Final answer:
A protein, like the lac repressor, is allosteric because it changes structure and function when it binds lactose.
Step-by-step explanation:
A protein, like the lac repressor, is said to be allosteric because when it binds lactose, it structurally changes and its chemical activity also changes. Allosteric regulation is a type of enzyme regulation where the binding of a molecule to a specific site on the protein causes a change in the protein's shape and function at a distant site. In the case of the lac repressor, when lactose binds to it, it undergoes a conformational change that allows RNA polymerase to transcribe the lac operon genes involved in lactose metabolism.