G Protein Coupled Receptors - Online Tutor, Practice Problems & Exam Prep

G Protein Coupled Receptors (GPCRs) are the largest class of cell surface receptors, characterized by a single polypeptide chain that traverses the membrane seven times. They activate G proteins, which act as molecular switches, binding GTP to signal downstream pathways. Key signaling pathways include cyclic AMP (cAMP) production via adenylyl cyclase, calcium signaling mediated by calmodulin, and inositol phospholipid signaling involving IP3 and DAG. These pathways are interconnected, highlighting the complexity of cellular signaling mechanisms.

Structure and Signaling

Hi. In this video, we're going to be talking about G Protein Coupled Receptors. So, there's a lot of information that we're going to have to go over about G Protein Coupled Receptors. So, first, we're just going to start with the structure and the basics of the signaling. G Protein Coupled Receptors, they're the largest cell surface receptors, around 700 or more in humans. Generally, they work by signaling through G proteins. Their structure is composed of a single polypeptide chain. That polypeptide chain snakes back and forth through the bilayer. And, this number is really important, it does this 7 times. This number is super important. This means that there are 3 extracellular loops and 3 intracellular loops. The extracellular loops bind ligands, while the intracellular loops bind signaling proteins. The cytosolic side of the GPCR is bound to a G protein, which we talked about. This G protein acts as a molecular switch. The G protein is going to be a trimeric protein, which means it has 3 subunits. It's activated when it's bound to GTP and inactivated when it's bound to GDP. It's called a G protein because it has this interaction with GTP and GDP. This makes sense. This is not a new concept always in cell biology. Things are activated when bound to the TP version and inactivated when bound to the DP version. An activated G protein can then couple ligand and receptor binding to other enzymes. What this means is that once the G Protein Coupled Receptor activates that G Protein by having the GTP form, then that activated protein can go and do other things, and activate other things in the cell.

Here we have G protein activation, this is a signaling molecule, in this case it's a hormone. This is going to interact with a G protein, which is here. You can't really see, but this is actually going to snake 7 times throughout the membrane. You can see that this results in some type of conformational change here. The hormone is now bound here. This results in the G protein switching from GDP to GTP. That results in activation, which can then go on and signal downstream.

The regulation of GPR signaling involves regulating the G protein. Where the G protein is and how the G protein regulates is going to just regulate that signaling. Signaling can be affected by proteins that regulate GTP hydrolysis, which makes sense. If it can't hydrolyze, then that is going to remain active for a really long time. But if it's hydrolyzed really quickly, then it's going to only remain active for a short period of time. One process of this is called desensitization, and this is a process that blocks active receptors from activating G proteins. For instance, there's one type of protein called the G Protein Coupled Receptor Kinase 2. You don't necessarily need to memorize that, but just know it's a protein. This binds to GPCRs to compete for binding with the G protein. This means that when this protein is bound, the G protein cannot bind. And when the G protein can't bind, it can't be activated and then can't signal. This is one example of desensitization, which means that, you know, the ligand is still binding and activating this G protein coupled receptor, but it's not doing anything in the cell because the G protein itself is not being activated.

GPCRs can be regulated via receptor sequestration, which just means that they're all sequestered in one area and so less responsive to the ligand, or down regulation on the surface. So that's kind of an overview of the structure and signaling, let's now turn the page.

Common Pathways

Okay. In this video, we're going to talk about common GPCR signaling pathways. So, we're going to specifically talk about 3 different pathways that you're going to read about or hear about as good examples of GPCR signaling. So, the first is going to be cyclic AMP, or CAMP, I like to say in my head. And this, concentration of this molecule is very much regulated by a G protein signaling. So how this works, is that cAMP is a signaling molecule. It's present in every single cell that's been studied on Earth, so it's a very universal signaling molecule. And generally, it's kept at a low concentration in the cell. But, extracellular signals can result in a huge increase in this intracellular and how this happens is because cAMP is made by a certain enzyme called adenyl cyclase, and you're actually going to need to know cAMP and adenyl cyclase. And, it actually is degraded by this other enzyme called phosphodiesterase. Now, ligands come in, they activate a G protein coupled receptor, this activates a G protein, and different G proteins can stimulate or inhibit adenyl cyclase, which then controls cAMP levels. So, if it stimulates the adenyl cyclase then more CAMP will be made, so more. But, if it inhibits it, then less will be made. So here's an example of this. So here you have a ligand, it's binding to a GPCR, this is activating. A G protein, which is then going on to activate. Adenyl cyclase, which is then producing a lot of cAMP, which can then go on to signal. So that's going to be a really common one that you hear about, cAMP and adenyl cyclase.

Now calcium is another important signaling molecule. And so, G proteins can trigger an increase in cytosolic calcium concentration. So, calcium, like cAMP, is kept really low concentration in the cell, and so G proteins can trigger this increase. And so, calcium is really important because calcium goes on to affect the regulation of many enzymes and proteins. And so, for instance, one of these are CaM Kinases, and these are protein kinases that phosphorylate different proteins involved in gene transcription. So, they control transcription of a lot of genes, and they are controlled via calcium. So, you can imagine that if you increase the calcium, then you're going to activate these CaM kinases, which then go on to activate or inhibit various genes. Now, there's this vocab word here, calmodulin. So what this is, is this is a protein, and this mediates the animal cell response to calcium. And so, sometimes calcium can have this huge effect because it can interact with calmodulin, which then goes on to either inhibit or activate a variety of different genes. But just know here that calcium is this really important signaling molecule that can have a variety of effects. So, here's an example of calcium signaling. So, you can see there's calcium getting into the cell, there's all these calcium increases here, And all of them result in signaling through all these different receptors and pathways. And now, you don't actually at all need to understand all these different pathways or all these different abbreviations, but just realize that calcium signaling in this one particular cell type can affect all these things at the exact same time. So calcium is a huge signaling molecule.

And then, the final one that we are going to talk about is the Inositol Phospholipid signaling pathway. And these are regulated by GPCRs as well. So, how this happens is there's this special G protein called Gq, you might want to actually memorize that. And Gq activates this adenyl cyclase again. So, remember adenyl cyclase is involved in cAMP, but in this case, we're going to talk about it in a different way and how it affects lipids. And so, activated adenyl cyclase results in a cleavage of a lipid called PIP₂. Now, when PIP₂ is cleaved, that actually results in 2 molecules, so it's cut, and that splits it in 2. And these 2 molecules are called IP₃ and DAG, and each one of these have different functions. So, IP₃ goes to the ER, binds to it, and opens calcium channels. So, we're seeing these, like, interconnections of these signaling pathways already. They don't stand alone, they all interact with each other. So, IP₃ will go to the ER, open calcium channels. And then, you have DAG, which will act, to activate a variety of other signaling pathways. And so, in this case, we have calmodulin again, because remember, we're activating these calcium channels and calmodulating. Calmodulin is a protein that mediates cell response to calcium. So calmodulin will, once it, you know, this calcium is activated, it binds calcium, undergoes conformational change, and binds to other proteins in the cell to have their function. So, this is an example of the inositol phospholipid pathway. So, we have this lipid called PIP and then we have, this enzyme called phospholipase C, which I did not mention but is actually, you know, can help do this cleavage. And so, this results in the formation of DAG, once it's cleaved, and IP₃. DAG then goes on to activate a variety of different pathways, which you can see are going throughout the cell. And, IP₃ goes to the ER and activates calcium signaling, which can release calcium into the cell and then go on to do a variety of other things both in and outside the cell. So, these are the 3 pathways that are really important that you're going to read about in your textbook, and notice that they are all connected in all these different ways. And it's very common in signaling pathways. They usually don't stand alone, they all interact. And so these 3 are definitely interacting. So with that, let's now move on.

Which of the following is true regarding G-protein coupled receptors?

• A. It contains three transmembrane sections
• B. The G protein binds the intracellular side of the G protein coupled receptors
• C. There are four intracellular and extracellular loops
• D. G proteins respond only to calcium

cAMP levels are kept at high levels in the cell.

• A. True
• B. False

Here’s what students ask on this topic:

What is the structure of G Protein Coupled Receptors (GPCRs)?

G Protein Coupled Receptors (GPCRs) are composed of a single polypeptide chain that traverses the cell membrane seven times. This unique structure results in three extracellular loops and three intracellular loops. The extracellular loops are responsible for binding ligands, while the intracellular loops bind signaling proteins. The cytosolic side of the GPCR is bound to a G protein, which acts as a molecular switch. This G protein is a trimeric protein, meaning it has three subunits, and it is activated when bound to GTP and inactivated when bound to GDP.

How do G proteins function as molecular switches in GPCR signaling?

G proteins function as molecular switches in GPCR signaling by cycling between active and inactive states. When a ligand binds to a GPCR, it causes a conformational change that activates the G protein by exchanging GDP for GTP. The G protein, now bound to GTP, can activate downstream signaling pathways. The G protein remains active until the GTP is hydrolyzed to GDP, at which point it becomes inactive. This switching mechanism allows for precise regulation of cellular responses to external signals.

What are the key signaling pathways regulated by GPCRs?

GPCRs regulate several key signaling pathways, including:

• cAMP Pathway: GPCR activation can stimulate or inhibit adenylyl cyclase, which controls the levels of cyclic AMP (cAMP), a universal signaling molecule.
• Calcium Signaling: GPCRs can trigger an increase in cytosolic calcium concentration, which affects various enzymes and proteins, including CaM kinases and calmodulin.
• Inositol Phospholipid Pathway: Involves the cleavage of PIP2 into IP3 and DAG, which have distinct roles in cellular signaling, including the release of calcium from the ER and activation of other pathways.

What is the role of cAMP in GPCR signaling?

cAMP (cyclic AMP) is a crucial second messenger in GPCR signaling. When a ligand binds to a GPCR, it can activate a G protein that stimulates adenylyl cyclase, an enzyme that converts ATP to cAMP. Increased cAMP levels activate protein kinase A (PKA), which then phosphorylates various target proteins to elicit cellular responses. cAMP is involved in numerous physiological processes, including metabolism, gene transcription, and cell growth. Its levels are tightly regulated by adenylyl cyclase and phosphodiesterase, which degrades cAMP.

How does calcium signaling work in the context of GPCRs?

Calcium signaling in the context of GPCRs involves the activation of G proteins that trigger an increase in cytosolic calcium concentration. This is often mediated by the inositol phospholipid pathway, where IP3 binds to receptors on the endoplasmic reticulum (ER), causing the release of calcium into the cytosol. Elevated calcium levels activate various proteins, including calmodulin and CaM kinases, which regulate numerous cellular processes such as gene transcription, muscle contraction, and neurotransmitter release.