A G protein is on with GTP and off with GDP
- The idea
- Predict
- See it
- Why
- Check
The idea
“When a signal comes, the G protein splits GTP for energy, or a kinase adds a phosphate to GDP.”
Splitting ATP usually powers work, and kinases often switch proteins on with a phosphate. A G protein works like neither one.
The short version
The mistake: When a signal comes, the G protein splits GTP for energy, or a kinase adds a phosphate to GDP.
What's actually true: A G protein is on with GTP and off with GDP. The receptor swaps GDP for GTP, and splitting the GTP turns it off.
- An active receptor makes the G protein release its GDP. GTP, which is plentiful in the cytosol, binds in its place. No kinase is involved.
- Holding GTP, the alpha subunit activates a target like adenylyl cyclase, which makes the second messenger cAMP. No GTP has been split yet.
- The G protein slowly splits its GTP to GDP, which turns it off. That slow step sets how long it stays on.
On the exam: GTP on, GDP off. The receptor swaps GDP for GTP. Splitting GTP is the off switch.