Cell Surface Receptors – Modes of Cell Communication

00:01 So three different possibilities can happen when we consider cell surface receptors. Recall there are two classes that we will talk about. Cell surface receptors as well as intracellular receptors. Cell surface receptors are required when we have a hydrophilic molecule. And the intracellular receptors can work when we have like a steroid hormone or something that can pass across that hydrophobic region of the membrane. So in the cell surface receptor classes we have chemically gated channels. We have enzymatic receptors. And then we'll have G-protein coupled receptors. We'll take a brief introduction to the G-protein coupled receptors in this lecture, and move much more into depth in the second lecture in this sequence. Looking at these gated channels, we'll see them a lot in biological processes. We have a ligand that binds to the receptor, the receptor is closed. And when the ligand binds it could be something like sodium or it could be potassium or chloride or maybe even acetylcholine. That ligand binds to the receptor, causes the receptor to open and we have a pathway through that's favourable to hydrophobic or hydrophilic molecules that might want to pass into the cell. Often these are multi-pass transmembrane proteins. So we have that alpha helical model that this one in particularly we've got shown an alpha helix or helix turn helix protein binding motive. The next type of receptor we'll look at is the enzymatic receptor. Again a cell surface receptor. And this enzymatic receptor will actually have an effect on the inside of the cell. So rather than being a channel that lets things through we have a ligand that binds to the exterior of the protein and then have an effect on the inside of the cell. Generally those are single pass proteins. So the ligand binds and activates the protein. Again we'll look at this in much more detail when we look at receptor protein kinases in the following lecture. But the enzymatic action is then happening inside the cell. So this is an indirect effect with enzymatic receptors Each of these receptor types will have some sort of cellular response. The details of those cellular responses we'll certainly get into in much more detail later on. So before we move on I'm going to give you a quick introduction to G-protein coupled receptors. What on earth is a G-protein coupled receptor? Well this is the first instance we'll see of second messenger systems. Again, a cell surface receptor that's going to have an effect on the inside of the cell. There is a couple of different mechanisms that can happen but they're both indirect. We have indirect action where we have the receptor on the outside of the cell illiciting an effect on the inside of the cell. So G-protein coupled receptors are a second messenger system. Again, we'll be looking at several of these in a future lecture. They're activated by G-proteins which are a small class or fairly large class of proteins that are activated by GTP rather than ATP. Again, GTP is phosphorylating and it's phosphorylating perhaps even a different channel. In this case we have the receptor in one place and then the signal is moved inside the cell. The G-protein carries that signal to another transmembrane protein which opens up as a channel. More details to come on that shortly but these G-protein coupled pathways are the pathways that many of our therapeutic drugs are targeted at. There are so many different places in the cascade of events leading to a cellular response that there are lots of opportunities for us to target drugs at them. And so that's why it's important for us to have a good understanding of some of the major G-protein coupled pathways.