Myoglobin

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Myoglobin and hemoglobin are proteins that serve all of the oxygen transport and storage needs of animals. For the tissues of larger organisms, aerobic metabolism requires much more oxygen than could be supplied by simple diffusion of oxygen through tissues or dissolution of oxygen into the blood for transport to the tissues. Oxygen transport proteins are able to deliver much more oxygen to tissues for much greater efficiency of transport. Myoglobin is the oxygen storage protein for tissues in almost all animals, and is the oxygen transport protein in some invertebrates. For vertebrates, oxygen is brought to the tissues in specialized cells that contain large amounts of oxygen-binding hemoglobin (such as human erythrocytes). Tissues that require a large amount of readily available oxygen, such as muscle tissue, store the oxygen brought to them in myoglobin.

Myoglobin is comprised of a single polypeptide chain of helical secondary structure, which is wrapped around a functionally essential prosthetic group called a heme.

The heme molecule is a porphyrin ring complexed with Fe(II), and is responsible for the oxygen-binding properties of myoglobin. The heme is bound non-covalently in a hydrophobic cleft of myoglobin. The protoporphyrin ring (white) has four inward-facing nitrogen atoms (blue) that coordinate with the central Fe(II) atom (yellow). The Fe atom is also coordinated on top with the diatomic oxygen molecule (dark red).

The bound oxygen is also coordinated on its top by the side chain of histidine 64 of myoglobin, termed the distal histidine (light blue). The heme Fe is coordinated on the bottom by the proximal side chain of histidine 93 of myoglobin (light blue).

The myoglobin protein surrounds the heme prosthetic group like a cage, protecting the oxygen from release until the tissue needs it.

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