**Core Concept**
G-protein coupled receptors (GPCRs) are a large family of membrane-bound receptors that play a crucial role in signal transduction. They are involved in various physiological processes, including hormone secretion, neurotransmission, and cardiovascular regulation. GPCRs are characterized by their seven transmembrane alpha-helix structure and are activated by a wide range of ligands, including hormones, neurotransmitters, and light.

**Why the Correct Answer is Right**
GPCRs work by binding to their specific ligands, which triggers a conformational change in the receptor. This change activates a G-protein, a heterotrimeric complex composed of alpha, beta, and gamma subunits. The activated G-protein then dissociates into its alpha and beta-gamma subunits, which interact with downstream effectors to produce a biological response. For example, the activation of Gαs (a subtype of the alpha subunit) leads to the stimulation of adenylyl cyclase, resulting in the production of cyclic AMP (cAMP), a key second messenger in cellular signaling.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is incorrect because GPCRs do not directly interact with DNA; instead, they modulate gene expression through the activation of downstream signaling pathways.
**Option B:** This statement is incorrect because GPCRs are not ion channels; they do not directly regulate the flow of ions across the cell membrane.
**Option C:** This statement is incorrect because GPCRs are not enzymes; they do not catalyze chemical reactions.

**Clinical Pearl / High-Yield Fact**
GPCRs are the target of approximately 40% of all prescribed medications, making them a critical area of research in drug development. Understanding the structure and function of GPCRs can provide valuable insights into the development of new therapeutic agents for various diseases.

**Correct Answer: C. GPCRs are not enzymes.**