Myoglobin is a globular, iron-containing protein that stores molecular oxygen in muscles and delivers it to the oxidative cell systems. Structurally, it is similar to a single sub-unit of hemoglobin and it has a higher affinity for oxygen than hemoglobin of the blood. Thus, myoglobin stores and supplies oxygen to the cardiac and skeletal muscle fibers, acting as a local oxygen reserve. The serum levels of this protein are often measured to facilitate diagnosis of an acute myocardial infarction. It is released into the circulation within 2-4 hours after myocardial infarction. Also known as muscle hemoglobin, it is found only in striated muscles.
Myoglobin is the first protein whose structure was elucidated by X-ray diffraction analysis (J. Kendrew and his associates, 1957–60). It consists of a single polypeptide chain (about 70 percent of the 153 amino-acid residues are included in the spiral parts). As in hemoglobin, the active center of the myoglobin molecule that binds O₂ is heme. The molecular weight of myoglobin is 17,000. The spatial structure of myoglobin resembles the beta chain of hemoglobin. Myoglobin combines reversibly with oxygen even at low partial pressures of oxygen. This is of great physiological importance: the partial pressure of oxygen decreases sharply during muscle contraction as a result of compression of the capillaries, and it is at this very moment that the oxygen needed by the working muscle is released from myoglobin.
Animals with the highest concentration of myoglobin
The muscles of vertebrates contain about two percent myoglobin per dry tissue mass; the muscles of marine animals capable of staying under water a long time (seal, whale, dolphin) contain about 20 percent. Myoglobin determines the color of muscles in vertebrates, especially in mammals. High concentrations of myoglobin in muscle cells allow animals to hold their breath for a longer period of time. Sea mammals, such as whales, dolphins, and seals, are the animals with the highest concentration of myoglobin in their muscles. Dolphins can hold their breath underwater for as long as twenty minutes.
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| Myoglobin chemical structure. |