Which term describes a compound that binds to a site different from the agonist, but inhibits receptor function (allosteric mechanism)?

• A. Competitive antagonist
• B. Irreversible antagonist
• C. Noncompetitive antagonist
• D. Physiological antagonist

Answer: (C)

The key idea is allosteric antagonism: a compound binds to a site on the receptor that is different from where the agonist binds (an allosteric site) and still shuts down receptor function. Because the antagonist changes the receptor’s shape or signaling despite the presence of the agonist, increasing the agonist concentration cannot fully restore activity—the maximal response is reduced. That’s the hallmark of a noncompetitive antagonist.

Competitive antagonists bind at the same site as the agonist, so their inhibition can be overcome by enough agonist. Irreversible antagonists can permanently block the receptor, but the defining feature here is that the binding occurs at a different site, making it noncompetitive. Physiological antagonists act through opposing bodily systems, not by directly blocking the receptor’s function.