Myoglobin

Myoglobin is a protein found in muscles in humans and other mammals. It contains iron and is important because it attracts and holds oxygen for use by the muscles. Myoglobin does not normally appear in the bloodstream. Because of this, its presence can be used as an indicator of certain health problems. Physicians can find evidence of damage to muscles, including the heart, by ordering blood or urine tests to look for the presence of myoglobin.

Background

Myoglobin, which is often abbreviated Mb or MB, is part of the process that facilitates the storage and diffusion of oxygen within muscle tissue in humans and most other mammals. It is made up of eight of the most common forms of protein structures, the alpha helix, which is connected by loops. The grouping has a special kind of protein formation known as a porphyrin at its center. In humans, myoglobin contains about 154 different amino acids, which are compounds that are often called the building blocks of proteins.

Myoglobin is extremely sensitive to oxygen and attracts it from the blood supply into muscles, most specifically the cardiac muscle that makes up the heart and the muscles connected to the skeletal system. It is present in smooth muscle tissue, although at much lower concentrations than in skeletal and cardiac muscle. The oxygen-gathering properties of myoglobin are important to life in all animals, but they are especially important to deep-diving mammals such as whales, seals, and dolphins, as well as some marine birds, which rely on oxygen-rich myoglobin to sustain them while they are underwater. In these animals, the ability of myoglobin to store oxygen in the muscles is generally ten to thirty times above the levels found in animals that do not experience long periods without breathing due to submersion in water.

The presence of myoglobin is also what gives meat its red color. In fact, the liquid that is seen in a package of raw beef or other meat is not blood but actually a liquid containing myoglobin. The color of the meat is affected by how much oxygen is contained in the myoglobin in the meat; the more oxygenated myoglobin the meat contains, the redder it will appear.

Myoglobin was the first protein to be studied by X-ray crystallography. John Cowdery Kendrew, an English biochemist, and Max Ferdinand Perutz, an Austrian-born English biochemist, used x-ray diffraction to examine the three-dimensional structure of myoglobin. The pair shared the 1962 Nobel Prize in Chemistry for their work in unveiling the form of the protein. In spite of this, scientists are still not completely certain how the protein works because some animals that are bred to have less myoglobin than should be necessary to sustain life are able to survive. Research continues to determine the full nature and purpose of myoglobin as well as to discover possible ways to enhance and improve its performance.

Overview

Scientists may not fully understand the regulatory roles of myoglobin, but the primary functions of myoglobin are well established. A blood component called hemoglobin carries oxygen to the muscle tissue, where it is picked up by the myoglobin. Hemoglobin is also an iron-rich protein, but unlike myoglobin, hemoglobin circulates in the blood. Scientists believe that both myoglobin and hemoglobin may have roots in the same ancestral gene, which most likely changed during the evolutionary process to have two separate functions.

Hemoglobin picks up oxygen as the blood passes through the lungs and transports it to the other muscles where it is needed. While hemoglobin attracts oxygen, myoglobin is able to attract more oxygen than a similar amount of hemoglobin; therefore, myoglobin can serve as an oxygen storage site to provide reserves of oxygen for times of hypoxia (deficiency in the amount of oxygen reaching the tissues) or anoxia (the absence of oxygen). This function of myoglobin is what helps diving mammals such as whales remain submerged for long periods and allows humans to hold their breath even after the oxygen in the lungs is depleted.

Scientists have determined ways to use myoglobin to detect internal damage to muscles, including the heart. Myoglobin is not normally present in blood. However, when a muscle is damaged or undergoes a period of prolonged hypoxia, myoglobin is released into the blood. This means that physicians can use urine or blood tests sensitive to the presence of myoglobin to determine if a patient has suffered any internal damage to a muscle. This has historically been important in determining damage from a heart attack. Within two to three hours of suffering a heart attack, a patient's blood will contain myoglobin. It will reach its highest levels within about eight to twelve hours after an attack and diminish within twenty-four hours. During the time in between, however, the physician may use a test for myoglobin to determine if a heart attack has occurred, although it is less specific than modern diagnostic markers.

While myoglobin serves a very important and useful purpose in the body, it can also be harmful. Large amounts of myoglobin can prove toxic to the epithelial tubules of the kidneys. This happens most often when a person receives severe crushing injuries in an accident that damages a lot of muscles, a condition known as rhabdomyolysis. The damaged muscle tissue releases large quantities of myoglobin at once. As this is processed through the system, the myoglobin precipitates, or settles, out of the other body fluids contained in urine and clogs the tubes in the kidneys. This causes kidney damage and results in a condition known as myoglobinuria. This condition can also be caused by muscle damage from drug overdoses, seizures, metabolic disorders, and some diseases.

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