A globular protein occurring widely in muscle tissue as an oxygen carrier, especially in muscles containing slow-twitch fibres. It comprises a single polypeptide chain and a haem group, which reversibly binds a molecule of oxygen. This is only relinquished at relatively low external oxygen concentrations, e.g. during strenuous exercise when muscle oxygen demand outpaces supply from the blood. Myoglobin thus acts as an emergency oxygen store. It occurs in high concentrations in the muscles of diving mammals such as seals.