Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

Myoglobin (Mb)

  • Niharika Swain
  • Shilpa Patel
  • Rashmi Maruti Hosalkar
Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_102005

Synonyms

Historical Background

Myoglobin (Mb) is a monomeric, cytoplasmic hemoprotein with rich and varied history, thus becoming an object of renewed interest owing to its role beyond those previously described. In 1987, Mörner first identified and differentiated Mb from hemoglobin (Hb) by their differences in absorption spectra and further termed it as myochrome (Garry and Mammen 2007). Interestingly, it is the first protein upon which a three-dimensional structure was determined. The notation Mb was introduced by Günther, who confirmed the results of Mörner in 1921 (Hendgen-Cotta et al. 2010). The term Mb has been given owing to its functional and structural similarity with Hb. For a long time, it was believed that expression of Mb in vertebrates was restricted to muscle tissue like cardiomyocytes, oxidative skeletal myofibers, and vascular smooth muscle cells and its associated diseases (Hendgen-Cotta et al. 2010). However, recent studies have revealed nonmuscle Mb expression in normoxic as well as hypoxic conditions along with human tumors including those of epithelial origin such as breast, lung, ovary, and colon carcinomas (Fraser et al. 2006; Flonta et al. 2009).

Gene Transcription

Evidence suggests that Mb and Hb proteins of modern organisms have evolved from a common ancestral gene. The Mb gene arose by duplication of a primitive globin gene some 800 million years ago whereas phylogenetic analyses have suggested early appearance (500 million years ago) of Mb in evolution of vertebrates (Burmester et al. 2002). In contrast to the large number of genes encoding the various Hbs, a single gene encodes Mb of both skeletal and cardiac muscle in man. The linkage arrangement between the Hb and Mb genes in man was analyzed using man-rodent somatic cell hybrids. The single functional Mb gene was assigned to chromosome 22 in the region 22qll → • 22ql3 and is therefore not linked to the α- or β-like globin genes but represents a third dispersed globin locus (Jeffreys et al. 1984). The Mb gene structure of vertebrates consists of three exons and two introns. Intron 1 (5.8 kb) and intron 2 (3.6 kb) are much longer than their Hb counterparts (108–886 basepairs (bp) and 103–906 basepairs, respectively) and combine to make the human Mb gene, the longest globin gene so far described (10.4 kb) (Blanchetot et al. 1983). Exon 2 of Mb gene encodes amino acids 31–105, including the heme binding domain of the 154-residue monomeric cytoplasmic protein. Another unusual feature of Mb gene is that they have very long 3′ nontranslated mRNA sequences (Fig. 1). The cap and polyadenylation sites are located by homology with sites in the seal Mb gene. The 5′-flanking region of the human Mb gene contains only one of the three conserved elements (TATA box, CCAC box, and an A:T motif) frequently found near globin genes and required for promotion of transcription by RNA polymerase II (Grosveld et al. 1982; Dierks et al. 1983). A normal TATA box is located 33 bp before the cap site. In human Mb gene the CCAAT box located 70–90 bp upstream is absent, though not so well preserved CCATT sequence exists at position 67. The dimerized CACCC element in the 100 region of β-globin genes is also absent. Instead, an unusual purine rich sequence (90% AG) occupies 68–114 bp upstream region from the cap site. Several candidates cognate binding factors such as Sp1, muscle enhancer factor-2 (MEF2 factors of the MADS box family), nuclear factor of activated T-cell (NAFT), and Myo D family that recognizes the CCAC A:T motifs, NFAT response element, and E-box of the Mb gene and regulates transcriptional activity. Studies have shown Sp1, MEF2, and NAFT to respond to calcium regulated signaling pathways (calcineurin pathways) and thus being potent transcriptional regulators, while E-box in the 5′-untranslated region participated in fiber type specific transcription of Mb gene (Garry and Mammen 2007) (Fig. 2).
Myoglobin (Mb), Fig. 1

mRNA of myoglobin gene with three exons and two introns. Exon 2 of Mb gene (encodes 31–105 amino acids) is comprising of main functional domain

Myoglobin (Mb), Fig. 2

Regulation of myoglobin expression through calcineurin and noncalcineurin pathways in cell contraction and rest affecting prominent transcriptional factors (NFAT, nuclear factor of affected T cell; MEF2, myocyte enhancer factor-2) and motifs on promoter (CCAC box, cysteine rich region; NRE, NFAT response element)

Structure and Distribution

Nearly 40 years ago through X-ray diffraction experiment John Kendrew and coworkers resolved structure of Mb in anatomic detail through X-ray diffraction experiment. The backbone of Mb consists of a single polypeptide chain of 153 (in human) amino acids which is formed by eight alpha helices assigned letters A-H joined by short nonhelical regions. The folding occurs in such a way that almost all hydrophilic (polar) groups are outside of the protein facing aqueous environment, while the hydrophobic (nonpolar) groups extend almost entirely into the protein thus maintaining the stability by large number of van der Waals interaction (Kendrew et al. 1958). The rigid structure of polypeptide chains cradles the heme prosthetic group such that it is positioned in between two histidine residues, His 64 and His 93. The heme prosthetic group consists of heme residue, porphyrin ring (iron ion complex). It is to this heme residue that oxygen binds thus playing key role in functioning of Mb. The iron is ligated to four nitrogen of the protoporphyrin in a common plane. The fifth ligation is to imidazole side chain of His93 thus stabilizing the heme prosthetic group in the hydrophobic pocket and displacing iron slightly away from the plane of heme. This leaves a sixth ligation position, on the side of the heme plane opposite (distal) to the histidine, available for the binding of oxygen as well as for other potential ligands such as CO or NO (Ordway and Garry 2004). In addition, studies have identified four highly conserved internal cavities within Mb that may serve to concentrate and orient molecules for binding to the heme residue (Frauenfelder et al. 2001; Ordway and Garry 2004). Utilizing ultrastructural immunohistochemical techniques, Mb was localized to the A-band and the I-band in skeletal muscle. Mb is present in the heart and the skeletal muscle of virtually all mammals, and its concentration is dependent on the species, the environmental conditions (i.e., living at sea level vs. high altitude), and the level of activity. In the heart, the basal Mb concentration is approximately 200–300 μmol kg−1 wet weight; while in the skeletal muscle levels are much higher reaching approximately 400–500 μmol kg−1 wet weight. Physiologically, small concentrations of Mb are also expressed in smooth muscle cells. Additionally, significant levels of Mb have been measured in tumor cells even of nonmuscle origin (Kamga et al. 2012).

Myoglobin in Health

Since its inception, Mb has been proved to play various roles apart from its main function of oxygen storage and diffusion. Mb functions as an intracellular catalyst in NO homeostasis and in scavenging of reactive oxygen species (Fig. 3).
Myoglobin (Mb), Fig. 3

Established functions of myoglobin as oxygen transporter and intracellular catalyst (neutralization of free radicals including reactive oxygen/nitrogen species)

Oxygen Storage and Its Facilitated Diffusion

It is the primary function of Mb to store oxygen in striated and cardiac muscles. Its concentration is about 100–300 μM and is sufficient to sustain aerobic metabolism for few seconds (Brunori 2010). Mb also functions as a short-term O2 storage molecule in the human heart when blood supply is briefly interrupted during normal systole. The O2 storage function of Mb is much established in diving mammals and birds which largely depend on extremely high Mb concentrations to increase body O2 stores and to prolong aerobic dive performance (Helbo et al. 2013).

Facilitated diffusion of oxygen is an important function of Mb. In 1959, Wittenberg proposed that oxygen diffusion was enhanced within cell from the sarcolemma to mitochondria by the ability of Mb to rapidly and reversibly bind oxygen and translationally diffuse within the cell (Lin et al. 2007). Studies have also demonstrated that affinity of Mb to bind oxygen could compete with ability of oxygen to dissolve in solution thus proving the capability of Mb to capture oxygen while it crosses phases from capillary to cell. Thus, it transports oxygen within the cell, offloading the oxygen as it tries to come to equilibrium with deoxygenated Mb pool within the cell. It is believed that under hypoxic conditions or prolonged physiological exercise Mb is integral for oxygen supply to drive oxidative phosphorylation (Kamga et al. 2012).

Intracellular Catalyst

Many in vitro studies have demonstrated that Mb functions as physiological catalyst as it scavenges/neutralizes both reactive oxygen species (ROS) and reactive nitrogen species (RNS). Ferrous and ferric forms of Mb react with reactive oxygen species (mainly H2O2) oxidizing it to the ferryl form. Ferryl Mb, a strong oxidant, can oxidize proteins and lipids as well as interact with a protein radical, forming a cross-linked species capable of generating more hydrogen peroxide and initiating lipid peroxidation. However, this physiological role has shown to yield both protective and deleterious effects. Another physiological role was first proposed by Brunori which was based on the fact that NO reversibly binds and inhibits cytochrome c oxidase and hence depresses mitochondrial respiration. He hypothesized that in working muscle or heart, oxygenated Mb functions as a NO scavenger to prevent NO-mediated inhibition of mitochondrial respiration (Brunori 2001). Beyond its NO scavenging role and regulation of mitochondrial respiration/oxidative phosphorylation, Mb also influences ROS generation and apoptosis. Mb-dependent attenuation of respiratory inhibition also decreases mitochondrial ROS generation specifically O2 (superoxide), which can attenuate oxidative stress and modulate redox signaling in the heart and skeletal muscle. Though exact mechanism explaining its regulation on apoptosis is controversial, it has been suggested that low concentration of NO prevents cytochrome c release from mitochondria, a key molecule for initiation of apoptosis, while higher concentration favors its release (Brune 2003).

Mb in Pathological Conditions

In Parasitic Infections

In mammals, infection by a variety of protozoa and helminth parasites (Trypanosoma cruzi, Toxoplasma gondii, Leishamania donovani, Leishmania major, Schistosoma mansoni) upregulates NO synthesis via elevated activity of inducible nitric oxide synthetase through production of proinflammatory cytokines like IL-1α, IL-1β, and TNF-γ. Elevated NO production seems to be a protective phenomenon of host against the pathogens by direct cytotoxicity or their growth inhibition. Scavanging action of NO by Mb could be disastrous as it can favor colonization of parasites in cardiomyocyte leading cardiomyopathy or in Chagas disease (Hendgen-Cotta et al. 2010).

In Myocardial Ischemia and Reperfusion Injury

Mb could play a vital role in tissue damage due to ischemia and reperfusion in heart as it has the ability to exhibit peroxidatic activity leading to higher oxidation states of heme. Various experimental studies observed that generation of ferryl Mb through interaction of ferrous/ferric Mb with H2O2 enhances the lipid peroxidation. In addition, within this phase, formation of a cross-linked form of Mb (Mb-X), due to interaction of ferryl heme with a protein-based radical, is more cytotoxic as compared to ferryl Mb as it oxidizes both free and membrane bound lipids (Hendgen-Cotta et al. 2010).

In Renal Diseases

Cumulative evidences of detection of cytotoxic derivative of Mb in the urine of patients with rhabdomyolytic-associated acute renal failure implied a pathological involvement of Mb in this disease. In rhabdomyolysis, breakdown of muscle fibers occurs, resulting in the release of cellular components including Mb into the circulation and further accumulation of these detrimental substances leading to renal diseases. Furthermore, while filtration, the released Mb precipitates in kidney and causes obstructive cast formation. Besides, it can exert a direct cytotoxic effect through the enhancement of local oxidative stress via lipid peroxidation in the tubular cells (Hendgen-Cotta et al. 2010).

In Tumorigenesis

Exact role of Mb is not well explored though altered expression of Mb is observed in few tumors which not only include that originate from muscle tissue such as sarcomas and rhabdomyosarcomas or myoid differentiation but also epithelial tumors like lung, ovary, colon carcinomas, and a small sample of breast cancer cases. During progression, the tumor cell adapts to tissue hypoxia by induction of transcription factor hypoxia inducible factor-1α (HIF-1α) which is a key regulator in various cascades of cellular responses like angiogenesis, erythropoiesis, metabolism, and cell survival and proliferation. Mb is induced by mitogenic stimuli, oxidative stress, and hypoxia as the tumors progress via production of reactive oxygen/nitrogen species. Few authors supported the antitumorigenic role of Mb through two mechanisms: (1) Being an oxygen store house and transporter, NO tries to pacify oxygen craving of proliferative tumor cells and thus helps in the inhibition of HIF-1α-mediated protumoral responses. (2) Another possibility is that Mb affects the induction of HIF-1α and stabilization by modulating prolyl hydroxylase enzymes. (3) Mb-mediated tumor oxygenation also promoted differentiation of cancer cells thus suppressing both local and distal metastatic spreading (Michieli 2009) (Fig. 4).
Myoglobin (Mb), Fig. 4

Antitumoral role of myoglobin through mechanisms like direct effect, i.e., suppression of tumor growth by improving cell differentiation and scavenging free radicals and indirect effect reduction in hypoxia through oxygen transport

Summary

Mb seems to have pathophysiological significance not only for tissues that express it but also in nonmuscular tissue. Its multifaceted role ranging from as oxygen transporter to scavenger of free radicals has been attracting many researchers for exploration of possible role of Mb in pathogenesis of various diseases involving wide range of tissues. Specifically in mechanism of tumorigenesis, the contribution of Mb is yet to be unfolded. Although the underlying mechanisms of Mb-related effects are being analyzed thoroughly through various researches, it will be fascinating to follow its use in developing future trails and drug design programs.

References

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  • Niharika Swain
    • 1
  • Shilpa Patel
    • 1
  • Rashmi Maruti Hosalkar
    • 2
    • 3
  1. 1.MGM Dental College and HospitalNavi MumbaiIndia
  2. 2.Indian Association of Oral and Maxillofacial PathologistsMumbaiIndia
  3. 3.Maharashtra State Dental CouncilMumbaiIndia