The dissociation rate in chemistry, biochemistry, and pharmacology is the rate or speed at which a ligand dissociates from a protein, for instance, a receptor. It is an important factor in the binding affinity and intrinsic activity (efficacy) of a ligand at a receptor. The dissociation rate for a particular substrate can be applied to enzyme kinetics, including the Michaelis-Menten model. Substrate dissociation rate contributes to how large or small the enzyme velocity will be. In the Michaelis-Menten model, the enzyme binds to the substrate yielding an enzyme substrate complex, which can either go backwards by dissociating or go forward by forming a product. The dissociation rate constant is defined using Koff.