Members of the multi pass, G protein-linked family of receptors include the muscarinic cholinergic and J3-adrenergic receptors and rhodopsin. Which of the following characterizes these receptors?
## **Core Concept**
The question pertains to the structural and functional characteristics of G protein-linked receptors, also known as G protein-coupled receptors (GPCRs). These receptors are a large family of membrane receptors that respond to a variety of external signals, including hormones, neurotransmitters, and light. They are called "multi-pass" because they span the cell membrane multiple times, typically seven times.
## **Why the Correct Answer is Right**
G protein-coupled receptors (GPCRs) are characterized by their structure, which includes seven transmembrane alpha-helical segments. This structure is crucial for their function, as it allows them to interact with both the extracellular ligand and the intracellular G protein. The muscarinic cholinergic receptors, β3-adrenergic receptors, and rhodopsin are all examples of GPCRs. They activate G proteins upon binding to their respective ligands, leading to various downstream signaling effects.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but typically, incorrect options might suggest characteristics not specific to GPCRs, such as being ligand-gated ion channels or having a single transmembrane pass.
- **Option B:** Similarly, without the content, we can infer that any option not describing a seven-transmembrane structure or G protein coupling would be incorrect.
- **Option C:** This would be incorrect if it does not accurately describe the seven-transmembrane, G protein-linked nature of these receptors.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that GPCRs are targets for a significant proportion of currently approved drugs. Understanding their structure and function is crucial for pharmacology and developing new therapeutic agents. Rhodopsin, mentioned in the question, is particularly notable for its role in the visual cycle and as a prototypical GPCR.
## **Correct Answer:** D. They have a structure that includes seven transmembrane alpha-helical segments.