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.
Enzyme Regulation: Covalent Modification - Online Tutor, Practice Problems & Exam Prep
Covalent Modification/Zymogens Concept 1
Video transcript
Zymogens Example 1
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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.
Problem Transcript
Phosphorylation/Dephosphorylation Concept 2
Video transcript
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
Video transcript
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.
Problem Transcript
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