Positive Feedback: Study with Video Lessons, Practice Problems & Examples

Positive feedback in metabolic pathways occurs when the final product stimulates an earlier step, enhancing production. This contrasts with negative feedback, where the product inhibits earlier steps. Allosteric enzymes can act as both activators and inhibitors, facilitating communication between pathways. For instance, a product can activate one enzyme while inhibiting another, ensuring efficient production of a final product. Understanding these interactions is crucial for grasping metabolic regulation and the dynamics of cellular processes.

Positive Feedback

So now that we've covered negative feedback, in this video, we're going to introduce the opposite type of feedback regulation, which is positive feedback. So with positive feedback, it turns out that the final product of a metabolic pathway, instead of inhibiting an earlier step, it actually activates or stimulates an earlier step in the same exact metabolic pathway. And so, again, with positive feedback, the final product is going to act as an activator to again further stimulate the production and the build up of the final product. And so in our example down below, notice we're saying that positive feedback really acts like the green light to activate or stimulate metabolic pathways. And so over here notice that we have this green light to remind you that positive feedback acts as the green light to allow metabolic pathways to proceed faster. And so, notice here we have our metabolic pathway, and again most of the enzymes in our metabolic pathway display Michaelis-Menten kinetics. However, we do have one enzyme here, enzyme 1, that is an allosteric enzyme and displays allosteric kinetics. And so, notice, here we have product F that's able to come back in this pathway, and it's able to stimulate or activate enzyme number 1. So that enzyme number 1 essentially increases its initial reaction rate, and it converts reactant A into intermediate B here, faster. And that ultimately leads to the increase in the concentration of product F. And so you can imagine a scenario where, product F, in a cell needs to be maintained at high levels in order for the cell to survive. And so in order for F to ensure that its concentration is maintained at high levels, F can act as a positive feedback regulator for enzyme number 1 to constantly stimulate it so that it gives this metabolic pathway the green light to continue to produce F so that F is maintained at those high concentrations required for survival. And again, this is our example of positive feedback. And so, this here concludes our introduction to positive feedback and in our next couple of videos, we'll be able to get some practice. So I'll see you guys there.

Positive Feedback

So now that we've covered both positive and negative feedback regulation, in this video we're going to introduce the idea of metabolic pathway communication. It's actually pretty common within cells for metabolic pathways to communicate or interact with each other through these feedback regulation mechanisms that we already covered in our previous lesson videos, including negative and positive feedback. The main purpose of this biological communication or interaction between metabolic pathways is to ensure efficient production of a single final product. What's important to note as we move forward with our lesson of metabolic pathway communication is that some allosteric enzymes can actually bind both allosteric activators and allosteric inhibitors. This means that some allosteric enzymes are capable of supporting both positive and negative feedback, and we'll be able to see some examples of that later in our image.

It's also important to note that the same molecule could potentially act as both an activator for one allosteric enzyme and an inhibitor for a different allosteric enzyme, and again, we're going to see examples of that in our image. Now, taking a look at our image down below, at first glance, it looks pretty darn complicated, but it's definitely not as complicated as it looks. If you take a closer look, notice that we're showing you only three different metabolic pathways. We have metabolic pathway number 1 here with this light blue background. Then, we have metabolic pathway number 2 here with this yellow background, and we have metabolic pathway number 3 over here with this pink background. What's important to note is that these three metabolic pathways can actually communicate with each other to again ensure efficient production of a single final product. That single final product in this image here is going to be product k, which is all the way over here on the right.

What's also important to note is that within a cell, there are actually hundreds or thousands of metabolic pathways, and here in this image, we're only showing you three metabolic pathways. It's possible for a lot of these metabolic pathways to communicate with each other in different ways. Here, we're only showing you one example of how these metabolic pathways could potentially communicate. What's really important to note is that the final product of pathway number 1 here is actually molecule f, and the final product of pathway number 2 is molecule i. Molecule i and molecule f must actually interact with each other for pathway 3 to proceed. Pathway 3 is the one to directly form the final product of interest, which is again product molecule k here.

What I want you guys to notice is that the final product of pathway number 1, notice that it can actually act as both an inhibitor to enzyme number 1, but it can also act as an activator to enzyme number 6 in a different pathway. This is what we mean by metabolic pathway communication because the product of pathway number 1 is capable of acting as an allosteric effector for an enzyme in a different pathway, that is what we mean by communication or interaction between these metabolic pathways. You can imagine a scenario where product f is at very high concentrations. If those concentrations are simply too high, product f can actually go back and act as an allosteric inhibitor to inhibit enzyme number 1 and therefore decrease the concentration of itself. Recall that the positive signs here are going to represent activators, whereas the negative signs are going to represent inhibitors. Product f, from pathway number 1, can act as an inhibitor for enzyme number 1. But again, imagine a scenario where the final product of pathway number 2 here, molecule i, is at low concentrations. In that scenario, f could act as an activator to enzyme 6 to help increase the concentration of molecule i.

There is also some communication between pathway number 2 with i, and pathway number 1 up here with enzyme number 1. Notice that molecule i here can actually act as an activator for enzyme number 1 to help increase the concentration of f. You can see how there is this communication between these different pathways. Notice also that pathway number 3, the final product, can act as an inhibitor for enzyme number 1 as well as an inhibitor for enzyme number 6. The concentration of k here can also regulate its own activity through negative feedback because it is inhibiting. Anytime you see a positive sign, that would represent a form of positive feedback through activation. Anytime you see a negative sign, that would represent negative feedback through inhibitors. This concludes our lesson on how metabolic pathways can actually communicate with one another. In our next couple of videos, we're going to get some practice interpreting these arrows, activators, and inhibitors and the mechanism of communication between the pathways. This concludes our lesson, and I'll see you guys in our next video.

Use the image below showing interactions between 3 metabolic pathways to answer the following questions.

A) Which of the following best describes the role of molecule 'F'?

a) At low concentrations, molecule F acts as an inhibitor on enzyme-1 & an activator on enzyme-6.
b) At high concentrations, molecule F acts as an activator on enzyme-1 & an inhibitor on enzyme-6.
c) At low concentrations, molecule F acts as an activator on enzyme-1 & an inhibitor on enzyme-6.
d) At high concentrations, molecule F acts as an inhibitor on enzyme-1 & an activator on enzyme-6.

B) Which of the following best describes the role of molecule 'I'?

a) At high concentrations, molecule I acts as an inhibitor on enzyme-6 & an activator on enzyme-1.
b) At low concentrations, molecule I acts as an activator on enzyme-6 & an inhibitor on enzyme-1.
c) At high concentrations, molecule I acts as an activator on enzyme-6 & an inhibitor on enzyme-1.
d) At low concentrations, molecule I acts as an inhibitor on enzyme-6 & an activator on enzyme-1.

C) Which of the following best describes the role of molecule 'K'?

a) At low concentrations, molecule K acts as an inhibitor on enzyme-1 & an activator on enzyme-6.
b) At high concentrations, molecule K acts as an inhibitor on enzyme-1 & an inhibitor on enzyme-6.
c) At low concentrations, molecule K acts as an activator on enzyme-1 & an inhibitor on enzyme-6.
d) At high concentrations, molecule K acts as an inhibitor on enzyme-1 & an activator on enzyme-6.

Alright. So here's a 3-part practice problem broken up into part a and parts b and c down below. The problem says to use the image below showing the interactions between the 3 metabolic pathways to answer the following questions. If you look down below, notice we have this image here showing the interactions with a pink background. Looking at part a, it's asking which of the following best describes the role of molecule F. We've got these 4 potential answer options down below. Molecule F is right here on our image, and notice that molecule F actually has 2 different roles. Molecule F, if we follow this pink dotted line over here, can act as an allosteric inhibitor to enzyme 1, and it can also act as an allosteric activator to enzyme 6. We know it's an inhibitor because of the minus sign and an activator because of the plus sign. Reviewing these four answer options, we need to realize that molecule F can only play these roles as an inhibitor to enzyme 1 and an activator to enzyme 6 when the concentration of F is high enough. We can eliminate anything that suggests roles at low concentrations, so we can eliminate option a and option c. It turns out that option d is correct at high concentrations of molecule F, acting as an inhibitor on enzyme 1 and an activator to enzyme 6. Option b is incorrect because it reverses the roles. Moving on to part b, it's asking about the role of molecule I. Molecule I is represented here in our image. Molecule I also has dual roles: it acts as an inhibitor on enzyme 6 and an activator on enzyme 1, as traced by the pink dotted line. We can eliminate any answer that suggests roles at low concentrations, so options b and d are dismissed. It's option a here that matches the roles that molecule I has. At high concentrations, molecule I acts as an inhibitor on enzyme 6 and an activator on enzyme 1. Option c has the reverse roles, inaccurately suggesting activator on enzyme 6, which is false. Moving to part c, it's asking about the role of molecule K. Molecule K is over here on the far right and also has dual roles, both as an inhibitor on enzyme 6 and enzyme 1. The correct answer that matches this description is option b. At high concentrations, molecule K acts as an inhibitor on both enzyme 1 and enzyme 6. The other options are incorrect as they suggest roles at low concentrations or inaccurately describe it as an activator on enzyme 6. Therefore, option b is the correct answer for part c. This concludes our 3-part practice problem, and I'll see you guys in the next video.

Here’s what students ask on this topic:

What is positive feedback in metabolic pathways?

Positive feedback in metabolic pathways occurs when the final product of a pathway stimulates an earlier step in the same pathway. This activation enhances the production of the final product, creating a cycle that amplifies the pathway's output. Unlike negative feedback, where the final product inhibits earlier steps to regulate production, positive feedback acts as a 'green light' to accelerate the pathway. This mechanism is crucial for maintaining high levels of certain products necessary for cellular functions and survival.

How does positive feedback differ from negative feedback in metabolic regulation?

Positive feedback and negative feedback are two types of regulatory mechanisms in metabolic pathways. Positive feedback occurs when the final product of a pathway stimulates an earlier step, enhancing the production of the final product. In contrast, negative feedback happens when the final product inhibits an earlier step, reducing the pathway's activity to prevent overproduction. Positive feedback amplifies the pathway's output, while negative feedback maintains homeostasis by balancing the production levels.

What role do allosteric enzymes play in positive feedback mechanisms?

Allosteric enzymes play a crucial role in positive feedback mechanisms by acting as activators. These enzymes can bind to allosteric sites, which are distinct from the active site, allowing them to regulate the enzyme's activity. In positive feedback, the final product of a metabolic pathway can bind to an allosteric enzyme, enhancing its activity and accelerating the pathway. This ensures the efficient production and maintenance of high levels of the final product necessary for cellular functions.

Can a molecule act as both an activator and an inhibitor in metabolic pathways?

Yes, a molecule can act as both an activator and an inhibitor in different metabolic pathways. This dual role is facilitated by allosteric enzymes, which can bind both activators and inhibitors. For example, a molecule might activate one enzyme in a pathway to enhance production while inhibiting another enzyme in a different pathway to regulate the overall metabolic balance. This ability to act as both an activator and an inhibitor allows for complex regulation and communication between metabolic pathways, ensuring efficient production of final products.

Why is understanding positive feedback important in studying metabolic regulation?

Understanding positive feedback is crucial in studying metabolic regulation because it provides insights into how cells maintain high levels of essential products. Positive feedback mechanisms amplify the production of final products by stimulating earlier steps in the pathway. This knowledge helps in comprehending how cells respond to various stimuli and maintain homeostasis. Additionally, it aids in understanding the dynamics of cellular processes and the intricate regulation of metabolic pathways, which is essential for developing therapeutic strategies for metabolic disorders.