**Core Concept**
G protein-coupled receptors (GPCRs) are a large family of receptors that play a crucial role in cellular signal transduction. They are involved in various physiological processes, including hormone regulation, neurotransmission, and immune response.

**Why the Correct Answer is Right**
GPCRs are activated by the binding of an agonist, which causes a conformational change in the receptor, leading to the exchange of GDP for GTP on the associated G protein. This exchange activates the G protein, which then triggers a downstream signaling cascade. GPCRs are involved in a wide range of physiological processes, including vasodilation, vasoconstriction, and cardiac contraction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because GPCRs are not directly involved in DNA transcription. While GPCRs can influence gene expression indirectly through downstream signaling pathways, they do not directly interact with DNA.
**Option B:** This option is incorrect because GPCRs do not typically activate enzymes directly. Instead, they activate G proteins, which then activate downstream effectors.
**Option C:** This option is incorrect because GPCRs are not directly involved in the transmission of electrical signals in neurons. While GPCRs can influence neuronal excitability indirectly through downstream signaling pathways, they do not directly interact with ion channels or other electrical signaling molecules.

**Clinical Pearl / High-Yield Fact**
A key feature of GPCRs is their desensitization and internalization in response to prolonged agonist exposure. This process, known as homologous desensitization, helps to prevent overstimulation of downstream signaling pathways and maintain physiological homeostasis.

**Correct Answer: D. GPCRs are involved in a wide range of physiological processes, including vasodilation, vasoconstriction, and cardiac contraction.**