Model of helical domains in myoglobin.
|RNA expression pattern|
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. The only time myoglobin is found in the bloodstream is when it is released following 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.
Meat color 
Myoglobin forms pigments responsible for making meat red. The color that meat takes is partly determined by the oxidation states of the iron atom in myoglobin and the oxygen species attached to it. When meat is in its raw state, the iron atom is in the +2 oxidation state, and is bound to a dioxygen molecule (O2). Meat cooked well done is brown because the iron atom is now in the +3 oxidation state, having lost an electron, and is now coordinated by a water molecule. Under some conditions, meat can also remain pink all through cooking, despite being heated to high temperatures. If meat has been exposed to nitrites, it will remain pink because the iron atom is bound to NO, nitric oxide (true of, e.g., corned beef or cured hams). Grilled meats can also take on a pink "smoke ring" that comes from the iron binding to a molecule of carbon monoxide. Raw meat packed in a carbon monoxide atmosphere also shows this same pink "smoke ring" due to the same coordination chemistry. Notably, the surface of this raw meat also displays the pink color, which is usually associated in consumers' minds with fresh meat. This artificially induced pink color can persist in the meat for a very long time, reportedly up to one year. Hormel and Cargill are both reported to use this meat-packing process, and meat treated this way has been in the consumer market since 2003. Myoglobin is found in Type I muscle, Type II A and Type II B, but most texts consider myoglobin not to be found in smooth muscle.
Role in disease 
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 solubility 
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 heme[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.
See also 
- PDB 1MBO; Takano T (March 1977). "Structure of myoglobin refined at 2.0 Å resolution. II. Structure of deoxymyoglobin from sperm whale". J. Mol. Biol. 110 (3): 569–84. doi:10.1016/S0022-2836(77)80112-5. PMID 845960.
- Nelson, D. L.; Cox, M. M. (2000). Lehninger Principles of Biochemistry (3rd ed.). New York: Worth Publishers. p. 206. ISBN 0-7167-6203-X.
- Ordway GA, Garry DJ (September 2004). "Myoglobin: an essential hemoprotein in striated muscle". J. Exp. Biol. 207 (Pt 20): 3441–6. doi:10.1242/jeb.01172. PMID 15339940.
- (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. Bibcode:1958Natur.181..662K. doi:10.1038/181662a0. PMID 13517261.
- 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. doi:10.1152/ajpheart.00147.2003. PMID 12881221.
- Akaboshi E (1985). "Cloning of the human myoglobin gene". Gene 33 (3): 241–9. doi:10.1016/0378-1119(85)90231-8. PMID 2989088.
- McGee H (2004). On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner. p. 148. ISBN 0-684-80001-2.
- Fraqueza MJ, Barreto AS (September 2011). "Gas mixtures approach to improve turkey meat shelf life under modified atmosphere packaging: the effect of carbon monoxide". Poult. Sci. 90 (9): 2076–84. doi:10.3382/ps.2011-01366. PMID 21844276.
- Associated Press (2007-10-30). "Meat companies defend use of carbon monoxide". Business. Minneapolis Star Tribune.
- 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. doi:10.1186/cc3034. PMC 1175920. PMID 15774055.
- 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. doi:10.1016/j.clinbiochem.2005.07.011. PMID 16125162.
- 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. doi:10.1016/j.niox.2009.10.003. PMID 19836457.
- 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 ed.). W. H. Freeman. ISBN 0-7167-3136-3.
- 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. doi:10.1073/pnas.73.10.3333. PMC 431107. PMID 1068445.
- Figueiredo KC, Ferraz HC, Borges CP, Alves TL (June 2009). "Structural stability of myoglobin in organic media". Protein J. 28 (5): 224–32. doi:10.1007/s10930-009-9187-y. PMID 19629659.
Further reading 
- 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. doi:10.1021/cr0206059. PMID 14871135.
- 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. doi:10.1089/ars.2004.6.954. PMID 15548893.
- 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. doi:10.1152/ajpregu.00043.2003. PMID 14656764.
- 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. doi:10.1016/S0022-2836(77)80112-5. PMID 845960.
- 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. doi:10.1080/10715160310001638038. PMID 15104207.
- 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. doi:10.1016/j.cbpc.2004.01.007. PMID 15050527.
- Wu G, Wainwright LM, Poole RK (2003). "Microbial globins". Adv. Microb. Physiol. 47: 255–310. doi:10.1016/S0065-2911(03)47005-7. PMID 14560666.
- The Myoglobin Protein
- Protein Database featured molecule
- Online 'Mendelian Inheritance in Man' (OMIM) 160000 human genetics
- Which Cut Is Older? (It's a Trick Question) New York Times, February 21, 2006 article regarding meat industry use of carbon monoxide to keep meat looking red.
- Stores React to Meat Reports New York Times, March 1, 2006 article on the use of carbon monoxide to make meat appear fresh.