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'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.^[1] Unlike the blood-borne hemoglobin, to which it is structurally related,^[2] 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. In 1958, John Kendrew and associates successfully determined the structure of myoglobin by high-resolution X-ray crystallography.^[3] For this discovery, John Kendrew shared the 1962 Nobel Prize in chemistry with Max Perutz.^[4]

Contents

Meat color

Role in disease

Structure and bonding

See also

References

Further reading

External links

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.^[5]

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.^[6]
Myoglobin is a sensitive marker for muscle injury, making it a potential marker for heart attack in patients with chest pain.^[7] 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:
# 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.^[8]
# to prevent irreversible dimerization of the oxymyoglobin with another deoxymyoglobin species

See also

★ Hemoglobin

★ Neuroglobin

★ Hemoprotein

References

1.
2. Molecular Cell Biology, Harvey Lodish, Arnold Berk, Lawrence S. Zipursky, Paul Matsudaira, David Baltimore and James Darnell, , , W. H. FREEMAN, 2000, ISBN 0-7167-3136-3
3.
4. The Nobel Prize in Chemistry 1962
5. McGee, H: "On Food and Cooking: The Science and Lore of the Kitchen, page 148. Scribner: New York, 2004. ISBN 0-684-80001-2
6.
7. {{cite journal | title = Diagnostic utility of new immunoassays for the cardiac markers cTnI, myoglobin and CK-MB mass | author = M. Weber, M. Rau, K. Madlener, A. Elsaesser, D. Bankovic, V. Mitrovic and C. Hamm | journal = Clinical Biochemistry | year = 2005 | volume = 38 | pages = pages 1027–1030 | id =
8. Carbon Monoxide Binding to Iron Porphyrins, J. P. Collman, J. I. Brauman and K. M. Doxsee, , , Proceedings of the National Academy of Sciences of the United States of America, 1979

Further reading

★ Functional Analogues of Cytochrome c Oxidase, Myoglobin, and Hemoglobin, J. P. Collman, R. Boulatov, C. J. Sunderland and L. Fu, , , Chem. Rev., 2004

External links

★ The Myoglobin Protein

★ Protein Database featured molecule

★ human genetics