Biosignaling 2 - Online Tutor, Practice Problems & Exam Prep

cAMP acts as a secondary messenger, activating protein kinase A (PKA) by binding to its regulatory subunit, allowing PKA to phosphorylate serine or threonine residues in various proteins. This phosphorylation cascade, initiated by hormones like epinephrine, amplifies signals, potentially leading to the release of glucose from glycogen. The pathway also adapts through mechanisms like receptor phosphorylation by beta-adrenergic receptor kinase, which reduces receptor availability. Disruption of this signaling, as seen in cholera toxin, can cause severe dehydration due to excessive ion and water loss in intestinal cells.

Biosignaling 2

Let's turn our attention to cAMP, that secondary messenger generated by adenylyl cyclase. cAMP activates protein kinase A, which is a protein kinase that's very important in signaling pathways, and it activates protein kinase A by allosterically binding to the regulatory subunit. If you can picture protein kinase A for a second, here I'm drawing it as a sort of blue blobby thing. This is protein kinase A, and it has this regulatory subunit that is bound into the active site. So R, that's our regulatory subunit, is bound into the active site so that the enzyme cannot perform its function. Now, cAMP causes the regulatory subunit to be released from the active site when it binds, and this allows PKA or protein kinase A to phosphorylate stuff, which is basically what it does.

It's important to realize that cAMP is actually a secondary messenger for many different systems, including hormones and neurotransmitters. We're really only focusing on one role that it has, and don't let that give you the impression that it's not really involved in anything else. It's involved in a ton of other stuff. Additionally, if you think about it, if it's involved in so many signaling pathways, you really need a way to control how much cAMP is present, and so it's important that there is cyclic nucleotide phosphodiesterase, or PDE, sometimes the abbreviation for phosphodiesterase. Basically, this breaks down cAMP into AMP, the non-cyclized version. And it's super important to have this enzyme around because our cells really need to control the levels of signaling pathways to function normally.

Protein kinase A regulates a ton of different enzymes. And, it does this by covalently modifying serine or threonine residues with a phosphate group. It's a kinase. It's going to phosphorylate stuff at serine or threonine. It's important to note that we're talking about all these different things: the G Protein-coupled receptor, this protein kinase, the adenylyl cyclase. You might be wondering, well, how does all this stuff get together in the cell? The cell is way bigger than proteins. How does that happen? It's actually because there are these anchoring proteins, and they basically hold together the receptor, right? So here in our image, let's call this our receptor. So we're going to have anchoring proteins that are going to hold together the receptor. This is adenylyl cyclase. I'm just going to abbreviate it AC. And our protein kinase A. So these are all going to be held together by an anchoring protein. And basically, that's just going to join them together to ensure that they're able to interact easily. You know, we see this in metabolic pathways how enzymes that are carrying out reactions in a sequence will be linked together by some sort of scaffolding protein to ensure efficiency. Well, same thing with this. This is basically just a measure to ensure efficiency. You keep all the stuff that needs to interact with each other close together so that it's easy for it to interact.

Now, protein kinase A, you know, we said it phosphorylates a lot of stuff, and it basically leads to what we call phosphorylation cascades. And these, you know, this is where you have a cascading effect, where things are getting phosphorylated and dephosphorylated, and this activates and deactivates a series of proteins. So basically, you're just transferring around these phosphate groups, and this causes various proteins in a signaling pathway to basically turn on or turn off, and there is sort of a cascading effect to it. We'll look at an example right now.

So let's use epinephrine as our example. Epinephrine is another hormone. You probably know it as adrenaline, but in America, we call it epinephrine. I don't know why adrenaline is the commonly used name in the media, but it doesn't matter. So, epinephrine is a hormone. It's actually synthesized from tyrosine, and it binds to a G protein-coupled receptor and leads to the activation of protein kinase A. So, looking at our example here, you know we have our receptor, our adenylyl cyclase, protein kinase A. So, let's call this yellow molecule protein kinase A. So, cyclic AMP is going to allow for the activation of protein kinase A. Here it is in its active form. This is going to cause a phosphorylation cascade. So protein kinase A is going to phosphorylate phosphorylase kinase B which I'm just going to abbreviate PKB, and that b on the end there, that means it's the inactive form. It's going to get phosphorylated by protein kinase A into phosphorylase kinase A, the active form. So that's what we have over here. So, again, that B means inactive. The A means active form. And, then, phosphorylase kinase A is going to phosphorylate glycogen phosphorylase. So, that's what we have over here. This is glycogen phosphorylase, glycogen phosphorylase, and that's the B form, the inactive form, right? B inactive. Let me just jump out of the image here so it's easier to see, and then we have glycogen phosphorylase A, the active form. So that's a phosphorylation cascade, right? You see how it works now. We have one thing getting phosphorylated, leading to another thing getting phosphorylated, and you're turning on, in this case, we're turning on a series of proteins.

But it is possible, and you know, just as a sidenote, people very frequently confuse phosphorylation, dephosphorylation as always being, you know, if you phosphorylate something, you activate it. If you're dephosp

Here’s what students ask on this topic:

What is the role of cAMP in cellular signaling pathways?

cAMP (cyclic adenosine monophosphate) acts as a secondary messenger in cellular signaling pathways. It is generated by the enzyme adenylyl cyclase in response to extracellular signals like hormones and neurotransmitters. cAMP activates protein kinase A (PKA) by binding to its regulatory subunit, causing the release of the catalytic subunit. This activation allows PKA to phosphorylate serine or threonine residues on various target proteins, leading to a phosphorylation cascade. This cascade amplifies the initial signal, resulting in significant cellular responses such as the release of glucose from glycogen stores. Additionally, cAMP levels are tightly regulated by cyclic nucleotide phosphodiesterase, which breaks down cAMP into AMP, ensuring proper control of signaling pathways.

How does protein kinase A (PKA) get activated?

Protein kinase A (PKA) is activated by the secondary messenger cAMP (cyclic adenosine monophosphate). PKA is composed of two regulatory subunits and two catalytic subunits. In its inactive form, the regulatory subunits bind to the catalytic subunits, inhibiting their activity. When cAMP levels increase, cAMP molecules bind to the regulatory subunits, causing a conformational change that releases the catalytic subunits. These free catalytic subunits are now active and can phosphorylate serine or threonine residues on target proteins, initiating a phosphorylation cascade that leads to various cellular responses.

What is a phosphorylation cascade and how does it work?

A phosphorylation cascade is a series of events where one enzyme phosphorylates another, leading to a chain reaction of phosphorylation events. This cascade amplifies the initial signal, resulting in a significant cellular response. For example, in the epinephrine signaling pathway, epinephrine binds to a G protein-coupled receptor, activating adenylyl cyclase, which increases cAMP levels. cAMP activates protein kinase A (PKA), which then phosphorylates phosphorylase b kinase, converting it to its active form. This active kinase then phosphorylates glycogen phosphorylase, leading to the breakdown of glycogen into glucose-1-phosphate, which is eventually released as glucose into the blood. This cascade ensures a rapid and amplified response to the initial signal.

How does cholera toxin affect cellular signaling pathways?

Cholera toxin disrupts cellular signaling pathways by modifying the G protein involved in the cAMP pathway. The toxin catalyzes the ADP-ribosylation of the G protein, locking it in its active form. This leads to continuous activation of adenylyl cyclase, resulting in excessive production of cAMP. Elevated cAMP levels cause the overactivation of protein kinase A (PKA), which in turn leads to the excessive secretion of chloride and sodium ions into the intestinal lumen. Water follows these ions osmotically, resulting in severe watery diarrhea. The primary concern for cholera patients is dehydration due to the massive loss of fluids, which can be life-threatening if not treated promptly.

What mechanisms are involved in the adaptation of signal transduction pathways?

Adaptation in signal transduction pathways involves mechanisms that reduce the sensitivity of the pathway to continuous stimulation. One such mechanism is receptor phosphorylation by specific kinases, such as beta-adrenergic receptor kinase (βARK) in the epinephrine pathway. βARK phosphorylates the receptor, leading to the binding of beta-arrestin. Beta-arrestin facilitates the internalization of the receptor into endosomes, reducing the number of receptors available on the cell surface. This decreases the cell's responsiveness to the signal. Additionally, the internalized receptors can be either degraded or recycled back to the cell surface, allowing the cell to modulate its sensitivity to the signal over time.