Cholera toxin is a bacterial toxin that blocks the Gsα subunit from hydrolyzing GTP to GDP, locking it in the active state, so adenylyl cyclase keeps making cAMP and the signal transduction pathway can't shut off.
Cholera toxin is the textbook example of what happens when you break one piece of a signal transduction pathway. Normally a G protein-coupled receptor (GPCR) turns ON when its Gsα subunit swaps GDP for GTP, then turns OFF when Gsα hydrolyzes that GTP back to GDP. Cholera toxin sabotages the OFF switch. It chemically modifies Gsα so it can no longer hydrolyze GTP, which means Gsα stays locked in its active, GTP-bound state.
With Gsα stuck ON, it keeps activating the enzyme adenylyl cyclase, which keeps churning out cyclic AMP (cAMP). In intestinal cells, that runaway cAMP signal drives massive secretion of salt and water into the gut. The result is the severe diarrhea and dehydration of cholera. The big-picture lesson for AP Bio is that a single change to one component (here, the timer that normally shuts the G protein off) can hijack the entire downstream response.
This term lives in Unit 4: Cell Communication and Cell Cycle, specifically Topic 4.3 Signal Transduction Pathways. It's a clean illustration of two learning objectives at once. For [AP Bio 4.3.B], it shows how a change in the structure of a signaling molecule (a chemically modified Gsα) alters the activity of the pathway by killing its ability to self-terminate. For [AP Bio 4.3.A], it shows how that altered signaling produces a dramatic cellular response (uncontrolled salt and water secretion). Cholera toxin is exactly the kind of 'something interferes with one step' scenario the CED loves, because it forces you to trace a cause through every downstream step.
Keep studying AP® Biology Unit 4
Gsα (Unit 4)
Gsα is the exact protein cholera toxin attacks. Understanding cholera toxin is really just understanding what happens when Gsα can never hydrolyze its GTP. Same protein, broken timer.
cyclic AMP (cAMP) (Unit 4)
cAMP is the second messenger that piles up when cholera toxin keeps adenylyl cyclase running. The dehydration is downstream of cAMP, so high cAMP is the link between the toxin and the cellular response.
Epinephrine and glycogen breakdown (Unit 4)
Epinephrine uses the same GPCR-to-cAMP pathway that cholera toxin hijacks. Comparing them shows you the difference between a normal, self-terminating signal and one whose OFF switch is broken.
Cellular Response (Unit 4)
Cholera toxin is a case study in how a change far upstream (a modified G protein) reshapes the final cellular response. The CED wants you to follow that chain from receptor to phenotype.
Expect cholera toxin in multiple-choice stems that describe the toxin 'modifying a G protein so it cannot hydrolyze GTP to GDP' and then ask for the most direct consequence. The right answer chain is always: Gsα stays active, adenylyl cyclase stays on, cAMP keeps rising, and salt and water keep being secreted, which causes dehydration. You need to reason forward through the pathway, not just recall a definition. The 2022 Long FRQ Q1 built its prompt around the same GPCR mechanism, where GTP replaces GDP after ligand binding, so practicing that activation/deactivation cycle prepares you for both formats. When you see a question, identify exactly which step is broken and trace what that does downstream.
GTP hydrolysis is the normal OFF switch: Gsα converts its bound GTP to GDP and the signal stops. Cholera toxin is what happens when that step is blocked. Don't say the toxin 'activates' the G protein from scratch. It just prevents the G protein from turning itself off, so it stays active far longer than it should.
Cholera toxin blocks Gsα from hydrolyzing GTP to GDP, locking the G protein in its active state.
Because Gsα stays active, adenylyl cyclase keeps producing cAMP and the signaling pathway can never shut off.
In intestinal cells, the runaway cAMP drives excess salt and water secretion, causing the severe dehydration of cholera.
It illustrates [AP Bio 4.3.B]: a structural change to one pathway component alters the whole downstream response.
On the exam, trace the chain forward: modified Gsα, constant adenylyl cyclase, high cAMP, fluid loss, dehydration.
It chemically modifies the Gsα subunit so it can no longer hydrolyze GTP to GDP. That locks Gsα in its active, GTP-bound state, keeping adenylyl cyclase and cAMP production running nonstop.
Not exactly. It doesn't switch the G protein on from scratch; it prevents the G protein from turning itself OFF. By blocking GTP hydrolysis, it removes the deactivation step, so Gsα stays active far longer than normal.
Locked-on Gsα keeps adenylyl cyclase making cAMP. In intestinal cells, that excess cAMP triggers massive secretion of salt and water into the gut, producing the watery diarrhea and dehydration of cholera.
Normal GTP hydrolysis is the pathway's OFF switch, where Gsα converts GTP to GDP and the signal stops. Cholera toxin blocks that hydrolysis, so the OFF switch never works and the signal stays on.
Yes, it shows up in Unit 4 (Topic 4.3) as a classic example for learning objectives [AP Bio 4.3.A] and [AP Bio 4.3.B]. Questions describe the toxin blocking GTP hydrolysis and ask you to trace the consequences down the pathway.
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