Myoglobin

Function

Myoglobin is a protein found in muscle tissues that plays a critical role in the storage and transport of oxygen. It is structurally and functionally related to hemoglobin, the protein responsible for oxygen transport in red blood cells. While hemoglobin carries oxygen throughout the body, myoglobin is primarily found in muscle cells and facilitates the uptake and release of oxygen within the muscles themselves.

Structural highlights

Myoglobin is a globular protein whose function is to store molecular oxygen in muscles (myo = muscles)^[1]. It has two main components: a single polypeptide chain, and heme ligand. The heme ligand is only partially exposed to the surface; the majority of it is buried inside the protein. The overall shape of myoglobin is approximately disc-shaped with a diameter that is about twice its thickness. The overall fold of the protein is conserved, especially the hydrophobic core of the protein (shown in purple), but the sequence is more variable on the surface. Metmyoglobin (MMb) is the oxidized form of myoglobin.The globin consists mostly of alpha helices shown in pink; it has no beta sheets and its non-helical segments mostly serve as links that connect the helices. Look down the barrel of some of the longer helices. Are they all straight? The eight structurally conserved alpha helices are labelled A through H. The protein is colored as a N-->C rainbow in this view; the N terminus is blue, while the C terminus is red.

The heme ligand, and specifically the iron atom in the middle of the heme, is what binds oxygen in myoglobin. In this representation, the heme alone is shown in ball and stick form with its C, N and O atoms displayed as grey, blue, and red balls respectively. The iron atom is shown in orange, and is in spacefilling mode to better illustrate its interactions with the heme. The iron is bound by four nitrogen atoms found in the heme ring, as well as an amino acid from the protein chain. Which amino acid from the myoglobin protein binds to the iron? Notice that in the oxygenated state, the iron is in the plane of the heme ring. In the deoxy (no oxygen) state, the Fe atom is slightly above the plane of the heme, and a second amino acid coordinates with the iron in the heme ring.

Function

The main function of myoglobin is to store oxygen in muscle tissues and provide a readily available supply during periods of increased demand, such as during muscle contraction or exercise. When the muscle cells have an ample supply of oxygen, myoglobin binds to oxygen molecules, effectively storing them within the muscle fibers.

During periods of oxygen deficiency, such as during intense exercise when oxygen demand exceeds supply, myoglobin releases the stored oxygen to the surrounding muscle cells. This ensures that the muscle cells have a sufficient oxygen supply to support their metabolic needs, facilitating aerobic respiration and energy production.

The affinity of myoglobin for oxygen is higher than that of hemoglobin, which allows myoglobin to extract oxygen from hemoglobin in the bloodstream when oxygen levels are low. This property enables efficient oxygen transfer from the bloodstream to the muscle tissues.

The red color of muscle tissues is attributed to the presence of myoglobin, as it binds oxygen, giving the oxygenated muscle a characteristic reddish appearance.

Myoglobin is also used as a diagnostic marker for certain medical conditions. For example, elevated levels of myoglobin in the blood can indicate muscle damage or injury, such as in cases of heart attacks or skeletal muscle trauma.

In summary, myoglobin is a protein found in muscle tissues that functions as an oxygen reservoir, allowing the efficient storage and release of oxygen within muscle cells. Its ability to bind and release oxygen helps sustain aerobic respiration in muscles during times of increased demand, such as during exercise.

Additional details

Oxymyoglobin for myoglobin complex with O2
Porphyrin for porphyrin.
Molecular Playground/Myoglobin
Myoglobin of Physeter catodon: structure
Extremophile
Extremophiles

Myoglobin-Physeter-catodon-structure (Spanish)
Myoglobin (Hebrew)
Myoglobin (arabic).

3D Structures of Myoglobin

Myoglobin 3D structures