Enzyme Regulation: Allosteric Control - Online Tutor, Practice Problems & Exam Prep

Enzyme regulation allows cells to control enzyme activity through mechanisms like allosteric control, feedback control, and covalent modification. Allosteric enzymes possess an active site for substrates and an allosteric site for regulators. Positive regulators enhance reaction rates by opening active sites, while negative regulators decrease rates by closing them. Understanding these mechanisms is crucial for grasping metabolic pathways and enzyme kinetics, as they influence activation energy and reaction rates in biochemical processes.

Allosteric Control Concept 1

Enzyme regulation is a mechanism cells use to turn on or off enzymes as needed. We have three types which include allosteric control, feedback control, and covalent modification. Now, let's take a look at allosteric control. With allosteric control, it's achieved by allosteric enzymes that have two types of binding sites. The active site, which is known as the spot for our substrate to attach to the enzyme. Besides that, we have what's called an allosteric site, designated for what we call the regulator. The regulator, also known as our effector, binds to the allosteric site and it either opens or closes an active site.

Let's examine these two images. In both of them, we have our substrate, which we abbreviate as S. In the first image, we have this triangular green block, and it's attaching to this portion here. When it attaches there, it opens up and creates an active site on the enzyme for our substrate to attach. This type of regulator is called a positive regulator as it helps to increase the rate of reaction by making an active site available to the substrate. This first image represents a positive allosteric regulator. By attaching to the allosteric site, it helps to open up an active site elsewhere on the enzyme for the substrate to then attach to.

In the second image, we see that the regulator attaches to the allosteric site. Before this, there is an active site available for our substrate. However, after the regulator has attached, the active site disappears, eliminating a place for the substrate to attach. This represents a negative allosteric regulator. A negative regulator decreases the rate of reaction by making an active site unavailable to the substrate.

So, remember, we have our active site where the substrate wants to attach, but then we have our regulator or effector that can attach to the allosteric site somewhere else on the enzyme. This can either help to free up or open an active site or close it. This mechanism can help to either increase the rate of reaction by opening up an active site or decrease the rate of reaction by closing an active site. Keep in mind that this describes allosteric control.

Allosteric Control Example 1

Which of the following statements is incorrect about allosteric enzymes?

A. The activity of an allosteric enzyme can be controlled by a regulatory molecule. This is true. Remember, the regulatory molecule or effector molecule can either help to increase or decrease the rate of the reaction by affecting the active site of the enzyme. So this is true.

Allosteric enzymes have 2 types of binding sites. So this is also true. Remember, there are 2 ways in which we can affect the overall binding sites. We can have positive or negative.

The binding of an allosteric regulator to an enzyme can change the availability of an active site. Yes. A positive regulator can increase the rate of reaction by opening up an active site, whereas a negative one can close up an already present active site as soon as it binds to the allosteric site.

The overall shape of an allosteric enzyme always remains the same. No. Once a regulatory molecule or effector molecule attaches to the allosteric site, this can open up or close an active site. This would cause a change in the overall shape of the enzyme. So here, option d is a statement that's incorrect about allosteric enzymes.

Isoleucine can attach to the enzyme threonine deaminase and can decrease its activity. Isoleucine can be classified as:

• A. Positive allosteric regulator
• B. Negative allosteric regulator

Here’s what students ask on this topic:

What is allosteric control in enzyme regulation?

Allosteric control is a mechanism of enzyme regulation where an enzyme's activity is modulated by the binding of a regulator molecule at a site other than the active site, known as the allosteric site. This binding can either enhance or inhibit the enzyme's activity. Positive regulators, or activators, bind to the allosteric site and induce a conformational change that opens the active site, increasing the enzyme's activity. Conversely, negative regulators, or inhibitors, bind to the allosteric site and cause a conformational change that closes the active site, decreasing the enzyme's activity. This type of regulation is crucial for controlling metabolic pathways and ensuring that enzymes function optimally under varying cellular conditions.

How do positive and negative allosteric regulators affect enzyme activity?

Positive allosteric regulators, or activators, bind to the allosteric site of an enzyme and induce a conformational change that opens the active site, making it available for substrate binding. This increases the rate of the enzymatic reaction. On the other hand, negative allosteric regulators, or inhibitors, bind to the allosteric site and cause a conformational change that closes the active site, preventing substrate binding and thereby decreasing the rate of the enzymatic reaction. These regulatory mechanisms allow cells to finely tune enzyme activity in response to changing metabolic needs.

What is the difference between the active site and the allosteric site of an enzyme?

The active site of an enzyme is the region where the substrate binds and the catalytic reaction occurs. It is specifically shaped to fit the substrate, facilitating the conversion of the substrate into products. The allosteric site, on the other hand, is a separate region on the enzyme where regulatory molecules, known as effectors or regulators, bind. Binding at the allosteric site induces a conformational change in the enzyme that can either enhance or inhibit the enzyme's activity by altering the availability of the active site for substrate binding.

Why is allosteric control important in metabolic pathways?

Allosteric control is crucial in metabolic pathways because it allows for the fine-tuning of enzyme activity in response to cellular needs. By using allosteric regulators, cells can quickly adjust the rates of enzymatic reactions, ensuring that metabolic processes are efficient and balanced. This regulation helps maintain homeostasis, prevents the accumulation of intermediates, and ensures that energy and resources are used optimally. Allosteric control also allows for the integration of multiple signals, enabling coordinated regulation of complex metabolic networks.

Can you provide an example of an enzyme that is regulated allosterically?

An example of an enzyme regulated allosterically is phosphofructokinase-1 (PFK-1), a key enzyme in glycolysis. PFK-1 is allosterically activated by AMP and ADP, which indicate low energy levels in the cell, and inhibited by ATP and citrate, which signal high energy levels. When AMP or ADP binds to the allosteric site of PFK-1, the enzyme's active site becomes more accessible to its substrate, fructose-6-phosphate, thereby increasing the rate of glycolysis. Conversely, when ATP or citrate binds to the allosteric site, the active site is less accessible, decreasing the rate of glycolysis.