Now, a metabolic pathway is a series of biochemical reactions taking place within a cell. We're going to say that under feedback control, the end product of a pathway acts as an on-off switch for the enzyme in the first step. The end product is a negative allosteric regulator of the enzyme in the first step. So let's take a look at this image, and let's just ignore this first one for right now. We'll come back to it. Now, let's say we're starting out with this first portion here. This enzyme has an active site, and here, 'a' represents our substrate. It also has this allosteric site here. Right now, there's no regulator in sight to attach to it, so the active site is going to be unaffected. The substrate is free to come and attach to this enzyme, so we’d say that this is an active enzyme. Once the substrate attaches to the enzyme, we get our enzyme-substrate complex, which is here. This helps to create our intermediates. So it would help to create structure 'b,' which would eventually create intermediate 'c,' so that we can finally get to our product 'd.' So we've reached our product 'd' here. The whole point of this reaction is to help to make this product 'd.' And once we’ve reached an efficient number or a suitable amount of it, we no longer need to continue making it. So what happens is that this product 'd' here, this end product will come back and attach to the allosteric site of our enzyme. By attaching to the allosteric site of our enzyme, it's going to act as a negative allosteric regulator. It is going to basically change the active site enough that the substrate can no longer bind to it. So, we have, in essence, created an inactive enzyme. The substrate no longer fits in the active site, so the enzyme cannot be utilized. This will help us from continuing to make more of our end product 'd.' Now, once that amount deteriorates and goes down again, we could have this end product 'd' unattach itself from the allosteric site and begin the whole process again, right? So, this is what we talk about in our feedback control. We have our enzyme binding to the substrate to make our final product, our end product 'd.' Once we've made enough of it, some of it will come and attach to the enzyme, deactivating it or making that enzyme inactive. So, we stop making it. So this is the whole idea of enzyme regulation in terms of feedback control.
Enzyme Regulation: Feedback Control - Video Tutorials & Practice Problems
Feedback Control Concept 1
Video transcript
Feedback Control Example 1
Video transcript
Which of the following statements cannot be true about feedback control?
Regulation is achieved by negative allosteric control of an enzyme in the first step. This is true. The end product that we make at the end, some of it will come and attach to the enzyme. It'll attach to the allosteric site, thereby altering the size of the active site so that the substrate can no longer attach. This is negative allosteric regulation, so this is true. The end product affects its own production. Yes, if you're making more than enough of the product and you no longer want to make more, then it could come and attach to the allosteric side of the enzyme and basically deactivate or make that enzyme inactive. So in that way, it is controlling how much of it is being made.
The end product binds irreversibly to the enzyme in the first step. No, it's not irreversible. It is reversible. The end product will attach to the allosteric site and make that enzyme inactive, so it will no longer continue making the product. But, eventually, let’s say you consume all of that product and you want to start making it again. The end product will detach from the allosteric side of the enzyme, and thereby reactivate it so that it can continue to make more product. It's a way of controlling the amount of your end product being formed. When you have too much of it, an overabundance of it, you turn off the enzyme. When you have a low amount of it, you turn the enzyme back on. And this has to do with attaching and detaching to the allosteric side of the enzyme. So here, this is not true. It's not irreversible.
Feedback control can help to save the energy of the cell by shutting down an entire pathway. Yes, it costs a lot of energy to help to make these products. And if you have a ton of a product, what's the point of continuing this series of reactions? You already have enough, so it's good to shut it down until you need some more. So, yes, this would help to save the energy.
So, the only statement here that is not true about feedback control will be option c.
Is the following diagram a correct illustration of feedback control?
Yes
No