An enzyme or other protein is allosteric, if it changes activity or efficiency in response to the binding of an effector molecule. "Allostery" has its roots in Greek and means other site. In the special case of cooperative allostery, such as characterizes the binding of oxygen molecules to hemoglobin, oxygen is effectively both the substrate and the effector, and the "other site" is just the active site of an adjoining protein subunit. For an enzyme to be allosteric it must possess at least two native conformations, the activity or catalytic efficiency of which is different. Binding of the effector shifts the equilibrium between these conformational states to favor the more effective one, which it does by making it more thermodynamically stable.
