Some organisms can thrive in the absence of oxygen and biological activities can take place in the absence of oxygen. Examples are anaerobic bacteria or the anaerobic metabolism. For most organisms however oxygen is an essential ingredient of life. Hemoglobin in mammals transports oxygen from the lungs to peripheral tissues. There is however sometimes a need to be able to store oxygen. Marine mammals like whales for instance come to the water surface to pick up oxygen and can then dive without continued access to oxygen. Nature provided a protein called myoglobin for this and myoglobin is found mainly in muscle tissue. Myoglobin was only the second protein whose structure was determined and the source of myoglobin was muscle tissue of sperm whales. Myoglobin is a monomeric, single polypeptide chain protein which is very similar to the beta-subunit of hemoglobin. It binds oxygen in the same way as hemoglobin does. Attached to the molecule is a protoporphyrin IX (heme) group with a central iron atom that can bind oxygen. As in hemoglobin the heme can also bind other molecules like carbon monoxide, carbon dioxide or cyanide. The affinity for carbon monoxide or cyanide is greater than the affinity for oxygen causing death by preventing the binding of oxygen.
Myoglobin is shaped like a disk, about 20 A thick and about 35 A in diameter. The heme group is inserted into a surface pocket formed by two helices at an angle of about 90 degrees with the disk plane. Some of the effects of oxygen binding observed for hemoglobin are not present in myoglobin. Since myoglobin is a monomeric protein there can be no cooperativity of binding. This effect is observed in hemoglobin because it has four chains, each containing a heme group. Usually a histidine residue near the heme group, the so-called distal histidine, is present in myoglobins and hemoglobins. In one case however, elephant myoglobin, this histidine is replaced by a glutamine residue and an adjacent leucine residue is replaced by a phenylalanine residue (PDB code: 1EMY). Nevertheless the oxygen binding properties are very similar to conventional myoglobins.
Protein Data Bank (PDB)
Author: Arno Paehler