Myoglobin

Myoglobin is an oxygen-binding protein pigment found in the skeletal muscle. When the skeletal muscle is damaged, the myoglobin is released into the bloodstream. It is filtered out of the bloodstream by the kidneys. Myoglobin may occlude the structures of the kidney, causing damage such as acute tubular necrosis or kidney failure. Myoglobin, an extremely compact heme protein (MW ~ 17 800), found primarily in cardiac and red skeletal muscles, functions in the storage of oxygen and facilitates the transport of oxygen to the mitochondria for oxidative phosphorylation.1 Myoglobin is particularly abundant in diving mammals including the whale, seal, and porpoise, whose muscles are so rich, they are brown. These mammals are able to remain submerged for long periods due to the storage of oxygen by muscle myoglobin.

Myoglobin breaks down into potentially toxic compounds, which will also cause kidney failure. Necrotic (dead tissue) skeletal muscle may cause massive fluid shifts from the bloodstream into the muscle, reducing the relative fluid volume of the body and leading to shock and reduced blood flow to the kidneys.

Myoglobin is a protein in heart and skeletal muscles. When a muscle is exercised, it uses up available oxygen. Myoglobin has oxygen bound to it, thus providing an extra reserve of oxygen so that the muscle can maintain a high level of activity for a longer period of time. When muscle is damaged, the myoglobin is released into the bloodstream. It is filtered out of the bloodstream by the kidneys, and eliminated in urine. In large quantities, myoglobin can damage the kidney and break down into toxic compounds, causing kidney failure.

Myoglobin is a single-chain protein of 153 amino acids, containing an iron porphyrin group in the center. It 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. Instead, the binding of oxygen by myoglobin is unaffected by the oxygen tension in the surrounding tissue.

Myoglobin is most useful when combined with an ECG. Elevated myoglobin and ST segment changes on an ECG are very indicative of an AMI. Other cardiac markers (CK, CK-MB, Troponins) should be pursued if an elevated myoglobin and non-diagnostic ECG are present. Myoglobin is an attractive moiety because it is elevated very early after an AMI, when thrombolytics are most helpful. Since myoglobin is secreted by the kidneys, a positive urine dipstick for hemoglobin can prompt further investigation of the serum for myoglobin

Myoglobin is a tightly contained protein with very little empty space within the molecular structure. Approximately 75% of the main chain is an alpha-helix, and the heme group is found in a small crevice near the center of the protein. The oxygen-binding epitope is found externally, although the oxygen molecule binds directly with the heme group. Myoglobin has been obtained in pure crystalline form from many sources. It has a molecular weight of 16,700, about one-fourth that of hemoglobin. Though the heme portion of all myoglobins is the same, the protein portions vary considerably between species.

There is a close chemical similarity between myoglobin and hemoglobin, the oxygen-binding protein of red blood cells. Both proteins contain a molecular constituent called heme, which enables them to combine reversibly with oxygen. The heme group, which contains iron, imparts a red-brown colour to the proteins. The bond between oxygen and hemoglobin is more complex than that between oxygen and myoglobin and accounts for the dual ability hemoglobin has to transport oxygen as well as to store it. In contact with venous blood, oxygen combines more readily with myoglobin than it does with hemoglobin, favouring the transfer of oxygen from blood to muscle cells. Thus, the oxygen that the working muscle requires for the energy-producing biochemical reactions is provided.

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