Myoglobin is an iron- and oxygen-binding protein found within the muscle cells of vertebrates, specifically in skeletal and cardiac muscle. As a hemoprotein, its structure contains a heme group with an iron ion. This group is responsible for binding to oxygen and gives muscle its characteristic reddish-brown color.
Oxygen Storage and Transport Within Muscle Cells
The primary function of myoglobin is to serve as an oxygen reservoir and transporter within muscle cells. It is concentrated in muscles with high oxygen demands to power their contractions. During periods of rest, myoglobin binds to oxygen that diffuses into the muscle cell from the blood, creating a localized supply for when metabolic needs increase.
This stored oxygen becomes available during intense muscular work. When a muscle contracts vigorously, it consumes available oxygen to produce the energy molecule, ATP. If oxygen demand outpaces the supply from the bloodstream, the oxygen pressure within the cell drops. This drop triggers myoglobin to release its bound oxygen, which then diffuses to the mitochondria to support cellular respiration.
Myoglobin also facilitates the movement of oxygen throughout the muscle cell. By binding to oxygen, it helps maintain a steep diffusion gradient from the capillaries to the cytoplasm, speeding up oxygen transfer from the blood. This action is like a bucket brigade, with myoglobin molecules passing oxygen along to reach the mitochondria efficiently. The concentration of myoglobin can increase in response to endurance training, enhancing the muscle’s aerobic capacity.
Myoglobin vs. Hemoglobin
Both myoglobin and hemoglobin are oxygen-binding proteins, but they have distinct structures and functions. Hemoglobin, found in red blood cells, transports oxygen from the lungs to the body’s tissues. In contrast, myoglobin is located inside muscle cells, where it acts as an oxygen storage unit.
Their molecular structures also differ. Hemoglobin is a larger protein composed of four subunits, each with a heme group, allowing it to carry up to four oxygen molecules. In contrast, myoglobin is a smaller, monomeric protein with only a single heme group and can only bind to one oxygen molecule.
This structural variation leads to a functional difference in their affinity for oxygen. Myoglobin has a much higher affinity for oxygen than hemoglobin, binding it more tightly at lower oxygen concentrations. This property allows myoglobin to pull oxygen from hemoglobin when blood reaches the muscles. Additionally, hemoglobin’s binding is cooperative, meaning its affinity increases as more oxygen binds, which is not the case for myoglobin.
Clinical Significance of Myoglobin Levels
Myoglobin is normally confined within muscle cells. When muscle tissue is damaged, myoglobin is released into the bloodstream. Measuring myoglobin levels in the blood or urine can therefore serve as a sensitive, though not specific, indicator of muscle injury. Conditions that cause elevated myoglobin include physical trauma, overexertion, and certain muscle diseases.
Since the heart is a muscle, a surge in blood myoglobin can be an early marker for a heart attack. Myoglobin is one of the first biomarkers to appear in the bloodstream after chest pain begins, sometimes within hours. However, since any skeletal muscle injury can also elevate myoglobin, this test is not definitive and is used with more specific cardiac markers like troponin.
High concentrations of myoglobin in the bloodstream, known as myoglobinuria when detected in urine, can threaten other organs. The kidneys filter waste from the blood and can be overwhelmed by large amounts of myoglobin. The protein can precipitate in the kidney’s tubules, causing obstruction and cellular damage that may lead to acute kidney injury. This is a serious concern in cases of severe muscle breakdown, such as rhabdomyolysis.
Myoglobin’s Role in Diving Mammals
Myoglobin’s role as an oxygen store is well illustrated in marine mammals like whales and seals. Their ability to perform long, deep dives is due to exceptionally high concentrations of myoglobin in their muscles, up to 30 times greater than in land mammals. This dense concentration gives their muscles an almost black appearance.
This vast oxygen reservoir allows diving mammals to supply their muscles with oxygen during prolonged submersion when access to air is cut off. During a dive, blood flow is redirected to the brain and other organs, while the muscles rely on their internal, myoglobin-bound oxygen supply to function.
This high concentration of myoglobin enables these animals to maintain aerobic metabolism in their muscles for a significant portion of their dive. This delays the onset of anaerobic respiration and the buildup of lactic acid, a factor in their ability to hunt and navigate underwater for long durations.