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Cooperativity: Definition & Explanation

Instructor: Joseph Said
This lesson covers cooperative binding, which is associated with allosteric proteins. Examples and graphs showing the differences between cooperative and non-cooperative binding curves will be discussed.

Molecular Binding

In molecular biology and biochemistry, molecular binding of a substrate molecule to its targets, or ligands, is extremely important. Many biochemical processes depend on proteins binding to and releasing their target ligands. For example, hemoglobin is a protein found in red blood cells that binds oxygen and carbon dioxide. The protein must bind to oxygen, release it, and then bind to carbon dioxide and later release it. If the protein binds to either the oxygen or carbon dioxide indefinitely, the protein is not serving its purpose as a blood gas transporter.

But what makes the hemoglobin bind to and then later release either of these blood gases? If we say that A represents our binding protein and B represents the ligand that A binds to, we must have a a system where A can associate with B, forming AB, and then dissociate, returning A and B back to solution separately. We call this an association or binding constant and label it KI.

If there's a lot of A + B in solution, the reaction will favor the direction of AB; however, if AB is high, the reaction will favor the direction of A + B for the reaction. We can calculate the equilibrium of A + B and AB by using the following formula:

pKI

By taking the log of the concentrations of AB and dividing it by the concentrations of A and B separately, we can find the equilibrium concentrations of the reactants and products in solution. However, as we will see, the concentration is not the only factor that affects binding since cooperative binding depends on changes in allosteric conformations (protein shapes).

Cooperative vs. Non-Cooperative Binding

Often, proteins are made of dimers where structure A and structure B are different from one another on some sequence level but relatively have the same folds and bind to the same ligand. The best example of this is with hemoglobin, which is made up of two alpha and two beta dimers that compose its quaternary structure.

All four subunits bind oxygen and carbon dioxide as their principle ligands. If, as in the case of hemoglobin, the binding of the ligand to the first subunit affects the second subunit in a way that increases the binding affinity for the ligand, the binding is cooperative. This means that, after the binding of oxygen to the first subunit of hemoglobin, the second subunit has an increased affinity for oxygen, which, once bound, increases the affinity for the third subunit and so on. This increased affinity for the ligand causes a sigmoidal - or S-shaped - binding curve, as seen below.

hemoglobin sig curve

The sigmoidal curve shows a rapid increase in affinity and binding after the first and second ligands bind to their sites. This produces a sharp curve upward, forming an S shape. The binding curve levels out at the top, forming the top of the S, as ligand binding positions become occupied.

In non-cooperative binding, we see a logarithmic curve since the binding of one ligand doesn't have an effect on the binding of another ligand for any subunits that might be present. Below is an example of a logarithmic curve that we would see in myoglobin oxygen binding curves.

Logarithmic curve

Myoglobin consists of one hemoglobin subunit, and the binding of ligands to anything around it doesn't change its steady affinity for ligands. Thus, the binding curve is so that ligand binding to binding sites increases steadily until ligand sites become saturated and level off. This forms a dome-shaped curve that shows no sharp increases of binding affinity and doesn't exhibit any cooperative binding.

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