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Myoglobin is an oxygen storage protein that occurs in muscle tissue of vertebrates. It has a very similar structure to haemoglobin, since it shares the globin fold. It is a haemoprotein and has two liganding histidine residues.

While the protein myoglobin does store oxygen for a short time, its primary function is transport, not storage of oxygen. Myoglobin is, in fact, very different from hemoglobin. While it does have many features in common with hemoglobin (like the structure of its heme based binding site), one of the most noticable differences between myoglobin and hemoglobin is that myoglobin is roughly 1/4 the size and weight of hemoglobin. In fact, the four subunits of hemoglobin are very similar to myoglobin in structure and function (much more so than hemoglobin as a whole).

Biologically and biophysically, the most striking and important difference between the two proteins is that myoglobin does not exhibit cooperative binding and hemoglobin does due to the ability of allosteric change in one subunit to affect the binding of the other three (see allosteric change for more detail). While some animals survive using only forms of myoglobin (yes, there are different types of both myoglobin and hemoglobin), hemoglobin's cooperative binding is essential to larger mammals (like ourselves). Basically hemoglobin works well for transporting oxygen from high concentration (like where the the blood surrounds the little sacks of air (alveoli) in the lungs) to low concentration. Myoglobin generally has a higher affinity for oxygen and thus is good at picking up oxygen at concentrations too low for hemoglobin, and transporting it to even lower concentrations (like that found in parts of active muscle tissue). This added step in transport is essential for large animals, because simply waiting for the oxygen to diffuse through thick layers of sarcomeres is too slow.

High myoglobin concentration in tissue makes it appear darker and more red than normal. This is why areas with more active muscle tissue in chickens will produce "dark meat" while lighter areas ("white meat", containing less myoglobin) are likely to come from other parts of the chicken. For those who are not on special diets, the resulting high protein content of dark meat may make it better for you. If you are at risk or heart disease or heart failure, you may wish to avoid meat (except fish) all together or you may find out the hard way Why people die on the toilet.

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