myoglobin

Myoglobin is a heme-containing globular protein that serves as an oxygen-binding reservoir in the muscle tissue of most vertebrates. It plays a key role in storing and facilitating the diffusion of oxygen from the cell membrane to mitochondria during periods of muscle activity.

Structure: Myoglobin consists of a single polypeptide chain of about 153 amino acids, adopting the globin fold. It contains a heme prosthetic group with an iron ion in the ferrous (Fe2+) state, capable of reversibly binding one molecule of oxygen. The binding site is exposed in a hydrophobic pocket that protects the iron-oxygen complex from solvent.

Function: By maintaining a high intracellular oxygen concentration, myoglobin supports sustained oxidative metabolism in muscles, especially during strenuous activity. It has a relatively high affinity for oxygen, yielding a hyperbolic binding curve with a P50 of roughly 2-3 mmHg, which makes it an efficient oxygen store at low tissue PO2. Myoglobin also facilitates diffusion of oxygen to nearby mitochondria and can act as a scavenger for reactive oxygen species in some contexts.

Distribution and expression: Myoglobin is abundant in cardiac and skeletal muscle fibers, with higher concentrations in oxidative (type I and some type IIa) fibers and in tissues of diving mammals that rely on stored oxygen.

Genetics and clinical significance: The MB gene encodes myoglobin. In humans, elevated plasma myoglobin can indicate muscle injury such as rhabdomyolysis and can contribute to kidney damage. Myoglobin is generally not present in significant amounts in blood under normal conditions but can be released following muscle damage.