myoglobin

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 eight alpha helices and a hydrophobic core. 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.

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.

Structure

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:

 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.

Pathology

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 cTnT is used in combination with ECG, and the clinical signs to diagnose Acute Myocardial Infarction (AMI).