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Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. Myoglobin is only found in the bloodstream after muscle injury. It is an abnormal finding, and can be diagnostically relevant when found in blood. 
Myoglobin is the primary oxygen-carrying pigment of muscle tissues. High concentrations of myoglobin in muscle cells allow organisms to hold their breaths longer. Diving mammals such as whales and seals have muscles with particularly high myoglobin abundance.
Myoglobin was the first protein to have its three-dimensional structure revealed. In 1958, John Kendrew and associates successfully determined the structure of myoglobin by high-resolution X-ray crystallography. For this discovery, John Kendrew shared the 1962 Nobel Prize in chemistry with Max Perutz. Despite being one of the most studied proteins in biology, its true physiological function is not yet conclusively established: mice genetically engineered to lack myoglobin are viable, but showed a 30% reduction in volume of blood being pumped by the heart during a contraction. They adapted to this deficiency through natural reactions to inadequate oxygen supply (hypoxia) and a widening of blood vessels (vasodilation). In humans myoglobin is encoded by the MB gene.
Role in diseaseEdit
Myoglobin is released from damaged muscle tissue (rhabdomyolysis), which has very high concentrations of myoglobin. The released myoglobin is filtered by the kidneys but is toxic to the renal tubular epithelium and so may cause acute renal failure. It is not the myoglobin itself that is toxic (it is a protoxin) but the ferrihemate portion that is dissociated from myoglobin in acidic environments (e.g., acidic urine, lysosomes).
Myoglobin is a sensitive marker for muscle injury, making it a potential marker for heart attack in patients with chest pain. However, elevated myoglobin has low specificity for acute myocardial infarction (AMI) and thus CK-MB, cTnT, ECG, and clinical signs should be taken into account to make the diagnosis.
Myoglobin (abbreviated Mb) is a single-chain globular protein of 153 or 154 amino acids, containing a heme (iron-containing porphyrin) prosthetic group in the center around which the remaining apoprotein folds. It has eight alpha helices and a hydrophobic core. It has a molecular weight of 17,699 daltons (with heme). Unlike the blood-borne hemoglobin, to which it is structurally related, this protein does not exhibit cooperative binding of oxygen, since positive cooperativity is a property of multimeric/oligomeric proteins only.
Structure, bonding and solubilityEdit
Myoglobin contains a porphyrin ring with an iron center. There is a proximal histidine group attached directly to the iron center, and a distal histidine group on the opposite face, not bonded to the iron.
Many functional models of myoglobin have been studied. One of the most important is that of picket fence porphyrin by James P. Collman. This model was used to show the importance of the distal prosthetic group. It serves three functions:
- To form hydrogen bonds with the dioxygen moiety, increasing the O2 binding constant
- To prevent the binding of carbon monoxide, whether from within or without the body. Carbon monoxide binds to iron in an end-on fashion, and is hindered by the presence of the distal histidine, which forces it into a bent conformation. CO binds to hemeTemplate:Which? 23,000 times better than O2, but only 200 times better in hemoglobin and myoglobin. Oxygen binds in a bent fashion, which can fit with the distal histidine.
- To prevent irreversible dimerization of the oxymyoglobin with another deoxymyoglobin species
In chemistry studies, which mostly deal with organic compounds, myoglobin can be dissolved in protic solvents by taking advantage of its structural and bonding characteristics. Dr. Katia C. S. Figueiredo and colleagues have studied myoglobin's structural stability in organic media. In this study they studied the effect of pH, organic solvents, and hydrophobic ion pairing on myoglobin's stability. This study has proved that the structure of myoglobin is least altered at range of pH=5 to pH=7. Study of different solvents effect on myoglobin's structure demonstrated that protic compounds have better performance as myoglobin solvents compared to aprotic ones. Dr. Figueiredo studied three main organic functional groups of protic solvent including alcohols, glycols, and amide. The behavior of myoglobin's solution in alcohols demonstrated a direct proportionality between chain branching and an inverse proportionality to the hydrocarbonic content. This study also showed that alcohols dissolve myoglobin with minor modifications in the heme environment. Ethylene glycol and glycerol were the best solvents when making 50% of the volume of an aqueous solution. Study of aprotic solvents demonstrated that high polar compounds such as N-methylpyrrolidone and dimethyl sulfoxide dissolved myoglobin. However, they damaged the secondary structure of myoglobin. The hydrophobic ion pairing technique showed that the superficial moiety of the protein can be altered by adding very low amounts of SDS, or sodium dodecyl sulfate, which increased the solubility of myoglobin in hexane.
- ↑ 1.0 1.1 Nelson, D. L.; Cox, M. M. (2000). Lehninger Principles of Biochemistry, 3rd, New York: Worth Publishers.
- ↑ 2.0 2.1 Ordway GA, Garry DJ (September 2004). Myoglobin: an essential hemoprotein in striated muscle. J. Exp. Biol. 207 (Pt 20): 3441–6.
- ↑ (U.S.) National Science Foundation: Protein Data Bank Chronology (Jan. 21, 2004). Retrieved 3.17.2010
- ↑ JC Kendrew, G Bodo, HM Dintzis, RG Parrish, H Wyckoff, and DC Phillips (1958). A Three-Dimensional Model of the Myoglobin Molecule Obtained by X-Ray Analysis. Nature 181 (4610): 662–6.
- ↑ The Nobel Prize in Chemistry 1962
- ↑ Mammen PP, Kanatous SB, Yuhanna IS, Shaul PW, Garry MG, Balaban RS, Garry DJ (2003). Hypoxia-induced left ventricular dysfunction in myoglobin-deficient mice. American Journal of Physiology. Heart and Circulatory Physiology 285 (5): H2132–41.
- ↑ Akaboshi E (1985). Cloning of the human myoglobin gene. Gene 33 (3): 241–9.
- ↑ Naka T, Jones D, Baldwin I, Fealy N, Bates S, Goehl H, Morgera S, Neumayer HH, Bellomo R (April 2005). Myoglobin clearance by super high-flux hemofiltration in a case of severe rhabdomyolysis: a case report. Crit Care 9 (2): R90–5.
- ↑ Weber M, Rau M, Madlener K, Elsaesser A, Bankovic D, Mitrovic V, Hamm C (November 2005). Diagnostic utility of new immunoassays for the cardiac markers cTnI, myoglobin and CK-MB mass. Clin. Biochem. 38 (11): 1027–30.
- ↑ Hendgen-Cotta UB, Kelm M, Rassaf T (February 2010). A highlight of myoglobin diversity: the nitrite reductase activity during myocardial ischemia-reperfusion. Nitric Oxide 22 (2): 75–82.
- ↑ Lodish H, Berk A, Zipursky LS, Matsudaira P, Baltimore D, Darnell J (2000). "Evolutionary tree showing the globin protein family members myoglobin and hemoglobin" Molecular Cell Biology, 4th, W. H. Freeman.
- ↑ Collman JP, Brauman JI, Halbert TR, Suslick KS (October 1976). Nature of O2 and CO binding to metalloporphyrins and heme proteins. Proc. Natl. Acad. Sci. U.S.A. 73 (10): 3333–7.
- ↑ Figueiredo KC, Ferraz HC, Borges CP, Alves TL (June 2009). Structural stability of myoglobin in organic media. Protein J. 28 (5): 224–32.
- Collman JP, Boulatov R, Sunderland CJ, Fu L (February 2004). Functional analogues of cytochrome c oxidase, myoglobin, and hemoglobin. Chem. Rev. 104 (2): 561–88.
- Reeder BJ, Svistunenko DA, Cooper CE, Wilson MT (December 2004). The radical and redox chemistry of myoglobin and hemoglobin: from in vitro studies to human pathology. Antioxid. Redox Signal. 6 (6): 954–66.
- Schlieper G, Kim JH, Molojavyi A, Jacoby C, Laussmann T, Flögel U, Gödecke A, Schrader J (April 2004). Adaptation of the myoglobin knockout mouse to hypoxic stress. Am. J. Physiol. Regul. Integr. Comp. Physiol. 286 (4): R786–92.
- Takano, T (1977). Structure of myoglobin refined at 2-0 A resolution. II. Structure of deoxymyoglobin from sperm whale. J. Mol. Biol. 110 (3): 569–584.
- Roy A, Sen S, Chakraborti AS (February 2004). In vitro nonenzymatic glycation enhances the role of myoglobin as a source of oxidative stress. Free Radic. Res. 38 (2): 139–46.
- Stewart JM, Blakely JA, Karpowicz PA, Kalanxhi E, Thatcher BJ, Martin BM (March 2004). Unusually weak oxygen binding, physical properties, partial sequence, autoxidation rate and a potential phosphorylation site of beluga whale (Delphinapterus leucas) myoglobin. Comp. Biochem. Physiol. B, Biochem. Mol. Biol. 137 (3): 401–12.
- Wu G, Wainwright LM, Poole RK (2003). Microbial globins. Adv. Microb. Physiol. 47: 255–310.
- The Myoglobin Protein
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