G Protein Coupled Receptors - Video Tutorials & Practice Problems
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concept
Structure and Signaling
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Hi in this video we're gonna 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. So G protein coupled receptors, they're the largest family of cell surface receptors around 700 or more in humans. And generally they work by signaling through G proteins. And so their structure is well they're composed of a single polyp peptide chain but that polyp peptide chain snakes back and forth through the bi layer. And this number is really important. It does the seven times this number is super important. So this means that there's three extra cellular loops and three intracellular loops and extra cellular loops. They bind Liggins while the intracellular loops biden signaling proteins. So the side of solid side of the G. P C. R is bound to a G. Protein which we talked about and this G protein acts as a molecular switch. So the G protein is going to be a trim eric protein which means there's three sub units. And um it's activated when it's bound to G. T. P. And inactivated when it's bound to G. D. P. And it's called a G protein. Because it has this interaction with GTP and GDP and this makes sense. This is not a new concept always in cell biology, things are activated when bound to the T. P. Version and inactivated when bound to the D. P. Version. And so um an activated G protein can then couple ligand and receptor binding to other enzymes. So what this means is that once the G protein coupled receptor activates that G protein by having the GTP form that activated protein can go and do other things and activate other things in the cell. So here we have G protein activation. So this is a signaling molecule in this case it's a hormone and so this is going to interact with a G. Protein which is here and you can't really see but this is actually gonna snake seven times throughout the membrane. And so you can see that this results in some type of confirmation will change here. So the hormones now bound here and this results in the G protein G protein switching from G. D. P. To G. T. P. And that resulted activation which can then go on and signal downstream. So the regulation of GpR signaling involves regulating the G protein so where the G. Protein is and how the G protein regulates is going to just regulate that signaling. So signaling can be affected by proteins that regulate GTP high tral icis. Which makes sense. If it can't hydraulic size then that is going to remain active for a really long time. But if it's hydraulic sized really quickly then it's gonna only remain active for a short period of time. So one process of this is called desensitization and this is a process that blocks active receptors from active gene activating gene proteins. So for instance, there's one type of protein called the G protein coupled receptor Cranes to don't necessarily need to memorize that, but just know it's a protein and this binds to G p C. R. S to compete for binding with the gene protein. So this means that when this proteins bound, the G protein cannot bind and when the G protein can't bind, it can't be activated and then can't signal. And so this is one example of desensitization, which means that um 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. And so G P. C. R. S. Can be regulated via receptor and activation sequestration, which just means that they're all sequestered in one area and so less responsive to the Liggan or down regulation on the surface. So that's kind of an overview of the structure and signaling. Let's not turn the page
2
concept
Common Pathways
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okay in this video we're going to talk about common GPC are signaling pathways. So we're going to specifically talk about three different pathways that you're going to read about or hear about as good examples of GPC are signaling. So the first is going to be cycling KMp 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 extra cellular signals can result in a huge increase in this intracellular concentration. And how this happens is because Camp is made by a certain enzyme called a dino cyclist. And you're actually gonna need to know Camp and a dino cyclists and it actually is degraded by this other enzyme called fossil dinosaur race. Now Liggins come in, they activated G protein coupled receptor. This activates a G. Protein and different G proteins can stimulate or inhibit a dino cyclists which then controls CAMP levels. So if it stimulates the adrenal cyclists then more camp will be made more. But if it inhibits it then less will be made. So here's an example of this. So here you have a Liggan, it's binding to a G. P. C. R. This is activating a G. Protein which is then going on to activate a dino cycle Ace which is then producing a lot of C. A. M. P. Which can then go on to signal. So that's gonna be a really common one that you hear about camp and a dino cyclist. Now calcium is another important signaling molecule. And so G proteins can trigger an increase inside a solid calcium concentration. So calcium like CAMP is kept really low concentration in the south 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 campiness is um and these are protein kindnesses that for like different proteins involved in gene transcription. So they control transcription of a lot of jeans and they are controlled via calcium. So you imagine that if you increase the calcium then you're going to activate these campiness is which then go on to activate or inhibit various genes. Now there's this vocab word here cal module in 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 caL module in 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 different 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 same exact time calcium is a huge signaling molecule and then the final one that we're going to talk about is this in a subtle phosphor lipid signaling pathway. And these are regulated by G. P. C. R. S. As well. So how this happens is there's this special G. Protein called G. Q. You might want to actually memorize that and G. Q. Um activates this additional cyclist again. So remember dino cyclist 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 female cyclist this results in a cleavage of a lipid called Pip two. Now when pIP two is cleaved that actually results in two molecules so it's cut and that splits it into and these two molecules are called I. P. Three and Dag and each one of these have different functions. So I. P. Three 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 I. P. Three 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 um in this case we have cal module in again because remember we're activating these calcium channels and cal modulating can modulate is a protein that mediates cell response to calcium. Socal modulation will once it you know this calcium is activated, binds calcium undergoes conformational change and bind to other proteins in the cell to have their function. So this is an example of the enormous tidal phosphor lipid pathway. So we have this lipid called pip and then we have um this enzyme called foss Philip K. C. Um which I didn't mention but it actually can help to do this cleavage. And so this results in the formation of dag once it's cleaved and I. P. Three dag then goes on to activate a variety of different pathways which you can see are going throughout the cell. And I. P. Three 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 three pathways um that are really important that you're gonna read about in your textbook and notice that they are all connected in all these different ways and it's very common and signaling pathways they usually don't see it alone, they all interact. And so these three are definitely interacting. So with that let's sounds fun.
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Problem
Problem
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