Binding Free Energy vs. Binding Affinity: Understanding the Relationship

Binding free energy and binding affinity are closely related concepts in the field of molecular interactions.

'Binding affinity' refers to the strength of the interaction between a ligand (molecule) and its receptor (target molecule). It quantifies how well the ligand binds to the receptor and is typically expressed as the dissociation constant (Kd), which is the concentration of ligand required to occupy 50% of the receptor sites.

On the other hand, 'binding free energy' measures the thermodynamic stability of the ligand-receptor complex. It represents the energy difference between the unbound state (ligand and receptor separate) and the bound state (ligand bound to the receptor). Binding free energy is typically expressed in terms of ΔG (delta G), where a negative value indicates favorable binding.

The relationship between binding free energy and binding affinity can be understood as follows:

1. Strong binding affinity generally corresponds to a more negative binding free energy. When a ligand has a high affinity for a receptor, it means that the ligand binds tightly to the receptor, resulting in a stable complex. This stability is reflected in a more negative binding free energy value.

2. Weak binding affinity corresponds to a less negative or positive binding free energy. If a ligand has a low affinity for a receptor, it binds less tightly and the resulting complex is less stable. This leads to a less negative or even positive binding free energy value.

In summary, the stronger the binding affinity between a ligand and its receptor, the more negative the binding free energy, indicating a more stable complex.