Cooperative binding

Cooperative binding is when the number of small molecules binding to a macromolecule depends on the concentration of those small molecules around the macromolecule.

This is a type of molecular binding, which is where molecules attach to each other in a stable way without decaying or breaking that bond rapidly.

Example

A popular example of cooperative binding is between haemoglobin and oxygen. In the lungs, there is a very high concentration of oxygen in the alveoli of the lungs. So the oxygen bonds to haemoglobin to form oxyhaemoglobin. When this travels around the body in the blood and reaches areas of low oxygen concentration, the oxygen is released from the haemoglobin.

Cooperative Binding Media

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  Original figure from Christian Bohr, showing the sigmoidal increase of oxyhemoglobin as a function of the partial pressure of oxygen.

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  Hill plot of the Hill equation in red, showing the slope of the curve being the Hill coefficient and the intercept with the x-axis providing the apparent dissociation constant. The green line shows the non-cooperative curve.

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  Monod-Wyman-Changeux model reaction scheme of a protein made up of two protomers. The protomer can exist under two states, each with a different affinity for the ligand. L is the ratio of states in the absence of ligand, c is the ratio of affinities.

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  Energy diagram of a Monod-Wyman-Changeux model of a protein made up of two protomers. The larger affinity of the ligand for the R state means that the latter is preferentially stabilized by the binding.

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  Hill plot of the MWC binding function in red, of the pure T and R state in green. As the conformation shifts from T to R, so does the binding function. The intercepts with the x-axis provide the apparent dissociation constant as well as the microscopic dissociation constants of R and T states.

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  Cartoon representation of the protein hemoglobin in its two conformations: "tensed (T)" on the left corresponding to the deoxy form (derived from PDB id:11LFL) and "relaxed (R)" on the right corresponding to the oxy form (derived from PDB id:1LFT).

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  Cartoon representation of the protein Calmodulin in its two conformation: "closed" on the left (derived from PDB id: 1CFD) and "open" on the right (derived from PDB id: 3CLN). The open conformation is represented bound with 4 calcium ions (orange spheres).

Other websites

• Khan Academy-Haemoglobin