myoglobin

 

(mī'əglō'bĭn) , protein molecule isolated from the cells of vertebrate skeletal muscle that is both a structural and functional relative of hemoglobin, the oxygen-transport protein of the blood of higher animals. Myoglobin, which is composed of a single polypeptide chain of 153 amino acid residues, has the ability to store oxygen by binding it to an iron atom; iron is part of myoglobin's essential chemical composition. The complete amino acid sequence of myoglobin has been determined; it is a relatively small protein with a molecular weight of approximately 17,000 grams per mole. The distribution of myoglobin among the higher animals is a reflection of its physiological function. It is found abundantly in the tissues of diving mammals, e.g., the whale, the seal, and the dolphin. High concentrations of myoglobin in these animals presumably allows them to store sufficient oxygen to remain underwater for long periods. Myoglobin is found abundantly in man only in cardiac muscle, which, by virtue of its essential function, must possess the capacity for continued activity when environmental oxygen concentrations are low. Myoglobin has been investigated intensely and is the first protein molecule to have been completely described in terms of its three-dimensional geometry. This achievement won the British scientist John Kendrew a share in the 1962 Nobel Prize for Chemistry.

Myoglobin
Model of helical domains in myoglobin.
Gene code:	HGNC symbol: MB[1]
Structure:	molecular structure[2]
Recent publications:	role in human pathologies,[3] gene knockout[4]
protein type:	Hemoprotein
Functions:	oxygen storage/transport{100 times more affinity towards oxygen}
Domains:	globin[5]
Diseases:	kidney disease, vasospasm
Taxa expressing:	many metazoan phyla, Archaea?[6] protozoan/eubacterial?[7]
Cell types:	muscle cells
Subcellular localization:	cytoplasm
covalent modifications	glycation?[8] phosphorylation in whales?[9]
Other names:	myoglobin-like proteins in microorganisms[10]
Molecular interactions	oxygen, heme, carbon monoxide, nitric oxide
related articles:	X-ray crystallography, Secondary structure

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.^[11] Unlike the blood-borne hemoglobin, to which it is structurally related,^[12] 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.^[13] For this discovery, John Kendrew shared the 1962 Nobel Prize in chemistry with Max Perutz.^[14]

Meat color

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.^[15]

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.^[16]

Myoglobin is a sensitive marker for muscle injury, making it a potential marker for heart attack in patients with chest pain.^[17] Its lack of specificity and the cost of the analysis has prevented its widespread use.

Structure and bonding

Myoglobin consists of 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:

1. to form hydrogen bonds with the dioxygen moiety, increasing the O₂ binding constant
2. 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.^[18]
3. to prevent irreversible dimerization of the oxymyoglobin with another deoxymyoglobin species

See also

• Hemoglobin
• Neuroglobin
• Hemoprotein

References

Further reading

• 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. 

External links

• The Myoglobin Protein
• Protein Database featured molecule
• Mendelian Inheritance in Man (OMIM) 160000 human genetics

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