Let's begin our discussion of biochemistry topics with biosignaling. Now, signal transduction actually has 5 features to it. And those are specificity and this is specificity just like we've seen with basically every other protein that we've been talking about. Usually, the signals are chemical in nature. However, for example, they can also be electrical. We're not going to deal with anything outside of chemical signals though. So, again, specificity just like the specificity we've seen with all the other proteins we've talked about. Amplification is basically the concept of how one molecule can lead to a 1000 downstream molecules. So one molecule binding to a receptor can activate a number of enzymes and those enzymes will produce a number of molecules. And so the signal gets amplified along the way as it progresses through the path. Modularity is basically how some proteins can interact with multiple components of a signaling pathway. So, a protein can interact with or bind something like that, interact with a signal and we also have adaptation which is how feedback modulates receptor presence or activity. And this is kind of like any type of feedback regulation we've seen with enzymes. Of course, it is going to work a little differently with signal transduction because the feedback isn't necessarily going to go But the But the point is that these signal transduction pathways can be attenuated. Some downstream signal will be able to affect how the upstream part of the signal pathway like the receptor behaves. It will attenuate its activity or it adapts its activity. You also have integration which is basically the idea that all signals are integrated with other signals to give the appropriate response. And we're going to see a figure a little later that should hopefully illustrate just how crazy and complicated, all of the signals a cell has to integrate are. I mean, you're going to see this figure and be like, oh my god, there's so much going on in a cell and this figure is only showing a little bit honestly. It's not even like the full picture. The main idea is signals don't exist alone. We're going to talk about single signal pathways. But you have to remember that there are a bunch of other signal pathways going on at the same time. And all of these pathways are integrating together so that the cell is producing the appropriate responses.
Now the first type of receptor we're going to talk about is a G protein-coupled receptor. Basically, with G Protein-Coupled Receptor, the defining feature is that it uses this protein, that we see right here. This is G and it uses this protein G to carry out its signal basically. Now the receptor has a sort of defining feature about it and that it has these 7 transmembrane domains. G protein-coupled receptors have these 7 transmembrane domains. Just a little interesting feature about them. But again, the most important part is that they use this protein G. So in our figure here, we have our receptor. This is G right here. It is both the yellow and the blue part. This whole thing is G. G actually has 3 subunits. You can see it better in this image of G here. The red, yellow, and blue parts. And this over here in green, this is adenylyl cyclase. And we're going to talk about what that does in just a second. So basically, when you have a ligand binding to your G protein-coupled receptor. And in the case of our example, we're going to be taking a look at, hormone binding and this is very common. So we have our ligand or in this case hormone binding into our receptor here in red. And then what's going to happen is G as you see right here, G actually already has inside of it. So back here, G already has inside of it, just drawing like a blob.
GDP and when, when the hormone binds, what's going to happen is where we see here where GTP is actually going to come in and be exchanged for that GDP and the GDP is going to be released. And this reaction is actually catalyzed by, stuff that we don't see here in this picture. They're called guanine exchange factors. So they're going to help carry out that reaction, that swap of GTP and GDP. And just to be precise, this is happening in the alpha subunit. So alpha subunit there. And once GTP is bound, that's sort of when G becomes activated. So I'm actually going to hop out of the image here just so you can see it better. And basically, when G is activated, it is going to cruise over to Adenylyl Cyclase, like you see here. I should be drawing my arrow coming from the alpha subunit there. Anyways, so as you can see, here we have G and it is now interacting with and activating Adenylyl Cyclase. So again this is adenylyl Cyclase. And what adenylyl cyclase does is it actually converts ATP into this molecule called cyclic AMP. You guys know AMP adenosine monophosphate. Well this is cyclic adenosine monophosphate. Basically, it's just a cyclized version. And this cyclic AMP which I'm just going to call camp from now on just because it's easier to say. So, camp is a second messenger probe secondary messenger. It's not a protein. It's a secondary messenger and it's a nucleoside. So it's going to do a variety of things that we're going to talk about momentarily. But for now, I just want to finish up, by talking about G a little bit more because you see G is actually a GTPase. And so on its own, it'll slowly break down the GTP inside of it and leave the alpha subunit inactive. So let me just say that again. G is a GTPase. And so the GTP that it has bound inside of it will, be broken down by G very slowly. And this will result in GTP being turned into GDP, right? And that's going to leave alpha inactive. That's basically going to take us back to where we were over here. What's interesting is there are these GTP activator proteins or gaps and they actually can increase the rate of G of the breakdown of GTP into GDP. And this is going to be important in terms of, you know, modulating signals and attenuating signals.
