Glycogen Phosphorylase - Online Tutor, Practice Problems & Exam Prep

Glycogen Phosphorylase is an enzyme that catalyzes the breakdown of glycogen into glucose 1-phosphate, crucial for energy production in liver and muscle cells. It exists in two isozymes: liver and muscle phosphorylase, each regulated differently by allosteric effectors. The enzyme has two forms, phosphorylase a (active) and phosphorylase b (inactive), interconverted through covalent regulation via phosphorylation. In the liver, phosphorylase a predominates, while in muscle, phosphorylase b is typically inactive until activated during muscle contraction, highlighting the enzyme's role in energy metabolism and regulation.

Glycogen Phosphorylase

In this video, we're going to introduce the enzyme Glycogen Phosphorylase. Glycogen Phosphorylase is an enzyme that catalyzes glycogen breakdown. If we take a look at our image below, notice on the left-hand side, we have this large molecule that is a glycogen molecule. Recall from our previous lesson videos that glycogen is a polymer consisting of individual glucose monomers. You can see that this one diamond represents a glucose monomer, and if we link a bunch of glucose molecules together in a chain like this, we create our glycogen molecule. Notice that the glycogen molecule is not only a single chain but also has branches coming off the main chain. 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, the enzyme that catalyzes the breakdown of glycogen, is right here. It uses an inorganic phosphate to break down our glycogen. Notice that we have a shortened glycogen chain here because the glucose at the end has been removed as glucose 1-phosphate.

The reason we're talking about this enzyme, glycogen phosphorylase, now is that Glycogen Phosphorylase is 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 concepts learned in our previous lesson videos. We'll be able to talk more about this covalent and allosteric regulation later in our course. What's important to know about Glycogen Phosphorylase is that it has two different subunits, each with a specific serine amino acid residue, specifically serine 14, that can be phosphorylated. Phosphorylation is a post-translational modification and a type of covalent regulation because it involves the covalent attachment of a phosphate group. We'll discuss this phosphorylation and covalent regulation more later in our course.

The Glycogen Phosphorylase enzyme, as we have already seen in our image, uses glycogen polymer as a substrate and catalyzes the removal of a single glucose monomer. This enzyme is primarily expressed in liver cells and in muscle tissue where glycogen breakdown is very critical, and we'll talk more about glycogen breakdown in the liver and muscle later in our course. What's important to know is that the glucose released by glycogen phosphorylase, through subsequent reactions, can be used in cellular respiration to generate energy in the form of ATP. We know from our previous biology courses that cellular respiration is a long process with many different reactions, and we'll be able to discuss cellular respiration further later in our course. For now, what we can see is that the glucose released can be used in many reactions to generate ATP, which is energy. Thus, glycogen phosphorylase's activity can lead to energy for the cell through releasing this glucose.

Now that we understand the fundamentals of this enzyme, Glycogen Phosphorylase, in our next lesson video, we'll be able to introduce the two different isozymes of glycogen phosphorylase. So, I'll see you guys in that video.

Glycogen Phosphorylase

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 isozyme, and on the right, over here, we can see we have the muscle Glycogen Phosphorylase isozyme. And so, again, these are going to be catalytically similar or even catalytically identical isozymes 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 isozymes, 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 isozyme, 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 isozymes 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 isozyme. So, I'll see you guys in that video.

Glycogen Phosphorylase

So now that we've introduced both isozymes of Glycogen Phosphorylase, the liver Glycogen Phosphorylase isozyme, 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 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 a, is that the serines are not phosphorylated in phosphorylase b, whereas the serines are phosphorylated in phosphorylase 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 serin...

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.

Here’s what students ask on this topic:

What is the function of Glycogen Phosphorylase in the body?

Glycogen Phosphorylase is an enzyme that catalyzes the breakdown of glycogen into glucose 1-phosphate. This process is crucial for energy production, especially in liver and muscle cells. In the liver, the glucose released can be used to maintain blood sugar levels, while in muscle cells, it provides energy for muscle contractions. The enzyme's activity is regulated by both covalent and allosteric mechanisms, ensuring that glycogen breakdown occurs in response to the body's energy needs.

What are the two isozymes of Glycogen Phosphorylase and how do they differ?

The two isozymes of Glycogen Phosphorylase are liver glycogen phosphorylase and muscle glycogen phosphorylase. While they are catalytically and structurally similar, they differ in their genetic makeup and allosteric regulation. Liver glycogen phosphorylase is usually active and helps maintain blood glucose levels, whereas muscle glycogen phosphorylase is typically inactive and becomes active during muscle contractions to provide energy. These differences reflect their distinct biological roles in energy metabolism.

How is Glycogen Phosphorylase regulated?

Glycogen Phosphorylase is regulated through both covalent and allosteric mechanisms. Covalent regulation involves the phosphorylation and dephosphorylation of serine residues, converting the enzyme between its active (phosphorylase a) and inactive (phosphorylase b) forms. Allosteric regulation involves the binding of effectors that either activate or inhibit the enzyme. For example, in the liver, glucose can act as an allosteric inhibitor, while in muscle, AMP can act as an activator during muscle contraction.

What is the difference between phosphorylase a and phosphorylase b forms of Glycogen Phosphorylase?

Phosphorylase a is the active form of Glycogen Phosphorylase, characterized by phosphorylated serine residues. It favors the R state, which is catalytically active. Phosphorylase b is the inactive form, with unphosphorylated serine residues, favoring the T state, which is less active. The interconversion between these forms is regulated by covalent modification, specifically the addition or removal of phosphate groups, allowing the enzyme to respond to the cell's energy needs.

Why is Glycogen Phosphorylase important in muscle cells?

In muscle cells, Glycogen Phosphorylase is crucial for providing energy during muscle contractions. The enzyme breaks down glycogen into glucose 1-phosphate, which can then enter glycolysis to produce ATP, the energy currency of the cell. This process is especially important during intense physical activity when the demand for ATP is high. The muscle isozyme of Glycogen Phosphorylase is typically inactive and becomes active in response to signals such as increased AMP levels, ensuring that energy is available when needed.