Glycogen Phosphorylase - Video Tutorials & Practice Problems
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Glycogen Phosphorylase
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In this video we're going to introduce the enzyme Glycogen Phosphorylase. So, Glycogen Phosphorylase is an enzyme that catalyzes Glycogen breakdown. And so if we take a look at our image down below, notice on the left hand side over here, we have this large molecule that is a glycogen molecule and recall from our previous lesson videos that glycogen is a polymer that consists of individual glucose monomers. And so you can see that this one diamond right here represents a glucose monomer and if we take a bunch of glucose molecules and we link them together in a chain like this, we can create our glycogen molecule. And notice that the glycogen molecule is not only a single chain, but it also has branches coming off of the main chain here. And so we'll be able to talk more about glycogen structure later in our course. But for now, we can see that this is the glycogen molecule and glycogen phosphorylase, which is, the enzyme that catalyzes the breakdown of glycogen, is right here. And so you can see it uses an inorganic phosphate here to break down our glycogen. And so notice that we have a shortened glycogen chain here because the glucose here at the end has been removed as, a glucose 1 phosphate. Now the reason that we're talking about this enzyme, glycogen phosphorylase, now now is because Glycogen Phosphorylase is actually a classic example of an enzyme whose activity is controlled by both covalent and allosteric regulation, making it a perfect opportunity for us to apply a lot of the concepts that we learned about in our previous lesson videos. And so, we'll be able to talk more about this covalent and allosteric regulation later in our course. But what's important to know about this enzyme Glycogen Phosphorylase is that it actually has 2 different subunits, each of which has a specific serine amino acid residue, specifically serine 14, that can be phosphorylated. And of course, phosphorylation, we know is a post translational modification, and it's a type of covalent regulation because there's covalent attachment of a phosphate group in phosphorylation and we'll be able to talk more about this phosphorylation and covalent regulation later in our course. Now, this Glycogen Phosphorylase enzyme, as we already have seen down below in our image, it uses Glycogen polymer as a substrate, and it catalyzes the removal of a single glucose monomer. And so you can see it uses glycogen as the substrate, and it catalyzes the removal of a single glucose monomer. Now this glycogen phosphorylase enzyme is primarily expressed in liver cells and in muscle tissue where glycogen breakdown is very critical and we'll be able to talk more about glycogen breakdown in the liver and in the muscle later in our course. Now what's important to know is that this glucose that is released by glycogen phosphorylase, through subsequent reactions, the released glucose can be used in cellular respiration to generate energy in the form of ATP. And of course, we know from our previous biology courses that cellular respiration is a very long process with many different reactions, and we'll be able to talk about cellular respiration later in our course but for now, what we can see is that this glucose that's released ultimately, it can be used in many, many, many different reactions to generate ATP, which is energy. And so ultimately, what we're saying is that glycogen phosphorylase, its activity can lead to energy for the cell, through releasing this, glucose. And so, now that we understand the fundamentals of this enzyme glycogen phosphorylase, in our next lesson video, we'll be able to introduce the 2 different isozymes of glycogen phosphorylase. So I'll see you guys in that video.
2
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Glycogen Phosphorylase
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4m
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So now that we know that glycogen phosphorylase breaks down glycogen, in this video we're going to introduce the 2 different isozymes of glycogen phosphorylase, which are liver glycogen phosphorylase or liver phosphorylase for short, and muscle glycogen phosphorylase or muscle phosphorylase for short. Now these two different isozymes are going to be catalytically and structurally similar to each other, however, they are going to be genetically and allosterically different from one another. And so, this allosteric, difference means that they're going to be regulated differently by different allosteric effectors. And so we'll be able to talk about the allosteric regulation of these 2 isozymes a little bit later in our course. But for now, if we take a look at our image down below, we can see the 2 different isozymes. On the left over here, what we have is the liver Glycogen Phosphorylase isosyme, And on the right, over here, we can see we have the muscle Glycogen Phosphorylase isosyme. And so, again, these are going to be catalytically similar or even catalytically identical isosyme because they both catalyze the same exact reaction that we talked about in our last lesson video of breaking down glycogen. And you can also see that they're structurally similar to each other because notice that they both have 2 subunits, as we can see here. And then, they both also have these, 2 serine amino acid residues that can be phosphorylated. And so you can see that the liver has phosphorylated serine residues and the muscle has, serine residues that are not phosphorylated. But we'll be able to talk more about this phosphorylation later in our course. But for now, what I want you guys to notice is that the liver and muscle phosphorylase isosymes, they're going to be regulated differently, through different allosteric regulation, due to their different biological roles of glycogen breakdown. Now, later in our course, we'll talk about the exact biological roles of glycogen breakdown in the liver and the role of glycogen breakdown in the muscle. But for now, what I want you guys to notice is that the liver glycogen phosphorylase isozyme over here on the left is usually going to be in an active form unless it's allosterically signaled to stop, being active. And, the muscle glycogen phosphorylase isozyme on the other hand over here on the right is usually inactive. So it's usually off unless it's allosterically signaled to turn on and make ATP for a muscle contraction. And so if we take a look down below, notice the liver glycogen phosphorylase and, isosyme, we're saying is usually active or on. And it's usually active or turned on, except when we eat a high carbohydrate meal. And we'll talk more about this exception a little bit later in our course. But for now, I want you to think liver glycogen phosphorylase is normally active or on, whereas glycogen phosphorylase over here is usually inactive or, turned off, except during a muscle contraction and again, we'll talk about this exception more as we move forward in our course. But for now, we have a better understanding of these 2 isosymes of glycogen phosphorylase. And in our next lesson video, we'll be able to talk more about the activity of liver, versus muscle glycogen phosphorylase isosyme. So, I'll see you guys in that video.
3
concept
Glycogen Phosphorylase
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Video transcript
So now that we've introduced both isozymes of Glycogen Phosphorylase, the liver Glycogen Phosphorylase isosyme, and the muscle Glycogen Phosphorylase isozyme. In this video, we're going to introduce the phosphorylase a and the phosphorylase b forms of both isozymes. And so what's important to know is that both the liver and the muscle glycogen phosphorylase isozymes have 2 different forms. The first is going to be the phosphorylase a form, and the second is going to be the phosphorylase b form. Now each of these forms, phosphorylase a and phosphorylase b, exist in equilibrium with their own r state and their own t state. And so if we take a look down below at our image, we can see exactly what we've described up above. So on the left, what we have is the liver glycogen phosphorylase isozyme, and on the right, what we have is the muscle glycogen phosphorylase isozyme. And notice that for both liver and muscle, we have the phosphorylase a form here at the top, which exists in equilibrium with its own r state and t state. And then on the bottom, what we have is the phosphorylase b form, which exists again in its own equilibrium with its own r state and its own t state. And, notice if we take a look at the muscle glycogen phosphorylase, the same exact thing applies where on the top what we have is phosphorylase a in equilibrium with its own r state and its own t state. And then we have phosphorylase b on the bottom, which exists in its own equilibrium with its own R state and its own T state. And so, what you'll notice is that the phosphorylase a and the phosphorylase b forms are actually interconvertible. Meaning that we can convert Phosphorylase A into Phosphorylase B and vice versa. And so really this interconversion occurs through the addition and removal of phosphate groups on the serine residues, the 2 serine residues. And so, phosphate group addition and removal is a form of covalent regulation. And so covalent regulation is what converts phosphorylase a into phosphorylase b and vice versa. And so if we take a look down below at our image, notice that the phosphorylase a form on top can be converted into phosphorylase b form on bottom through covalent regulation. And so you can see that covalent regulation, these up and down equilibrium arrows, will convert phosphorylase a into phosphorylase b. And notice that really the only difference between phosphorylase b and phosphorylase is, the fact that the serines are not phosphorylated in phosphorylase b, whereas the serines are phosphorylated in phosphorylase a. And so, the same applies over a. And so, the same applies over here with muscle, glycogen phosphorylase. Covalent regulation regulates the conversion of phosphorylase a into phosphorylase b and vice versa. Now, what I want you guys to notice is that phosphorylase a is going to have, again, phosphorylated serines just as we see up above at the top here. They're all phosphorylated, and that makes phosphorylase a catalytically more active. And so you can think a here is for active. And that's because the equilibrium between the t state and r state, in the phosphorylated form actually favors the r state. And so if we take a look down below at our phosphorylated a at the top up here, notice that the equilibrium between the r state and t state actually favors the r state, which is why we have this bottom arrow so much larger, than the reverse equilibrium arrow. And so with the phosphorylase a form, it's normally going to be in this R state. And the same applies over here with muscle isoson. The phosphorylase A form, the equilibrium here is going to favor the r state over here. And so, when we take a look at the phosphorylase B form on the other hand on the bottom, notice that it is unfosporrelated. So it's not phosphorylated as we can see down below. All of these serines here are not phosphorylated. And, also notice that this makes phosphorylase b in the catalytically less active form, and that's because the equilibrium between the t state and the r state in the phosphorylase b form favors the t state. And so if we focus on the bottom of our image down here in the b form, notice that the equilibrium between the r state and the t state, is favoring the t state. And so, that means that in Phosphorylated B form, it's this T state that will exist more often. And the same applies over here as well with the muscle isazyme. When it's in the Phosphorylase B form, the equilibrium favors the T state. And so, what you'll notice here in this image is that, one of the differences between the two sides is the covalent regulation. And so you'll notice that the covalent regulation in liver, favors phosphorylation. And so you can see that the equilibrium here, for covalent regulation is favoring, the upwards direction here. And so that means that normally in liver cells, the covalent regulation is going to be lead to phosphorylation and so the Phosphorylase phosphorylase a form is going to predominate in the liver. And so, this yellow background that we have here is really suggesting the form that it's normally in and so the liver, as we can see, if we follow these equilibrium arrows no matter which way we follow them using the, the larger arrow, it will always lead us back to this, form up here, the phosphorylase A R state. And so in the liver, it will exist in this form right here, and this form is almost like the on switch for the enzyme. So when it's in the phosphorylase a r state, it's in the most active, form of the enzyme. So it's pretty much like the switch is on for the enzyme. And then notice, down below, over here, the phosphorylated bt state is like the switch is off. And so normally in liver, it's on, but it can be allosterically regulated and covalently regulated to turn off. And if we take a look at the muscle over here, notice that, we have the yellow background behind the phosphorylated BT state and that's because in the muscle, it's normally off. And so, we can also follow each of these equilibrium arrows and notice that they will always lead to, this form down here, the phosphorylated BT state in the muscle. And so it's normally off. However, again, through allosteric allosteric and covalent regulation, we can turn it on when necessary. Example, during a muscle contraction. And we'll be able to talk more specifically about the, allosteric regulation of both the muscle and liver isozymes as we move forward in our course. But for now that concludes our introduction to the Phosphorylase A and B forms, and again, we'll be able to talk more about these forms as we move forward in our course. So, I'll see you guys in our next video.
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Problem
Problem
Glycogen phosphorylase is an enzyme involved in glycogen metabolism that's regulated by phosphorylation. Phosphorylation on serine residues results in more enzyme activity, while the dephosphorylated enzyme has little to no activity. What result would you expect on activity if the serine residues that served as phosphorylation sites on glycogen phosphorylase were mutated to aspartate residues?
A
No effect on activity, since aspartate residues can be phosphorylated similarly to serine residues.
B
The enzyme would be completely inactive if it has an Asp residue, since it will no longer recognize its substrate.
C
The enzyme likely has some activity, since Asp is negatively charged like a phosphoryl group, but activity would not be regulated by phosphorylation.
D
The enzyme would be mostly inactive, since the enzyme can no longer be phosphorylated.