Hey everyone, this is where I talk about the inositol triphosphate or IP3 pathway. So the IP3 pathway involves a first messenger response leading to a second messenger response leading to a cellular response and I'll get into more detail on what each of these mean in a moment. So the first messenger is an extracellular molecule or an extracellular ligand, whether that be a hormone or some other molecule.
And in this pathway, in the IP3 pathway, the extracellular ligand binds to a G-protein coupled receptor. Now when the first messenger aspect of this pathway becomes activated, it leads to the activation of a second messenger response. Now the second messenger is an intracellular molecule, and in this IP3 pathway, those intracellular or second messenger molecules are either IP3 or triphosphate. diacylglycerol or DAG and calcium and I'll get into more detail on what each of these does a little bit later.
So I'm going to show you this molecule here PIP2 or phosphatidyl inositol 4,5-bisphosphate and I show you this molecule because PIP2 actually acts as a precursor molecule for the formation of DAG and IP3. So you if you look at this molecule here, PIP2 can get cleaved essentially in half. The first top or the first top half of this molecule can get cleaved off of PIP2 and this is essentially what DAG is.
It forms DAG. And the bottom part of this molecule can get also get removed and this is IP3. So essentially PIP2 acts as a precursor molecule for other second messengers including diacylglycerol and inositol triphosphate. So here is the cell membrane with a G protein couple receptor and a phospholipase C.
We'll talk about what phospholipase C does in a moment. So with the G protein couple receptor, if the cell is presented with a ligand, a ligand that binds to a G protein couple receptor, The G protein couple receptor can become activated and it can lead to an activating response and lead to the activation of phospholipase C. Now phospholipase C is, as its name suggests, it is an enzymatic reaction that involves phospholipids. And what it does is it actually acts on PIP2.
And what it does is phospholipase C is the enzyme that is responsible for cleaving PIP2 in half, it will actually chop PIP2 into IP3 and DAG. So IP3 enters into the cytosol wall, DAG remains associated with the plasma membrane. Now the endoplasmic reticulum becomes very important in this pathway, and there is a channel located on the endoplasmic reticulum, and that is an IP3 calcium channel.
And the endoplasmic reticulum contains high concentrations of calcium. And what happens is that IP3 that has been liberated from PIP2 actually travels, it binds to the IP3 calcium channel, and it actually opens that channel. releasing or allowing calcium to travel down its concentration gradient from the endoplasmic reticulum into the cytosol. So now cytosolic calcium levels begin to increase in concentration. Now that we have increased calcium and we also have DAG, there is also another kinase that's important in this pathway, protein kinase C.
And what happens is that DAG and the calcium actually lead to the activation of protein kinase C. And what happens is once protein kinase C becomes activated, as its name suggests, it's a kinase. It leads to phosphorylation of target proteins within the cell. This ultimately leads to that cellular response.
So I'm going to quickly go over... what are the first messengers, what are the second messengers, and how it leads to the cellular response once more. So in this pathway, the extracellular ligand represents the first messenger. It binds to G protein coupler receptor.
PIP2 becomes cleaved into two second messengers. One is IP3, the other is DAG, and another second messenger is the calcium. The calcium also becomes released from the endoplasmic reticulum, and then with DAG leads to the activation of protein kinase C, which then leads to phosphorylation of target proteins.
This is the cellular response as part of this pathway. Anyways guys, that was a quick lesson on the IP3 calcium pathway. I hope you found this lesson helpful.
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