Myoglobin is a single-chain globular protein of 153 amino acids, containing a heme (iron-containing porphyrin) prosthetic group in the center around which the remaining apoprotein folds. With a molecular weight of 16,700 daltons, it is the primary oxygen-carrying pigment of muscle tissues. 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 reserved for multimeric proteins. Instead, the binding of oxygen by myoglobin is unaffected by the oxygen pressure in the surrounding tissue. Myoglobin is often cited as having an "instant binding tenacity" to oxygen given its hyperbolic oxygen dissociation curve. High concentrations of myoglobin in muscle cells allow organisms to hold their breaths longer. 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. The human version of this gene is MB.
Myoglobin forms pigments responsible for making meat red. The color that meat takes is partly determined by the charge of the iron atom in myoglobin and the oxygen attached to it. In its raw state, the iron atom has a charge of +2 and is bound to O2, an oxygen molecule. Meat cooked well done is brown because the iron atom has a charge of +3, having lost an electron, and is now bound to a water molecule (H2O). 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 a molecule of carbon monoxide. Raw meat packed in a carbon monoxide atmosphere also shows this same pink "smoke ring" due to the same molecular process. Notably, the surface of the raw meat also displays the nice 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 to not 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.
Myoglobin is a sensitive marker for muscle injury, making it a potential marker for heart attack in patients with chest pain. Its lack of specificity and the cost of the analysis has prevented its widespread use.
Structure and bonding
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 are that of picket fence porphyrin by James 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 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
^ Takano, T. "Structure of myoglobin refined at 2-0 A resolution. II. Structure of deoxymyoglobin from sperm whale". J. Mol. Biol.110: 569-584.
^ George A. Ordway and Daniel J. Garry (2004). "Myoglobin: an essential hemoprotein in striated muscle". Journal of Experimental Biology207: pages 3441–3446. doi:10.1242/jeb.01172.
^ Harvey Lodish, Arnold Berk, Lawrence S. Zipursky, Paul Matsudaira, David Baltimore and James Darnell (2000). "Evolutionary tree showing the globin protein family members myoglobin and hemoglobin", Molecular Cell Biology, Fourth Edition, W. H. FREEMAN. ISBN 0-7167-3136-3.
^ 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". Nature181 (4610): pages 662-666. doi:10.1038/181662a0 PMID 13517261.
^ McGee, H: "On Food and Cooking: The Science and Lore of the Kitchen, page 148. Scribner: New York, 2004. ISBN 0-684-80001-2
^ Minneapolis Star Tribune, Nov. 14, 2007 http://www.startribune.com/10223/story/1548852.html
^ Minneapolis Star Tribune, October 31, 2007 http://www.startribune.com/535/story/1518775.html
^ Toshio Naka, Daryl Jones, Ian Baldwin, Nigel Fealy, Samantha Bates, Hermann Goehl, Stanislao Morgera, Hans H. Neumayer and Rinaldo Bellomo (2005). "Myoglobin clearance by super high-flux hemofiltration in a case of severe rhabdomyolysis: a case report". Critical Care9: pages R90–R95. doi:10.1186/cc3034.
^ M. Weber, M. Rau, K. Madlener, A. Elsaesser, D. Bankovic, V. Mitrovic and C. Hamm (2005). "Diagnostic utility of new immunoassays for the cardiac markers cTnI, myoglobin and CK-MB mass". Clinical Biochemistry38: pages 1027–1030. Entrez PubMed 16125162.
^ J. P. Collman, J. I. Brauman, T. R. Halbert, and K. S. Suslick (1976). "Nature of Oxygen and Carbon Monoxide Binding to Metalloporphyrins and Heme Proteins". Proceedings of the National Academy of Sciences of the United States of America73 (10): 3333-3337.
J. P. Collman, R. Boulatov, C. J. Sunderland and L. Fu (2004). "Functional Analogues of Cytochrome c Oxidase, Myoglobin, and Hemoglobin". Chem. Rev.104 (2): 561-588. doi:10.1021/cr0206059.
Reeder, BJ; Svistunenko DA, Cooper CE, Wilson MT (Dec 2004). "The radical and redox chemistry of myoglobin and hemoglobin: from in vitro studies to human pathology". Antioxid Redox Signal6 (6): 954-66. PMID 15548893.
Schlieper, G; Kim JH, Molojavyi A, Jacoby C, Laussmann T, Flogel U, Godecke A, Schrader J (Apr 2004). "Adaptation of the myoglobin knockout mouse to hypoxic stress". Am J Physiol Regul Integr Comp Physiol286 (4): R786-92. PMID 14656764.
Takano, T. "Structure of myoglobin refined at 2-0 A resolution. II. Structure of deoxymyoglobin from sperm whale". J. Mol. Biol.110: 569-584.
Roy, A; Sen S, Chakraborti AS (Feb 2004). "In vitro nonenzymatic glycation enhances the role of myoglobin as a source of oxidative stress". Free Radic Res.38 (2): 139-46. PMID 15104207.
Stewart, JM; Blakely JA, Karpowicz PA, Kalanxhi E, Thatcher BJ, Martin BM (Mar 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 Biol137 (3): 401-12. PMID 15050527.