
J.N.
,
Horvath
,
R.
,
Kennaway
,
N.G.
et al.
(
2003
)
Mutations in COX10 result in a defect in mitochondrial heme a biosynthesis and account for multiple, early-onset clinical phenotypes associated with isolated COX deficiency
.
Hum. Mol. Genet.
12
,
2693
2702
https://doi.org/10.1093/hmg/ddg284
94
Oquendo
,
C.E.
,
Antonicka
,
H.
,
Shoubridge
,
E.A.
,
Reardon
,
W.
and
Brown
,
G.K
. (
2004
)
Functional and genetic studies demonstrate that mutation in the COX15 gene can cause Leigh syndrome
.
J. Med. Genet.
41
,
540
544
https://doi.org/10.1136/jmg.2003.017426
95
Reiter
,
L.T.
,
Murakami
,
T.
,
Koeuth
,
T.
,
Gibbs
,
R.A.
and
Lupski
,
J.R
. (
1997
)
The human COX10 gene is disrupted during homologous recombination between the 24 kb proximal and distal CMT1A-REPs
.
Hum. Mol. Genet.
6
,
1595
1603
https://doi.org/10.1093/hmg/6.9.1595
96
Adams
,
P.L.
,
Lightowlers
,
R.N.
and
Turnbull
,
D.M
. (
1997
)
Molecular analysis of cytochrome c oxidase deficiency in Leigh’s syndrome
.
Ann. Neurol.
41
,
268
270
https://doi.org/10.1002/ana.410410219
97
Bugiani
,
M.
,
Tiranti
,
V.
,
Farina
,
L.
,
Uziel
,
G.
and
Zeviani
,
M
. (
2005
)
Novel mutations in COX15 in a long surviving leigh syndrome patient with cytochrome c oxidase deficiency
.
J. Med. Genet.
42
, e28 https://doi.org/10.1136/jmg.2004.029926
98
Tiranti
,
V.
,
Hoertnagel
,
K.
,
Carrozzo
,
R.
,
Galimberti
,
C.
,
Munaro
,
M.
,
Granatiero
,
M.
et al.
(
1998
)
Mutations of SURF-1 in leigh disease associated with cytochrome c oxidase deficiency
.
Am. J. Hum. Genet.
63
,
1609
1621
https://doi.org/10.1086/302150
99
Coenen
,
M.J.H.
,
van den Heuvel
,
L.P.
,
Ugalde
,
C.
,
Ten Brinke
,
M.
,
Nijtmans
,
L.G.J.
,
Trijbels
,
F.J.M.
et al.
(
2004
)
Cytochrome c oxidase biogenesis in a patient with a mutation in COX10 gene
.
Ann. Neurol.
56
,
560
564
https://doi.org/10.1002/ana.20229
100
Piekutowska-Abramczuk
,
D.
,
Magner
,
M.
,
Popowska
,
E.
,
Pronicki
,
M.
,
Karczmarewicz
,
E.
,
Sykut-Cegielska
,
J.
et al.
(
2009
)
SURF1 missense mutations promote a mild leigh phenotype
.
Clin. Genet.
76
,
195
204
https://doi.org/10.1111/j.1399-0004.2009.01195.x
101
Poyau
,
A.
,
Buchet
,
K.
,
Bouzidi
,
M.F.
,
Zabot
,
M.T.
,
Echenne
,
B.
,
Yao
,
J.
et al.
(
2000
)
Missense mutations in SURF1 associated with deficient cytochrome c oxidase assembly in Leigh syndrome patients
.
Hum. Genet.
106
,
194
205
https://doi.org/10.1007/s004390051028
102
Teraoka
,
M.
,
Yokoyama
,
Y.
,
Ninomiya
,
S.
,
Inoue
,
C.
,
Yamashita
,
S.
and
Seino
,
Y
. (
1999
)
Two novel mutations of SURF1 in Leigh syndrome with cytochrome c oxidase deficiency
.
Hum. Genet.
105
,
560
563
https://doi.org/10.1007/s004399900191
103
Chiabrando
,
D.
and
Tolosano
,
E
. (
2010
)
Diamond blackfan anemia at the crossroad between ribosome biogenesis and heme metabolism
.
Adv. Hematol.
2010
, 790632 https://doi.org/10.1155/2010/790632
104
Vaughan
,
D.P.
and
Costello
,
D.J
. (
2022
)
Extending the phenotype of posterior column ataxia with retinitis pigmentosa caused by variants in FLVCR1
.
Am. J. Med. Genet. A
188
,
1259
1262
https://doi.org/10.1002/ajmg.a.62612
105
Hooda
,
J.
,
Shah
,
A.
and
Zhang
,
L
. (
2014
)
Heme, an essential nutrient from dietary proteins, critically impacts diverse physiological and pathological processes
.
Nutrients
6
,
1080
1102
https://doi.org/10.3390/nu6031080
106
Petrillo
,
S.
,
De Giorgio
,
F.
,
Bertino
,
F.
,
Garello
,
F.
,
Bitonto
,
V.
,
Longo
,
D.L.
et al.
(
2023
)
Endothelial cells require functional FLVCR1a during developmental and adult angiogenesis
.
Angiogenesis
26
,
365
384
https://doi.org/10.1007/s10456-023-09865-w
107
Bertino
,
F.
,
Mukherjee
,
D.
,
Bonora
,
M.
,
Bagowski
,
C.
,
Nardelli
,
J.
,
Metani
,
L.
et al.
(
2024
)
Dysregulation of FLVCR1a-dependent mitochondrial calcium handling in neural progenitors causes congenital hydrocephalus
.
Cell Rep. Med.
5
, 101647 https://doi.org/10.1016/j.xcrm.2024.101647
108
Chiabrando
,
D.
,
Fiorito
,
V.
,
Petrillo
,
S.
and
Tolosano
,
E
. (
2018
)
Unraveling the role of heme in neurodegeneration
.
Front. Neurosci.
12
, 712 https://doi.org/10.3389/fnins.2018.00712
109
Lalani
,
S.R.
,
Liu
,
P.
,
Rosenfeld
,
J.A.
,
Watkin
,
L.B.
,
Chiang
,
T.
,
Leduc
,
M.S.
et al.
(
2016
)
Recurrent muscle weakness with rhabdomyolysis, metabolic crises, and cardiac arrhythmia due to Bi-allelic TANGO2 mutations
.
Am. J. Hum. Genet.
98
,
347
357
https://doi.org/10.1016/j.ajhg.2015.12.008
110
Mingirulli
,
N.
,
Pyle
,
A.
,
Hathazi
,
D.
,
Alston
,
C.L.
,
Kohlschmidt
,
N.
,
O’Grady
,
G.
et al.
(
2020
)
Clinical presentation and proteomic signature of patients with TANGO2 mutations
.
J. Inherit. Metab. Dis.
43
,
297
308
https://doi.org/10.1002/jimd.12156
111
Miyake
,
C.Y.
,
Lay
,
E.J.
,
Soler-Alfonso
,
C.
,
Glinton
,
K.E.
,
Houck
,
K.M.
,
Tosur
,
M
, et al.
(
2023
)
Natural history of TANGO2 deficiency disorder: baseline assessment of 73 patients
.
Genet. Med.
25
,
100352
https://doi.org/10.1016/j.gim.2022.11.020
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