Practice - Biosignaling: Videos & Practice Problems

Receptor-ligand interactions are crucial for understanding G protein-coupled receptors (GPCRs), where GDP is replaced by GTP upon ligand binding, activating the G protein. Protein kinase A is allosterically activated by cyclic AMP, leading to the release of its catalytic subunit. Phospholipase C converts PIP2 into diacylglycerol and inositol triphosphate, which activate protein kinase C. Receptor tyrosine kinases undergo autophosphorylation upon dimerization and ligand binding, while insulin signaling activates glycogen synthase and GLUT4 transporters. Steroid hormones require carrier proteins for blood transport due to their hydrophobic nature.

Practice - Biosignaling

As we move through the following questions, it's important that you try each one on your own before you listen to the answer. I will pause briefly between each question, but you should actually pause the video and try to answer each question yourself or attempt all the questions and then watch the video to get the explanations. Now, let's begin with question 1 and again, if you haven't tried to answer this question on your own, pause the video now. The answer to question 1 is E, receptor ligand interactions. Now, the analysis is not something that you need to understand the specifics of. Basically, all you need to know is it's sort of like what we talked about with enzyme kinetics. There are some differences here, some important ones too. But you don't need to worry about that. The main point is that the Scratcher analysis is a way to find K_D, which is like Kilometers for enzyme kinetics, except K_D has to do with binding affinities. Or I should say receptor ligand binding. So the Scratcher analysis provides data on receptor ligand interactions. Specifically, it's there so that you can find that K_D which is again sort of like the equivalent of Kilometers from enzyme kinetics. Now, let's move on to question 2. When G protein-coupled receptors bind their ligand, GDP is replaced with GTP in protein G. So, essentially when the ligand binds to the G protein-coupled receptor, there's a GDP already bound in the alpha subunit of protein G, right? We have our alpha subunit and in there, we've got GDP. When we bind ligand, GDP gets switched with GTP and that makes our protein G active. And also remember that, that G, the protein G, is a GTPase. So it's going to very slowly break down its bound GTP back into GDP. So it's going to inactivate itself. But also remember that that can be sped up with those GAP proteins or GTPase activator proteins. And just looking at the wrong answer traces here, these G protein-coupled receptors are not ion channels. When the ligand binds to the receptor, we activate G. That's how they carry out their function. Other receptors, for example, are ion channels, but we're not going to talk about those really. Adenyl cyclase is not immediately activated. G is going to go over and activate adenyl cyclase. So it's not the immediate result of ligand binding, but it is a downstream result. It's what G does once it's activated. There's not going to be any sort of enzymatic reaction performed on the ligand. And, the receptor itself doesn't synthesize G. G is already hanging out there. G is made, not made by the receptor. Now, let's move on to question 3. So, protein kinase A is allosterically activated by cyclic AMP and you might remember that protein kinase A has that regulatory subunit R and the regulatory subunit is going to be bound into the active site. So here's our active site. And when cAMP binds allosterically to the regulatory subunit, it's going to cause the regulatory subunit to leave the catalytic subunit and free it up to do its job. So, in essence, protein kinase A is allosterically activated by cyclic AMP. And the wrong answer choices here are, you know, should be fairly obvious why they're wrong. It's not covalent binding, right? This is allosteric binding so it has to be easily reversible. Covalent modification would not be easily reversible. Affected by cyclic AMP at the phosphorylation site. No. Completely inhibited. That's the opposite and hydrolyzed by cyclic AMP. No. It's activated. Alright. Let's move on to question number 4. The hormone-activated phospholipase C can convert phosphatidylinositol 4,5-bisphosphate to diacylglycerol and inositol triphosphate. Now, the abbreviated names for these are we have PIP₂ and diacylglycerol that's DAG, DAG and inositol triphosphate that's IP₃. So you might be more familiar with those names, or those abbreviated names, but these are the actual names of the molecule and, you might recall that, that phospholipase C is going to snip the bond between the inositol phosphate and the glycerol of the phosphatidylinositol. And again, what you wind up with are these 2 molecules and recall that IP₃ and calcium are going to be used to activate protein kinase C and inositol triphosphate is going to open up those intracellular calcium channels. Alright. Let's talk about question 5. The autophosphorylation of receptor tyrosine kinases, remember, receptor tyrosine kinases are special because they can phosphorylate themselves. So that autophosphorylation depends on everything you see here. So dimerization of the receptor, right? We have to have those 2 components come together, dimerize, and then they will be activated or like ready to go. We'll need ATP to provide phosphate, for the phosphorylation, right? We're going to use ATP for that. Ligand binding. We need ligand to bind our receptor dimer in order for it to get the signal that it needs to, do the autophosphorylation and that ligand binding is going to lead to conformational changes in the receptor through the membrane, right? Ligand binding is going to change the conformation of the receptor. So first we have dimerization then ligand binding then conformational in the receptor, and then we're going to use ATP to phosphorylate the receptor. So then, you know, it can carry out its various functions, and remember insulin was the model we used for our receptor tyrosine kinase. Now, speaking of insulin, let's move on to question 6. After insulin binds to its receptor, both A and B happen. Glycogen synthase becomes, and it should be noted that both A and B happen due to protein kinase B. So, glycogen synthase becomes activated, and this was that, I don't want to say terribly confusing but you know slightly complex thing that's going on here. So protein kinase B is going to inactivate GSK 3. And remember that GSK 3 normally inactivates glycogen. I'm sorry. Glycogen synthase is about phosphorylase glycogen synthase. So, if protein kinase B inactivates GSK 3, then glycogen synthase won't be inactivated anymore and it will turn on. So that's how protein kinase B leads to the activation of glycogen synthase. And protein kinase B also signals the cell to move glucose transporters, specifically GLUT4 transporters from internal membrane vesicles, right? They're going to be stored in those endosomes and protein kinase B will signal for them to be moved from those internal membranes to the plasma membrane so that more glucose can be transported into the cell. Alright. Let's finish up by taking a look at question 7. So steroid hormones are carried on specific carrier proteins in the blood because the hormones cannot dissolve in the blood, right? Steroid hormones, those have that sterol backbone, and I'm not going to draw it out. It's the four fused rings, right? Super hydrophobic. Those sterols are superhydrophobic. They can't move through the blood on their own. They can't; it's not that they're too unstable. It's that they're too hydrophobic. Targeting cells, that's not what it's about either. They don't need any help passing through the cell membrane. In fact, they'll readily diffuse through the membrane because they're so, they're so hydrophobic or lipophilic maybe I should say. So, they don't need help passing through the membranes and, they don't need carrier proteins to help them bind their receptors in the nucleus. They'll do that just fine once they get in. The problem is that they can't move through the blood on their own. They need carrier proteins because they're too hydrophobic to be in that aqueous environment. Alright. Let's flip the page.

Here’s what students ask on this topic:

What is the role of GTP in G protein-coupled receptor (GPCR) signaling?

In GPCR signaling, GTP plays a crucial role in activating the G protein. When a ligand binds to the GPCR, it causes a conformational change that allows the GDP bound to the alpha subunit of the G protein to be replaced by GTP. This exchange activates the G protein, enabling it to interact with downstream effectors such as adenyl cyclase. The G protein is a GTPase, meaning it will eventually hydrolyze the bound GTP back to GDP, thereby inactivating itself. This process can be accelerated by GTPase-activating proteins (GAPs).

How does cyclic AMP (cAMP) activate protein kinase A (PKA)?

cAMP activates PKA through allosteric binding. PKA is composed of regulatory and catalytic subunits. In the inactive state, the regulatory subunits bind to the catalytic subunits, inhibiting their activity. When cAMP binds to the regulatory subunits, it induces a conformational change that releases the catalytic subunits. These free catalytic subunits are then able to phosphorylate target proteins, thereby propagating the signal within the cell. This activation is reversible, as cAMP binding is non-covalent.

What are the products of Phospholipase C (PLC) activity on PIP2?

Phospholipase C (PLC) hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) into two secondary messengers: diacylglycerol (DAG) and inositol trisphosphate (IP3). DAG remains in the membrane and activates protein kinase C (PKC), while IP3 diffuses into the cytoplasm and triggers the release of calcium ions from intracellular stores. These calcium ions further activate PKC and other calcium-dependent signaling pathways.

Why do steroid hormones require carrier proteins in the blood?

Steroid hormones are highly hydrophobic due to their sterol backbone, which makes them insoluble in the aqueous environment of the blood. To be transported effectively, they bind to specific carrier proteins that solubilize them and facilitate their movement through the bloodstream. Once they reach their target cells, they can readily diffuse through the cell membrane due to their lipophilic nature and bind to intracellular receptors to exert their effects.

What is the significance of autophosphorylation in receptor tyrosine kinases (RTKs)?

Autophosphorylation is a critical step in the activation of receptor tyrosine kinases (RTKs). Upon ligand binding, RTKs dimerize, bringing their intracellular kinase domains into close proximity. This allows them to phosphorylate each other on specific tyrosine residues. These phosphorylated tyrosines serve as docking sites for downstream signaling proteins, initiating various cellular responses. Autophosphorylation thus amplifies the signal and ensures precise regulation of cellular activities such as growth, differentiation, and metabolism.