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Place the events of the G-protein signaling cycle in the correct order, starting from the inactive state.
AGTP is hydrolyzed first, then it binds the receptor and splits into subunits
BIt binds the receptor, releases GDP for GTP, splits into subunits, then GTP is hydrolyzed back
CThe subunits split apart first, then it binds the receptor and picks up GTP
DIt picks up GTP first, then binds the receptor before splitting into subunits
Answer & Solution
Correct answer: B. It binds the receptor, releases GDP for GTP, splits into subunits, then GTP is hydrolyzed back
1. The inactive G-protein first binds to a newly revealed site on the receptor.
2. This binding causes a shape change that activates the G-protein, which releases GDP and picks up GTP.
3. The G-protein subunits then split into the alpha subunit and the beta-gamma subunit.
4. After a while, the GTP on the active alpha subunit is hydrolyzed back to GDP and the beta-gamma subunit is deactivated.
5. The subunits then reassociate to form the inactive G-protein again, completing the cycle.
6. The option starting with GTP hydrolysis places the final step first, which is out of order.
7. The option starting with subunit splitting skips receptor binding, which must happen before the subunits can split.
8. The option that has GTP binding occur before receptor binding reverses the order, since receptor binding is what triggers GDP release and GTP uptake.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 9 "Cell Communication", section 9.1 Signaling Molecules and Cellular Receptors_
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