Myoglobin, a protein found in both invertebrate and vertebrate animals. In vertebrates it occurs exclusively in red muscle and is the pigment responsible for the red colour. There is a close chemical similarity between myoglobin and hemoglobin, the red pigment of vertebrate blood. Like hemoglobin, myoglobin combines reversibly with oxygen. It is thought that myoglobin functions by storing molecular oxygen for use by the working muscle. In invertebrates the function is apparently similar to that in vertebrates.
In venous blood, myoglobin combines with oxygen more readily than does hemoglobin; this favours the transfer of oxygen from blood to muscle cells, in which oxygen is consumed during chemical reactions that provide energy for the exercising muscle. The combination of oxygen with myoglobin occurs in such a way that the ratio of the concentration of oxygenated myoglobin, or oxymyoglobin, to unoxygenated myoglobin is equal to a constant times the oxygen pressure. This simple relationship follows from the fact that there is one heme group (i.e., iron-containing chemical group with which oxygen combines) in each molecule of myoglobin, and all the heme groups are alike but act independently.

Hemoglobin, on the other hand, has four heme groups per molecule; and the heme groups within a molecule interact and differ in their affinity for oxygen. As a result, the combination of hemoglobin with oxygen is more complex than the combination of myoglobin with oxygen.
Myoglobin has been obtained in pure crystalline form from many sources. It has a molecular weight (based on the weight of hydrogen as 1) of 16,000, about one-fourth that of hemoglobin. Though the heme portion of all myoglobins is the same, the protein portions vary considerably from species to species.
Myoglobin has been of great importance in the elucidation of protein structure. The complete amino acid sequence of sperm whale myoglobin is known, In 1962 the Nobel Prize for Chemistry was awarded jointly to the English biochemist John Cowdery Kendrew and to Max Ferdinand Perutz, an Austrian-born molecular biologist, for their work on the structure of myoglobin and hemoglobin, respectively. Kendrew’s work, utilizing the technique of X-ray diffraction, permitted construction of a three-dimensional model of crystalline sperm whale myoglobin.

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