Myoglobin

Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. Myoglobin is only found in the bloodstream after muscle injury. It is an abnormal finding, and can be diagnostically relevant when found in blood.

Myoglobin is the primary oxygen-carrying pigment of muscle tissues. High concentrations of myoglobin in muscle cells allow organisms to hold their breaths longer. Diving mammals such as whales and seals have muscles with particularly high myoglobin abundance.

Myoglobin was the first protein to have its three-dimensional structure revealed. 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. 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, but showed a 30% reduction in volume of blood being pumped by the heart during a contraction. They adapted to this deficiency through natural reactions to inadequate oxygen supply (hypoxia) and a widening of blood vessels (vasodilation). In humans myoglobin is encoded by the MB gene.

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. It is not the myoglobin itself that is toxic (it is a protoxin) but the ferrihemate portion that is dissociated from myoglobin in acidic environments (e.g., acidic urine, lysosomes).

Myoglobin is a sensitive marker for muscle injury, making it a potential marker for heart attack in patients with chest pain. However, elevated myoglobin has low specificity for acute myocardial infarction (AMI) and thus CK-MB, cTnT, ECG, and clinical signs should be taken into account to make the diagnosis.

Chemistry
Myoglobin (abbreviated Mb) is a single-chain globular protein of 153 or 154 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 17,699 daltons (with heme). 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.

Structure, bonding and solubility
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 P. 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

In chemistry studies, which mostly deal with organic compounds, myoglobin can be dissolved in protic solvents by taking advantage of its structural and bonding characteristics. Dr. Katia C. S. Figueiredo and colleagues have studied myoglobin's structural stability in organic media. In this study they studied the effect of pH, organic solvents, and hydrophobic ion pairing on myoglobin's stability. This study has proved that the structure of myoglobin is least altered at range of pH=5 to pH=7. Study of different solvents effect on myoglobin's structure demonstrated that protic compounds have better performance as myoglobin solvents compared to aprotic ones. Dr. Figueiredo studied three main organic functional groups of protic solvent including alcohols, glycols, and amide. The behavior of myoglobin's solution in alcohols demonstrated a direct proportionality between chain branching and an inverse proportionality to the hydrocarbonic content. This study also showed that alcohols dissolve myoglobin with minor modifications in the heme environment. Ethylene glycol and glycerol were the best solvents when making 50% of the volume of an aqueous solution. Study of aprotic solvents demonstrated that high polar compounds such as N-methylpyrrolidone and dimethyl sulfoxide dissolved myoglobin. However, they damaged the secondary structure of myoglobin. The hydrophobic ion pairing technique showed that the superficial moiety of the protein can be altered by adding very low amounts of SDS, or sodium dodecyl sulfate, which increased the solubility of myoglobin in hexane.