What protein stores oxygen in muscle tissue?

Myoglobin is the protein that stores oxygen in muscle tissue. It is found mainly in cardiac and skeletal muscle, where its iron-containing heme group binds oxygen and helps make it available to mitochondria during activity.

Myoglobin resembles hemoglobin, but the two proteins have different jobs. Hemoglobin circulates inside red blood cells and transports oxygen through the bloodstream, while myoglobin remains inside muscle cells. Myoglobin binds oxygen more tightly than hemoglobin, helping muscle fibers maintain an oxygen reserve when demand rises.

The distinction is especially important in diving mammals. Whales and seals have unusually high concentrations of myoglobin, which helps their muscles keep working during long dives. A common mix-up is ferritin, which stores iron, and transferrin, which transports iron in the blood. Myoglobin stores oxygen, not iron itself, although its heme group contains iron. When muscle is damaged, myoglobin can leak into the blood and urine; high levels may occur in rhabdomyolysis and can injure the kidneys.