An amino acid-based hormone typically induces a response in a cell by binding to a receptor on the outside of the cell, often a GPCR (G-protein coupled receptor), and initiating a signaling cascade inside the cell. And we said that the word cascade is meant to evoke something like a waterfall, where it's picking up speed, picking up momentum, getting bigger as it goes. That idea of getting bigger as it goes, we're going to call amplification, and we're going to look at that in more detail here. So first, let's just remember that a signaling cascade is this series of linked chemical messengers where they pass the signal off from one to another, and we looked at an example of that when we looked at this cyclic AMP signaling cascade.
Well, this idea of amplification is going to be when one molecule in that pathway passes the signal to multiple molecules and that's going to increase, we'll signify that with an up arrow, the total amount of signal. Now when we looked at how that cyclic AMP pathway works, we just said, you know, this molecule activates the next molecule, but there's going to be times when it activates actually multiple molecules, and we're going to illustrate that using these sort of personified happy little molecules here. And these are the molecules of the cyclic AMP signaling pathway. And here, they're all holding a cell phone and they're sending a message from one to another, a text message or a DM or something like that. And we're going to say there's going to be a couple of key places in this pathway where the signal is amplified. We're going to say that the cyclic AMP secondary messenger systems are amplified when, and then we have three specific places here.
Let's take a look. We're going to start with our happy little hormone here. The hormone starts the signal by sending a message to the GPCR, this blue GPCR here, but it sends that signal by actually physically binding to the GPCR. And a hormone can only bind to one GPCR at a time, so we're not going to get any amplification here. But that GPCR is going to activate multiple G proteins. And so our next little figure in the line here, this green G-protein, well this GPCR is going to get the signal, but it's actually going to pass it off to multiple G proteins. Now these G proteins have the message and they're going to go out and diffuse across the membrane and they're going to pass the message off to their friend here, adenylate cyclase that we see in pink. We'll label that as AC. But again, G proteins have to actually bind to adenylate cyclase, and adenylate cyclase is only active when the G-protein is bound to it. So again, we're not going to get any amplification. Each G-protein just sort of sends this message off to one adenylate cyclase. But adenylate cyclase is an enzyme, and so it's catalyzing chemical reactions really, really fast. So adenylate cyclase produces many, many cyclic AMP molecules. So here, adenylate cyclase is going to send this message on to the cyclic AMP that we see here. But again, it's going to amplify because it's sending it to each individual adenylate cyclase, it's sending that message to many. It's making many cyclic AMPs.
Well, cyclic AMPs are going to send the message off to its little blue friend here, a kinase. But again, cyclic AMP acts actually have to bind to the kinase for the kinase to be active, and you actually need a few cyclic AMPs bound to the kinase for it to work. So you're not going to get any amplification here again. It's going to stay relatively the same. But again, our kinase, our kinase is an enzyme, so protein kinase is going to phosphorylate many, many proteins. So you can see already, from one hormone, we have many kinases, and those kinases are now going to go out and phosphorylate whole bunches of proteins. And then those proteins, some of those are going to go off and activate even more molecules. So we can start with very few hormones. Here we have one, but, like, a countable number of hormones binding to receptors on a cell, and that can, in turn, induce this change that results in literally millions or billions of molecules being activated inside the cell, and that can induce a huge physiological response. So again, this process of amplification, because at certain steps in this process, the message gets amplified, it gets sent on to multiple molecules, you can have a very low hormone signal induce a very large physiological change. Alright. We're going to practice that more in examples and practice problems of fall. You should give them a try.