Myoglobin as an oxygen store in muscle
Myoglobin is a haem protein (haem is the iron-containing ring that binds oxygen) found only in muscle, where it stores oxygen for use when oxygen runs low.
Remember
Myoglobin has a very high affinity for oxygen (it holds oxygen tightly), so it keeps its oxygen as a store in muscle and releases it mainly when muscle oxygen is very low.
Myoglobin is a haem protein found only in muscle. Haem is the iron-containing ring that binds oxygen. Myoglobin is monomeric, meaning it is a single polypeptide chain, and that chain holds one haem group whose iron ion (Fe2+) binds one oxygen molecule.
Its job is to act as an oxygen store in muscle cells. Myoglobin has a very high affinity for oxygen, which means it holds oxygen tightly. It loads oxygen readily at normal oxygen levels and holds on to it until oxygen levels fall. Oxygen comes off myoglobin mainly during oxygen deprivation, for example in vigorous exercise, which lets muscle cells carry on making energy to contract.
Myoglobin has one chain and one binding site, so it cannot show cooperative binding, which needs several subunits (separate chains) that interact. As a result, a graph of saturation against oxygen pressure gives a hyperbolic curve: a curve that rises steeply and then flattens out. A low P50, the oxygen pressure needed for 50% saturation, marks the high affinity.
Haemoglobin is the contrast: a tetramer (four subunits) with cooperative binding and an S-shaped curve. Its affinity is lower, so it hands oxygen on to myoglobin in muscle.
Key facts
- Structure
- a single polypeptide chain carrying one haem; the iron ion (Fe2+) in that haem binds one oxygen molecule.
- Site and role
- found only in muscle, where it acts as an oxygen store.
- Affinity
- very high, so oxygen is loaded readily at normal levels and released mainly in oxygen deprivation, such as vigorous exercise.
- Curve shape
- hyperbolic, because there is one binding site and no cooperation, unlike the S-shaped curve of haemoglobin.
- Haemoglobin
- has a lower affinity than myoglobin, so it hands oxygen on to myoglobin in muscle.
- Reserve or transport
- myoglobin is an oxygen reserve rather than a transport molecule; haemoglobin does the transporting.
Exam traps
- A protein with four subunits that travels in red blood cells and carries oxygen from the lungs to the tissues is haemoglobin. Myoglobin has one chain, and it stays inside muscle cells.
- Cooperative binding needs several interacting subunits, and that is a feature of haemoglobin, not myoglobin. Myoglobin has a single chain, and cooperative unloading would release oxygen too early for a store.
- Oxygen binds haem iron in the ferrous (Fe2+) state, not the ferric (Fe3+) state, so ferric iron is not how myoglobin stores oxygen.
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