Drugs & Toxins Affecting GPCR Signaling - Online Tutor, Practice Problems & Exam Prep

Bacterial toxins like cholera toxin and pertussis toxin affect GPCR signaling pathways by targeting G proteins. Cholera toxin inhibits GTPase activity of the stimulatory G protein, leading to excessive adenylate cyclase activation and causing cholera. In contrast, pertussis toxin inhibits GDP-GTP exchange in the inhibitory G protein, preventing it from inhibiting adenylate cyclase, resulting in whooping cough. Understanding these mechanisms highlights the role of agonists and antagonists in receptor activity, exemplified by caffeine acting as an antagonist to the adenosine receptor, blocking its effects and increasing heart rate.

Drugs & Toxins Affecting GPCR Signaling

In this video, we're going to revisit our map of the lesson on biosignalling pathways, which we have down below right here. And of course, we know that we've been exploring this map by following the leftmost branches first, and so we've talked about G protein-coupled receptors or GPCRs, and we've talked more specifically about a specific GPCR signaling pathway, the adenylate cyclase GPCR signaling pathway, in terms of the stimulatory pathway that involves cAMP and PKA, as well as the inhibitory pathway that we introduced in our previous lesson videos. And so now in this video, we're going to continue to talk about the Inhibitory pathway as we talk about drugs and toxins that can affect the adenylate cyclase GPCR signaling. And so let's get started talking about that. So here we're going to introduce some drugs and toxins affecting GPCR signaling. And more specifically, we're going to talk about 2 bacterial toxins that you guys should know. And we have both of these bacterial toxins numbered down below, number 1 and number 2. Now these 2 bacterial toxins will both target G proteins, although they target different G proteins. One of them targets the stimulatory G protein and the other targets the inhibitory G protein. And both of these bacterial toxins will also indirectly increase the activity of the effector enzyme adenylate cyclase. However, they will increase the activity of Adenylate Cyclase in different ways that we'll get to talk about here very shortly. Now the very first bacterial toxin that you guys should know is cholera toxin, And Cholera toxin inhibits the GTPase activity of the G_s G protein, which is the stimulating G protein alpha subunit. And this causes the disease cholera, which is characterized by extreme diarrhea and dehydration, and so it's definitely not something that you want to have. Now what we need to recall from our previous lesson videos is that the GTPase activity of a G protein is when it cleaves the high-energy GTP into the low-energy GDP. And so, GTPase activity inactivates the G Protein. But if we inhibit something that inactivates the G protein, then essentially what we're doing is permanently activating, or we have a permanently active G protein alpha subunit. And this permanently active G protein alpha subunit will then thus be able to overstimulate adenylate cyclase activity, essentially increasing the activity as we already mentioned. And so if we take a look at our image down below, over here on the left-hand side, notice that we're focusing on cholera toxin and its ability to overstimulate the G protein. And so notice that we have our, extracellular ligand, our hormone epinephrine, that binds to the beta-adrenergic GPCR causing a conformational shift in this GPCR, and that is going to activate the G protein because it's going to cause it to exchange the low-energy GDP with the high-energy GTP. And then the G protein alpha subunit will dissociate and be able to activate adenylate cyclase so it can create, essentially it can catalyze the reaction that converts ATP into cAMP, our secondary messenger. Now normally the G protein alpha subunit for G_s, the stimulating G protein, would only remain active for a relatively short period of time because over time eventually this GTP would get cleaved down to GDP, the low energy, inactive form. However, notice that the bacterial toxin cholera toxin will actually inhibit this GTPase activity. So it will not allow the G protein to convert GTP into GDP. And so that means that GTP will remain associated with this alpha subunit and it will continue to activate adenylate cyclase. Essentially, it will continue to overstimulate, the adenylate cyclase. And so what we can say is that the stimulatory G protein alpha subunit G_s and therefore adenylate cyclase AC are always going to remain active in the presence of cholera toxin. And again, this will lead to the disease cholera which is characterized by extreme diarrhea and dehydration which is why we have this toilet over here. And so really, you can think of the bacterial toxin cholera toxin, almost like having the gas pedal down to the metal. So it's almost like having the pedal to the metal where, you're pretty much accelerating the, activity of adenylate cyclase to its maximum. Again because, the GTP, GTPase activity is being inhibited by cholera toxin. And so if we move on to our second bacterial toxin that we're going to cover here, it is the toxin called pertussis toxin. And pertussis toxin differs from cholera toxin really in 2 major ways. The first is that it doesn't inhibit GTPase activity. Instead, it inhibits the GDP GTP exchange, which is different, and also it does not inhibit the stimulating G protein alpha subunit G_s. Instead, pertussis toxin inhibits the inhibiting G protein alpha subunit or GI. So it's almost like inhibiting an inhibitor. And so this causes a different disease called whooping cough instead of causing cholera. Now recall from our previous lesson videos that the GDP, GTP exchange is really what activates the G protein. And so if we inhibit something that activates the G protein, then that means it's going to permanently remain inactive. And so we have a permanently inactive GI protein alpha subunit. But recall that when GI is active, what it does is it inhibits adenylate cyclase. And so if we have an inactive GI, that means that we're inhibiting an inhibitor. And so what we're doing is we're preventing adenylate cyclase from being inhibited, which is kind of a strange way of increasing its activity. And so it's increasing the activity in a different way than what cholera toxin increases the activity. So let's take a look down below at our image, and, we're focusing this time on the right-hand side of our image, and we can see that the bacterial toxin, pertussis toxin, is really going to inhibit the inhibitor and the inhibitor is again the inhibiting G protein alpha subunit, GI. And so what we have is our inhibiting ligand up here that's going to bind to the inhibitory GPCR causing a conformational shift that would normally allow for GTP to come in and replace GDP. And then when that would happen, what would normally happen is the alpha subunit would then dissociate to inhibit the adenylate cyclase, essentially, causing it to create less cAMP. However, notice that in the presence of pertussis bacterial toxin here, it actually inhibits the GDP GTP exchange. And so GTP is never actually able to get in and replace GDP. So that means that the G protein alpha subunit is gonna remain in its inactive state, and it will never be able to dissociate to inhibit adenylate cyclase. So down below, what we can say is that the GI is always going to remain inactive. And because it is the GI protein, if we have an inactive GI, of course, that means that the adenylate cyclase is never going to be inhibited. And so that's almost like a way of, again, increasing the activity of adenylate cyclase, again, by inhibiting the inhibitor. And so you could think of pertussis toxin almost like broken brakes on a car. You won't be able to slow down the rate. However, as long as the gas pedal is working properly then, it's going to create a different effect than, pedal to the metal over here. And so again, pertussis toxin is_associated with the disease whooping cough, which is why we have this character over here coughing. And so really, this concludes our lesson on these 2 bacterial toxins, cholera toxin and pertussis toxin, And as we move forward in our course, we'll be able to get some practice applying these concepts. So I'll see you guys in our next video.

Drugs & Toxins Affecting GPCR Signaling Example 1

Alright. So here we have an example problem that says cholera toxin blocks GTP hydrolysis of the stimulating G protein alpha subunit, whereas pertussis toxin prevents the interaction of the inhibiting G protein alpha subunit with adenylate cyclase. What is the effect of these toxins on the intracellular concentration of cAMP? And we've got these 4 potential answer options down below. And so what we need to recall from our last lesson video is that both cholera toxin and pertussis toxin will increase the activity of the effector enzyme adenylate cyclase. Now they do it in different ways, but ultimately, they both lead to an increase in the activity. And so of course, adenylate cyclase is responsible for converting ATP into cAMP. And so increasing the activity of adenylate cyclase will increase the concentration of cAMP within the cell. And so we would expect that both cholera toxin and pertussis toxin should lead to an increase in the cAMP concentration. And so we can go ahead and indicate that c here is the correct answer, and all of these other ones are just trick answers that are trying to tempt you. But, c here is correct, and that concludes this practice. So I'll see you guys in our next video.

Cholera toxin increases the cellular cAMP levels by:

• A. Binding to and activating GPCRs.
• B. Altering the activity of stimulatory G_s proteins.
• C. Inhibiting phosphodiesterase activity.
• D. Binding to and inhibiting adenylyl cyclase.
• E. Altering the activity of inhibitory G_i proteins.

Pertussis toxin is produced by Bordetella pertussis, the bacterium that causes whooping cough. Pertussis toxin catalyzes the addition of ADP-ribose to G_i which 'locks' it in the GDP-bound state. If the uninhibited, toxin free GPCR pathway normally results in decreased glycogen synthesis, then what would be the effect of pertussis toxin?

• A. It would decrease contraction.
• B. It would decrease glucose production.
• C. It would further decrease glycogen production.
• D. It would increase the rate of endocytosis.

Drugs & Toxins Affecting GPCR Signaling

In this video, we're going to distinguish between Agonists and Antagonists. Many clinical drugs are developed to act either as agonists or antagonists to various receptors. But what exactly are these Agonists and antagonists? Well, an agonist can be defined as a structural analog to a ligand or a molecule that is going to very closely resemble the structure of a ligand. This structural analog is going to bind receptors and mimic the effect of the original or natural ligand. Now antagonists on the other hand are also structural analogs that resemble the structure of the original ligand. However, the antagonist is going to bind the receptor without triggering the normal effect. And so if the normal effect is not going to be triggered, then that means that it is going to be blocking the effects of the agonist or the ligand. And so really, you can think of the antagonist as functioning very similarly to competitive enzyme inhibitors. Recall that competitive enzyme inhibitors would compete with the substrate for binding position to the active site and therefore, it would block the active site and block the substrate from binding to the active site. And so antagonists can work in a very similar way where they bind to the receptor and block the original ligand or the agonist from binding to the receptor.

If we take a look at our image down below, we can look at an example of how caffeine, a molecule found in your typical coffee, actually acts as an antagonist to the adenosine receptor. Notice on the left-hand side, what we're showing you is the chemical structure of the molecule adenosine, which is actually going to be the ligand, the original ligand to the adenosine receptor. And then on the right, notice that we're showing you the chemical structure of the molecule caffeine, which again you can find in your typical coffee that you might drink in the morning. Notice that the caffeine molecule, right here in this red region, is going to very closely resemble the structure of the ligand. However, it is not identical; you can see that the branching units here are going to be different.

On the right side of the image, notice that we have 2 regions. We have the top half and then we have the bottom half of the image. Notice that the top half of the image is showing you the adenosine molecule binding to the adenosine receptor, and here you can see the adenosine molecule binding to the adenosine receptor, and of course, adenosine is the normal or natural ligand for the adenosine receptor. When the adenosine molecule binds the adenosine receptor, it's going to trigger the normal effect. The normal effect of the adenosine molecule binding to the adenosine receptor is going to be decreased heart rate. This decreased heart rate is actually going to lead to a drowsiness type of effect where you're going to feel somewhat tired and sleepy.

Now, down below in the bottom half of the image we're showing you Caffeine, this antagonist, binding to the same adenosine receptor. Because caffeine is an antagonist, it is going to function very similarly to a competitive enzyme inhibitor. When caffeine binds to the adenosine receptor, it will block adenosine from binding. And so, therefore, when caffeine binds, it will block the normal effect that adenosine usually has. Instead of decreasing heart rate, when caffeine binds, it is actually going to lead to an increased heart rate. This increased heart rate is going to lead to a more wakefulness type of effect where you're going to feel more awake and more energized. This is why a lot of you drink coffee in the morning, to get that caffeine stimulation that leads to increased heart rate and a more wakefulness effect.

This concludes our brief example of how caffeine acts as an antagonist to the adenosine receptor and concludes our brief discussion on Agonists and Antagonists. We will be able to get some practice applying these concepts as we move forward, so I'll see you all in our next video.

Caffeine is a molecule that binds to ______________ receptors, inhibiting their response.

• A. Tyrosine.
• B. Adenosine.
• C. Adenine.
• D. Cytosine.
• E. Asparagine.
• F. Dopamine.

Caffeine is a(n) __________ of adenosine that blocks its receptors from activation.

• A. Inhibitor.
• B. Antagonist.
• C. Agonist.
• D. Activator.

Here’s what students ask on this topic:

What are the effects of cholera toxin on GPCR signaling pathways?

Cholera toxin affects GPCR signaling by inhibiting the GTPase activity of the stimulatory G protein (G_s). Normally, GTPase activity converts GTP to GDP, inactivating the G protein. However, cholera toxin prevents this conversion, leading to a permanently active G_s alpha subunit. This continuous activation overstimulates adenylate cyclase, increasing cAMP levels. The excessive cAMP causes severe diarrhea and dehydration, characteristic of cholera. Understanding this mechanism is crucial for comprehending how bacterial toxins can disrupt cellular signaling and lead to disease.

How does pertussis toxin interfere with GPCR signaling?

Pertussis toxin interferes with GPCR signaling by inhibiting the GDP-GTP exchange in the inhibitory G protein (G_i). Normally, this exchange activates G_i, which then inhibits adenylate cyclase, reducing cAMP levels. Pertussis toxin prevents this activation, keeping G_i in its inactive state. As a result, adenylate cyclase remains uninhibited, leading to increased cAMP levels. This disruption causes whooping cough. Understanding this mechanism highlights how bacterial toxins can manipulate cellular signaling pathways to cause disease.

What is the difference between agonists and antagonists in receptor signaling?

Agonists and antagonists are molecules that interact with receptors but have different effects. Agonists are structural analogs of natural ligands that bind to receptors and mimic the ligand's effect, activating the receptor. In contrast, antagonists also resemble the ligand's structure but bind to the receptor without triggering its normal effect, effectively blocking the ligand or agonist from activating the receptor. For example, caffeine acts as an antagonist to the adenosine receptor, blocking adenosine's effect and increasing heart rate, leading to wakefulness.

How does caffeine act as an antagonist to the adenosine receptor?

Caffeine acts as an antagonist to the adenosine receptor by binding to the receptor without triggering its normal effect. Adenosine, the natural ligand, typically binds to the adenosine receptor, leading to decreased heart rate and drowsiness. Caffeine, structurally similar to adenosine, competes for the same binding site on the receptor. When caffeine binds, it blocks adenosine from binding, preventing the decrease in heart rate and instead causing an increase in heart rate. This results in a wakefulness effect, which is why caffeine is commonly consumed to stay alert.

What diseases are associated with cholera toxin and pertussis toxin?

Cholera toxin is associated with cholera, a disease characterized by severe diarrhea and dehydration. The toxin inhibits the GTPase activity of the stimulatory G protein (G_s), leading to excessive activation of adenylate cyclase and increased cAMP levels. Pertussis toxin is associated with whooping cough, a respiratory disease. It inhibits the GDP-GTP exchange in the inhibitory G protein (G_i), preventing the inhibition of adenylate cyclase and resulting in increased cAMP levels. Both toxins disrupt GPCR signaling pathways, leading to their respective diseases.