What forms the most important class of receptors?

• A. Lipids
• B. Carbohydrates
• C. Proteins
• D. Nucleic acids

Answer: (C)

Proteins form the most important class of receptors because receptor function hinges on specific ligand binding and the ability to transduce that signal into a cellular response. Proteins provide the diverse, well-defined binding pockets and conformational flexibility needed to recognize a wide range of ligands with high specificity. Once a ligand binds, the receptor can trigger downstream effects through conformational changes, enzymatic activity, ion flow, or coupling to intracellular signaling partners. The major receptor families in biology and pharmacology—G-protein–coupled receptors, receptor tyrosine kinases, ligand-gated ion channels, and intracellular/nuclear receptors—are all proteins, illustrating why this class is central. Lipids, carbohydrates, and nucleic acids can participate in signaling or serve other roles, but they do not serve as the primary, functional receptors in the same way proteins do.