Descriptions of molecular binding events. (A) Simple model of ligand binding. Binding event between receptor (R) and ligand (L) to form receptor-ligand complex (R•L) represented as simple association processes governed by a single set of association and dissociation rates. When association is slower than diffusion-limited collision, additional steps are needed to describe binding. (B) Induced fit and conformational capture models of ligand binding. These models account for slower-than-diffusional binding. The tertiary capture model invokes conformational changes prior to the encounter between R and L, while the induced fit model represents the opposite extreme in which conformational changes occur after the initial encounter. (C) Complex model of ligand binding considering an ensemble of conformational states and series of conformational changes to the receptor during the association process. Even the complex representation in the figure is a vast simplification, as it depicts a linear order of conformation changes (and a limited number of conformational steps) rather than a larger and more realistic multi-dimensional energy landscape that can account for multiple orders for the local conformational changes and multiple possible binding pathways.
