Myoglobin

Myoglobin is a monomeric globular protein primarily responsible for storing oxygen in muscle tissue of vertebrates. It is structurally and functionally related to hemoglobin, the protein responsible for oxygen transport in blood.

Structure and Composition:

Myoglobin consists of a single polypeptide chain, approximately 153 amino acids in length, folded into a compact tertiary structure. Embedded within this polypeptide chain is a heme group, a porphyrin ring containing a central iron atom. This iron atom is the site of oxygen binding. The polypeptide chain shields the heme group from water, which is crucial for preventing the iron atom from being oxidized to its ferric state, rendering it unable to bind oxygen reversibly.

Function:

The primary function of myoglobin is to facilitate oxygen diffusion and provide an oxygen reserve within muscle cells. Oxygen diffuses from capillaries into muscle cells, where it binds to myoglobin. When cellular oxygen demand increases, such as during exercise, myoglobin releases its bound oxygen to supply the mitochondria for ATP production via oxidative phosphorylation. Myoglobin has a higher affinity for oxygen than hemoglobin at low oxygen concentrations, which allows it to effectively “draw” oxygen from hemoglobin in the blood and store it in the muscle.

Distribution:

Myoglobin is predominantly found in cardiac and skeletal muscle tissue, where it supports the high metabolic demands of these tissues. The concentration of myoglobin in muscle tissue varies depending on the species and the physiological demands of the muscle. Diving mammals, such as whales and seals, have exceptionally high concentrations of myoglobin in their muscles, allowing them to remain submerged for extended periods.

Clinical Significance:

Myoglobin is released into the bloodstream following muscle damage, such as that caused by trauma, ischemia (lack of blood supply), or rhabdomyolysis (breakdown of muscle tissue). Elevated levels of myoglobin in the blood can be used as an indicator of muscle injury. However, because myoglobin is rapidly cleared from the bloodstream by the kidneys, it is not a specific marker for cardiac damage. Elevated myoglobin can also cause kidney damage if present in high concentrations.