Question 3
In a signaling pathway, which of the following types of protein acts as a switch to release a second messenger?
Options:
A) Enzyme
B) Glycoprotein
C) G protein
D) Receptor
Answer:
Correct Answer: C) G protein
Explanation:
Receptors detect extracellular signals and can activate G proteins, but they do not directly release second messengers. The G protein is the key intermediary.
Role of G Proteins in Signaling Pathways
G proteins (guanine nucleotide-binding proteins) are molecular switches involved in transmitting signals from receptors on the cell surface to internal signaling pathways.
They are activated by receptors and, upon activation, trigger the release of second messengers (such as cyclic AMP).
This activation initiates a cascade of cellular responses.
Mechanism of Action
When a receptor on the cell surface binds to its specific ligand (signaling molecule), it activates an associated G protein.
The G protein then either activates or inhibits enzymes (e.g., adenylate cyclase) that produce second messengers.
Second messengers relay the signal to downstream effectors, amplifying the initial signal within the cell.
Why Other Options Are Incorrect
A) Enzyme
While enzymes can generate second messengers (e.g., adenylate cyclase), they do not act as the “switch” to release them. The G protein activates the enzyme.
B) Glycoprotein
Glycoproteins play roles in cell recognition and adhesion but are not typically involved in signal switching for second messenger release.
D) Receptor
Receptors detect extracellular signals and can activate G proteins, but they do not directly release second messengers. The G protein is the key intermediary.