myoglobin n : a hemoprotein that receives oxygen from hemoglobin and stores it in the tissues until needed

English

Noun

1. A small globular protein, containing a heme group, that carries oxygen to muscles

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. It has a molecular weight of 16,700 daltons, and 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 of multimeric/oligomeric proteins only. 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. 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 and show no obvious defects.

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. When meat is 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 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.

Myoglobin is a sensitive marker for muscle injury, making it a potential marker for heart attack in patients with chest pain. CK-MB and TnT is used in combination with ECG, and the clinical signs to diagnose AMI

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 is 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.
3. to prevent irreversible dimerization of the oxymyoglobin with another deoxymyoglobin species

See also

• Hemoglobin
• Neuroglobin
• Cytoglobin
• Hemoprotein

References

Further reading

External links

• The Myoglobin Protein
• [http://pdbdev.sdsc.edu:48346/pdb/molecules/mb1.html Protein Database featured molecule]
• human genetics
• Which Cut Is Older? (It's a Trick Question) New York Times, February 21, 2006 article regarding meat industry use of carbon dioxide 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.