Enzyme Regulation: Covalent Modification: Study with Video Lessons, Practice Problems & Examples

Enzymes, primarily proteins, can be activated or deactivated through processes like zymogen activation and phosphorylation. Zymogens are inactive enzyme precursors that become active upon hydrolysis, removing an extra polypeptide segment. Phosphorylation involves adding a phosphate group via kinases, activating the enzyme, while dephosphorylation, facilitated by phosphatases, can either activate or deactivate enzymes depending on their specific requirements. Understanding these mechanisms is crucial for grasping enzyme regulation and metabolic pathways.

Covalent Modification/Zymogens Concept 1

Now remember, enzymes can be represented as protein molecules. And we're going to say enzyme activity can be regulated by the addition or removal of groups on the enzyme polypeptide chain. Now here we have two types. We have what are called zymogens. Zymogens are what we call proenzymes. And then we have what's called phosphorylation and dephosphorylation. With zymogens, we're going to say, zymogens are enzymes produced in inactive forms with an extra polypeptide segment. Now they're activated by cleavage. Remember, just cutting off of the extra polypeptide segment by hydrolysis. So if we take a look here, in this first image, we have our enzyme. And we can see that a vast majority of this enzyme is purple. And we also have this grayed out portion here. This is the extra part of the polypeptide segment, and this would represent an inactive enzyme. Through hydrolysis, we're able to cut or cleave this gray portion of my polypeptide chain, so now it's gone. By removing it, we've just activated my enzyme. So this will represent my active enzyme. Right? So just remember, when we're talking about addition or removal, in this case, we're talking about the removal of a portion of this polypeptide segment that transforms our inactive enzyme into now an active enzyme.

Zymogens Example 1

Here it says, which of the following statements is not relevant to zymogens? Remember, zymogens represent inactive enzymes that have an extra polypeptide segment that needs to be cut or cleaved off in order to activate the enzyme. So, zymogens are inactive because the excess polypeptide chain alters its overall structure. This is true. This is definitely relevant to a zymogen. Enzyme activity by loss of some part of the peptide chain is a type of covalent modification. That is true. And this also directly relates to zymogens. They exist in an inactive form because they have that extra segment to their polypeptide sequence or chain. Cutting it off helps to make them active. All of this is a type of covalent modification. Now, trypsinogen is converted into active trypsin by hydrolysis of a hexapeptide segment from its backbone. Now here, this is showing us an example of a zymogen going from its inactive form to its active form. Here, trypsinogen is the inactive form of my enzyme. It's the zymogen. Through hydrolysis, we're able to cut off a portion of its polypeptide chain and give us the active enzyme in the form of trypsin. So this is giving us an exact example of a zymogen. Citric can attach to a non-active site, the phosphofructokinase enzyme, and decrease its activity. Remember, when we're talking about zymogens, we're talking about cleaving or cutting off a portion of the polypeptide chain in order to go from an inactive form to an active form. This isn't even talking about cutting off; it's talking about attaching. This is not what we're talking about when referring to zymogens. So here, this is not related to zymogens. So here, our final answer would be option d. We can see that options a through c are talking about what we expect from a zymogen: in inactive form, having a portion of it cleaved or cut off, changes it from an inactive form to an active form. Option d doesn't do that. So option d would be the one that's not related to zymogens.

Match the terms (a) allosteric control, (b) feedback control, and (c) zymogen activation with each of the following:

_______ Pepsinogen is converted into its active form (pepsin) by losing 44 amino acids from its primary structure.

_______ A small molecule attaches to the enzyme and makes an active site available to a substrate.

_______ The end-product of a metabolic pathway decreases the activity of the enzyme in the first step.

_______ Alanine binds to pyruvate kinase and reduces active site availability for the enzyme's substrate.

In this practice question, it says, match the terms. A, allosteric control, B, feedback control, and C, zymogen activation with each of the following. Alright. So here it says, Pepsinogen is converted into its active form Pepsin by losing 44 amino acids from its primary structure. This would be an example of a specific zymogen going from its inactive form to its active form. Remember, with zymogen activation, we have the inactive form of our enzyme and a portion of it needs to be cut or cleaved off in order to activate the enzyme. So this would be C. Next, a small molecule attaches to the enzyme and makes an active site available to a substrate. Alright. So this would be an example of positive allosteric regulation, where a regulator or effector attaches to the allosteric site of an enzyme and opens up an active site for an incoming substrate. This would help to increase the overall rate of the reaction. So, this would be A. Next, the end product of a metabolic pathway decreases the activity of the enzyme in the first step. This would be an example of feedback control. The end product that you make is going to come around and attach to E1, our initial enzyme. It's going to attach to its allosteric site and basically take out the active site, so that no incoming substrate can attach. You have changed an active enzyme to an inactive enzyme to help lower the amount of product being formed. And because you're basically taking away the active site, you're decreasing the activity of that enzyme. So, this would be again an example of feedback control. Then finally, alanine binds to Pyruvate Kinase and reduces the active site availability for the enzyme's substrate. Alright. So, we're attaching in order to decrease the enzyme's active site so that the substrate and it cannot interact. This would be an example of allosteric control. We know that it can't be zymogen activation because we're binding, we're not cutting anything off. It's not feedback control because the product being formed is not coming back around and attaching to the allosteric site. So here, this would have to be allosteric control and it would have to be negative allosteric control because we are decreasing the amount of active site, which would decrease the overall rate and activity of my enzyme. So here we'd say it'd be C A B A as the letters for each one of these statements.

Phosphorylation/Dephosphorylation Concept 2

In this video, we'll take a look at Phosphorylation versus Dephosphorylation. Now, phosphorylation and dephosphorylation deal with the addition or removal of phosphate groups to the enzyme polypeptide chain, and that alters the enzyme's activity. So if we take a look here at phosphorylation, that means that we're adding a phosphate group. Here we have the inactive form of our enzyme. We'd use a kinase as the enzyme to do this. The kinase would help to attach a phosphate group to my enzyme. Here, we'd show the phosphate group being represented by the letter P. By attaching that phosphate group to this enzyme, we are activating it. So in this case, we do phosphorylation in order to go from an inactive enzyme to an active enzyme. The opposite of phosphorylation is dephosphorylation. Here we have an inactive enzyme in this case. In this case, it's inactive because it has a phosphate group attached. Some enzymes work when a phosphate group is attached; others only work once a phosphate group is removed. In order to remove this phosphate group through dephosphorylation, we use what's called a phosphatase. So phosphatase is the enzyme of choice to remove a phosphate group. So using it on this inactive enzyme removes the phosphate group entirely, and we go from an inactive enzyme to an active enzyme. So just remember, adding a phosphate group can help to activate an enzyme. In other cases, other enzymes work once we remove their phosphate groups. Kinases are the enzyme of choice for adding a phosphate group. Phosphatase is the enzyme to help us remove phosphate groups. Right? So this is what the phosphorylation and dephosphorylation are in essence.

Phosphorylation/Dephosphorylation Example 2

In this example question, it asks, which of the following is true about covalent modification? Alright. So here, we're going to say a zymogen loses its activity when a peptide segment is removed from its primary structure. Remember, zymogen activation happens when we do remove a segment from the polypeptide chain. This is saying the exact opposite. It doesn't lose it, it gains. Next, additional removal of a phosphate group has no effect on enzyme activity. This is not true. Some enzymes are activated when we add a phosphate group, other enzymes are activated once we remove a phosphate group. So they do have an effect on the enzyme's overall activity. A zymogene is activated by the removal of hydroxyl groups from its polypeptide backbone. So we say that a segment of our polypeptide chain has to be removed, so we're talking about the amino acids. That's what has to be removed in order to facilitate zymogen activation. It's not just the hydroxyl groups that are removed. So this would not be true. That leaves the last one. Phosphatase removes a phosphate group. Yes. That's true. From an enzyme by breaking a covalent bond. In this case, yes. The phosphate group that is attached usually to the use of a kinase. In order to remove it, we use a phosphatase enzyme. This does result in the cleaving of the connection between the enzyme and phosphate group, and that involves breaking a covalent bond. So here, this would be true about this particular covalent modification, which has to do with the removal of a phosphate group from an enzyme in order to activate it.

Match the terms (a) allosteric control, (b) feedback control, (c) zymogen activation, and (d) phosphorylation/dephosphorylation with each of the following:

_______ Proline inhibits glutamate 5-kinase, the enzyme in the first step of the biosynthesis of proline from glutamate. 

_______ Glycogen synthase loses its catalytic activity when it is phosphorylated.

_______ Proelastase is converted to its active form elastase when it loses some part of its polypeptide backbone.

_______ Adenosine monophosphate binds to phosphofructokinase-1 and increases its activity.

Here in this practice question it says, match the terms. So a is allosteric control, b is feedback control, c is zymogen activation, and d is phosphorylation/dephosphorylation with each of the following. Right. So here it says, proline inhibits Glutamate 5 Kinase, the enzyme in the first step of the biosynthesis of proline from glutamate. Alright. So here, they're talking about proline inhibiting this particular enzyme. And they're saying the first step. So that means we're dealing with a biochemical pathway. We're dealing with a series of steps to help create our end product. We're going to say, Proline here is working under feedback control. So, the option here is b. Next, glycogen synthase loses its catalytic activity when it's phosphorylated. So it's kind of giving it away already. Phosphorylated means that we're adding a phosphate group. This is a type of covalent modification. We're adding a phosphate group, so this is phosphorylation, so d. Here, we're going to say, proelastase is converted to its active form elastase when it loses some part of its polypeptide backbone. Alright. So, proelastase is converted to the active form, Elastase. Proelastase is the zymogen, the inactive form. Through hydrolysis, we cut off a segment of its polypeptide chain to create its active form, which would be elastase. This is an example of zymogen activation. So this would be c. And then finally, we have adenosine monophosphate binds to phosphofructokinase-1 and increases its activity. Alright. So they're talking about adenosine monophosphate binding to this enzyme and the activity goes up. So let's see. Feedback control has to do with decreasing our overall activity of the enzyme by our end product attaching to the allosteric site, thereby affecting the active site so substrate can no longer attach. So that wouldn't work. Zymogen activation deals with cutting off or cleaving a portion of my polypeptide chain, activating an enzyme. That's not what's going on here. Phosphorylation/dephosphorylation, we're not talking about here adding a phosphate group, it doesn't say that. So we're going to say d wouldn't make the most sense. This here is an example of allosteric control. We're talking about a regulator binding to this enzyme here, and thereby opening up an active site. This will help to increase the overall activity of our enzyme. So this would be a. So the choices would be b, d, c, a when it comes to identifying each of the statements. So b, the first one would be feedback control, the second one would be phosphorylation because they say phosphorylation in it. C would be an example of zymogen activation. And then a would be an example of allosteric control in terms of positive allosteric regulation. Alright. So these will be our answers for each of these statements.

Here’s what students ask on this topic:

What are zymogens and how are they activated?

Zymogens, also known as proenzymes, are inactive precursors of enzymes. They contain an extra polypeptide segment that keeps them inactive. Activation occurs through hydrolysis, which involves the cleavage of this extra segment. This process transforms the zymogen into its active enzyme form. For example, in the digestive system, the zymogen pepsinogen is converted into the active enzyme pepsin by the removal of its extra polypeptide segment. This mechanism ensures that enzymes are activated only when and where they are needed, preventing potential damage to cells and tissues.

How does phosphorylation regulate enzyme activity?

Phosphorylation regulates enzyme activity by adding a phosphate group to the enzyme's polypeptide chain. This process is facilitated by enzymes called kinases. The addition of the phosphate group can either activate or deactivate the enzyme, depending on its specific function. For instance, in some enzymes, phosphorylation activates them, allowing them to catalyze reactions. In others, it may deactivate them. This reversible modification is crucial for controlling various cellular processes, including metabolism, cell division, and signal transduction.

What role do kinases and phosphatases play in enzyme regulation?

Kinases and phosphatases are essential for the regulation of enzyme activity through phosphorylation and dephosphorylation. Kinases are enzymes that add phosphate groups to other proteins, often activating them. Phosphatases, on the other hand, remove these phosphate groups, which can either activate or deactivate the enzyme depending on its specific requirements. This dynamic interplay between kinases and phosphatases allows cells to finely tune enzyme activity in response to various signals and conditions, ensuring proper cellular function and homeostasis.

What is the difference between phosphorylation and dephosphorylation?

Phosphorylation and dephosphorylation are processes that regulate enzyme activity by adding or removing phosphate groups, respectively. Phosphorylation involves the addition of a phosphate group to an enzyme, typically by a kinase, which can activate or deactivate the enzyme. Dephosphorylation, facilitated by phosphatases, involves the removal of a phosphate group. This removal can also either activate or deactivate the enzyme, depending on its specific function. These reversible modifications are crucial for controlling various cellular processes and ensuring proper metabolic regulation.

How does the removal of a polypeptide segment activate zymogens?

The removal of a polypeptide segment activates zymogens by converting them from their inactive form to their active form. Zymogens contain an extra polypeptide segment that inhibits their enzymatic activity. Through hydrolysis, this segment is cleaved off, resulting in a conformational change that activates the enzyme. This mechanism ensures that the enzyme is only active when needed, preventing potential damage to cells and tissues. For example, the zymogen trypsinogen is activated to trypsin by the removal of its extra segment, enabling it to digest proteins in the small intestine.