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).
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
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).
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).
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.
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