Glucose and Glycogen Regulation Practice 1 - Online Tutor, Practice Problems & Exam Prep

The most sensitive indicator of a cell's energetic status is AMP, which fluctuates significantly compared to ADP and ATP. Glycogen phosphorylase breaks down glycogen by cleaving alpha-1,4 bonds, releasing glucose as glucose-1 phosphate, which is converted to glucose-6 phosphate. It is allosterically inhibited by glucose. Phosphofructokinase-2, which produces fructose-2,6-bisphosphate to activate glycolysis, is inhibited by ATP, highlighting feedback inhibition in metabolic pathways. Understanding these regulatory mechanisms is crucial for grasping cellular metabolism and energy production.

Glucose and Glycogen Regulation Practice 1

The most sensitive indicator of the energetic status of a cell is AMP. Remember, AMP can fluctuate very greatly in concentration in the cell and therefore, it's a really good indicator of the energetic status of a cell. Whereas, ADP and ATP don't fluctuate that greatly by comparison. So, AMP is the most sensitive indicator of the energetic status of the cell.

Now, the enzyme glycogen phosphorylase is the enzyme that breaks down glycogen, and it does so by catalyzing the cleavage of alpha-1,4 bonds. Remember that it breaks off glucose as glucose-1 phosphate. Also recall that it needs the help of debranching enzyme, which is going to pick off those branch point glucoses. Now, the one last thing I want to note is it's a phosphorylase, so it is going to be cutting a phosphate group, and that's why the glucose comes off as glucose-1-phosphate, which of course has to be converted to glucose-6-phosphate.

Glycogen branching enzyme catalyzes the formation of alpha-1,6 bonds, right? Those are our branch point bonds, and it does this by transferring about 6 to 10 glucose subunits onto the 6th position of a glucose on another chain. Glycogenin is the primer for glycogen synthesis, right? It's the core of glycogen, and the initial sugars are attached to tyrosine residues on the glycogenin.

Now, glycogen phosphorylase can be allosterically inhibited by glucose. Remember that the active form of glycogen phosphorylase, phosphoylase A, can be allosterically inhibited by glucose. Now, remember that the active form of glycogen phosphorylase is the phosphorylated form and it has 2 phosphate groups attached. Glucose causes those phosphate groups to actually become exposed on the enzyme, allowing for another enzyme to very easily come in and clip them off and thereby deactivate glycogen phosphorylase. So, glucose is an allosteric inhibitor of glycogen phosphorylase.

Now, phosphofructokinase 2 is inhibited by ATP. And for this one, I hope that you had to reason it out. I hope you looked back in your notes and you didn't see anything in there that was intentional. This one, I wanted you to have to reason your way through and the way I was hoping that you would do it is think, okay, phosphofructokinase. And it does that by creating fructose 2,6-bisphosphate which activates PFK1 and this does glycolysis, right? This keeps glycolysis going and glycolysis produces ATP, right? Directly produces ATP and also indirectly produces a lot of ATP via the citric acid cycle and electron transport. So, you know, I want you to be thinking about this logic like what is the most likely inhibitor or what's the most likely negative feedback mechanism for a particular enzyme? So I hope that you saw this and you thought, geez, which one of these, right, which one of this list is a downstream product of phosphofructokinase or ultimately of, you know, what phosphofructokinase contributes to, and the answer is ATP. All right. Let's turn the page.

Here’s what students ask on this topic:

What is the role of AMP in cellular energy regulation?

AMP (adenosine monophosphate) is a crucial indicator of a cell's energetic status. Unlike ADP and ATP, AMP levels fluctuate significantly in response to changes in energy demand. When a cell's energy levels are low, AMP concentrations rise, signaling the need for increased ATP production. This makes AMP a sensitive marker for energy status. AMP activates AMP-activated protein kinase (AMPK), which then initiates pathways to generate ATP, such as glycolysis and fatty acid oxidation, while inhibiting energy-consuming processes. Understanding AMP's role helps in grasping how cells maintain energy balance.

How does glycogen phosphorylase function in glycogen breakdown?

Glycogen phosphorylase is an enzyme that catalyzes the breakdown of glycogen by cleaving α-1,4 glycosidic bonds. This process releases glucose-1-phosphate, which is then converted to glucose-6-phosphate for use in glycolysis or other metabolic pathways. The enzyme requires the assistance of a debranching enzyme to remove branch point glucoses. Glycogen phosphorylase is regulated allosterically; it is activated by phosphorylation and inhibited by glucose, which exposes its phosphate groups for removal, thereby deactivating the enzyme.

What is the function of phosphofructokinase-2 (PFK-2) in glycolysis?

Phosphofructokinase-2 (PFK-2) plays a regulatory role in glycolysis by producing fructose-2,6-bisphosphate (F-2,6-BP). F-2,6-BP is a potent activator of phosphofructokinase-1 (PFK-1), the enzyme that catalyzes a key step in glycolysis. By increasing the levels of F-2,6-BP, PFK-2 enhances the glycolytic flux, promoting ATP production. PFK-2 itself is inhibited by ATP, which serves as a feedback mechanism to prevent excessive ATP production when energy levels are already sufficient.

How does glucose act as an allosteric inhibitor of glycogen phosphorylase?

Glucose acts as an allosteric inhibitor of glycogen phosphorylase by binding to the enzyme and causing a conformational change that exposes its phosphate groups. This exposure allows another enzyme to easily remove the phosphate groups, thereby deactivating glycogen phosphorylase. This inhibition prevents further glycogen breakdown when glucose levels are high, helping to regulate blood sugar levels and maintain energy homeostasis in the cell.

What is the significance of glycogenin in glycogen synthesis?

Glycogenin is a protein that serves as the primer for glycogen synthesis. It initiates the formation of glycogen by attaching the first few glucose molecules to its tyrosine residues. This initial chain of glucose units provides a starting point for glycogen synthase to extend the glycogen molecule. Glycogenin's role is crucial because it ensures that glycogen synthesis can begin, allowing cells to store glucose for future energy needs.