Lecture 19b

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Questions and Answers

Which one of the following statements is correct regarding the regulation of carbohydrate metabolism?

Glycolysis and gluconeogenesis share 7 enzymes but have 3 different bypass reactions. (correct)

Glycolysis and gluconeogenesis are completely separate pathways with no shared enzymes.

Glycolysis and gluconeogenesis have no reciprocal regulation.

Glycolysis and gluconeogenesis are completely independent pathways with no overlap.

What is the primary purpose of the reciprocal regulation between glycolysis and gluconeogenesis?

To prevent the unnecessary expenditure of energy by running both pathways simultaneously. (correct)

To ensure that the pathways share as many enzymes as possible to conserve resources.

To allow the cell to switch between energy production and glucose synthesis as needed.

To ensure that the pathways are completely independent.

How many of the steps in glycolysis and gluconeogenesis are catalyzed by the same enzymes?

7 (correct)

Which enzyme is involved in the conversion of fructose 1,6-bisphosphate to fructose 6-P in gluconeogenesis?

FBPase-1 Signup and view all the answers

Which hexokinase isozyme is specific to the liver and responsive to higher glucose levels?

HK IV (Glucokinase) Signup and view all the answers

What is the role of hexokinase IV (Glucokinase) in glycolysis?

Converts glucose to glucose-6-phosphate in the liver Signup and view all the answers

What is a characteristic feature of hexokinase IV (Glucokinase) compared to other hexokinase isozymes?

Higher Km value Signup and view all the answers

What is the role of Glycogen Synthase Kinase 3 (GSK3) in the regulation of glycogen synthase activity?

GSK3 phosphorylates glycogen synthase a, the active form, converting it to the inactive glycogen synthase b form. Signup and view all the answers

How does insulin trigger the activation of glycogen synthase b?

Insulin blocks the activity of GSK3 and activates a phosphoprotein phosphatase (PP1), leading to the activation of glycogen synthase b. Signup and view all the answers

How do epinephrine (in muscle) and glucagon (in liver) affect the activity of glycogen synthase?

Epinephrine and glucagon activate PKA, which phosphorylates the glycogen-targeting protein GM, causing dissociation of PP1 from glycogen and inactivation of glycogen synthase. Signup and view all the answers

How does glucose 6-phosphate affect the activity of glycogen synthase?

Glucose 6-phosphate favors the dephosphorylation of glycogen synthase by binding to it and promoting a conformation that is a good substrate for PP1, leading to the activation of glycogen synthase. Signup and view all the answers

How do allosteric and hormonal signals coordinate carbohydrate metabolism globally in the liver?

Allosteric and hormonal signals coordinate carbohydrate metabolism in the liver by triggering a cascade of causal relationships between different metabolic events Signup and view all the answers

What is the key difference in the control of carbohydrate metabolism between the liver and other tissues?

The liver has a more complex and coordinated system of allosteric and hormonal signals for the control of carbohydrate metabolism, as shown in the diagram, compared to other tissues. Signup and view all the answers

What is the primary role of insulin in regulating glycolysis and gluconeogenesis?

Insulin increases fructose-2,6-bisphosphate (F2,6BP) levels, activating PFK-1 and inhibiting FBPase-1. Signup and view all the answers

Which of the following statements accurately describes the regulation of pyruvate kinase (PK) in glycolysis?

In the liver, PK is inactivated by phosphorylation in response to glucagon signaling. Signup and view all the answers

What is the feed-forward mechanism that activates pyruvate kinase (PK) during high glycolytic flux?

Fructose-1,6-bisphosphate accumulation Signup and view all the answers

Which of the following molecules acts as an allosteric inhibitor of pyruvate kinase (PK), indicating an abundant energy supply?

Acetyl-CoA Signup and view all the answers

In which tissue(s) is the liver isozyme (L form) of pyruvate kinase (PK) regulated by glucagon?

Liver only Signup and view all the answers

What is the physiological significance of alanine inhibiting pyruvate kinase (PK) activity?

It promotes the conversion of pyruvate to alanine for amino acid synthesis. Signup and view all the answers

Which of the following statements is correct regarding the role of insulin in the regulation of glycogen breakdown in the liver?

Insulin indirectly stimulates PP1 activity, leading to the conversion of phosphorylase a to phosphorylase b, slowing glycogen breakdown. Signup and view all the answers

Which of the following statements accurately describes the role of glucagon and epinephrine in the regulation of glycogen phosphorylase?

Glucagon and epinephrine stimulate the conversion of phosphorylase b to phosphorylase a, promoting glycogen breakdown. Signup and view all the answers

Which of the following statements accurately describes the role of glucose as a regulator of glycogen phosphorylase in the liver?

Glucose induces a conformational change in phosphorylase a that exposes the phosphorylated serine residues for dephosphorylation by PP1. Signup and view all the answers

Which of the following statements accurately describes the role of ATP and G6P in the regulation of glycogen phosphorylase?

ATP and G6P inhibit phosphorylase a, stabilizing the less active phosphorylase b form and slowing glycogen breakdown. Signup and view all the answers

Study Notes

Regulation of Carbohydrate Metabolism

Reciprocal regulation between glycolysis and gluconeogenesis ensures that one pathway is active while the other is suppressed, optimizing energy use.
Glycolysis and gluconeogenesis share only a few enzymes, meaning most steps are regulated independently.
Fructose 1,6-bisphosphate converts to fructose 6-phosphate in gluconeogenesis by the enzyme fructose-1,6-bisphosphatase.
Hexokinase IV, also known as glucokinase, is liver-specific and active at higher glucose concentrations.
Hexokinase IV has a lower affinity for glucose compared to other hexokinases, allowing it to act primarily when glucose is abundant.

Glycogen Metabolism

Glycogen Synthase Kinase 3 (GSK3) inhibits glycogen synthase, reducing glycogen synthesis in response to energy needs.
Insulin activates glycogen synthase b by promoting dephosphorylation, enabling glycogen storage.
Epinephrine in muscle and glucagon in the liver decrease glycogen synthase activity, promoting glucose release.
Glucose 6-phosphate enhances glycogen synthase activity by acting as an allosteric regulator.

Coordination of Metabolism

Allosteric and hormonal signals globally coordinate carbohydrate metabolism in the liver, allowing for responses to changes in energy demand.
Liver metabolism differs from other tissues in that it can both store and release glucose, adjusting responses based on blood glucose levels.

Insulin and Carbohydrate Regulation

Insulin plays a primary role in stimulating glycolysis and inhibiting gluconeogenesis, facilitating energy production and storage.
Pyruvate kinase (PK) regulation is sensitive to energy status, with key enzymes controlling its activity.
High glycolytic flux activates pyruvate kinase through a feed-forward mechanism, utilizing substrates from previous steps.
ATP acts as an allosteric inhibitor of pyruvate kinase, signaling a sufficient energy supply.

Specific Enzyme Regulation

The liver isozyme (L form) of pyruvate kinase is regulated by glucagon, impacting energy metabolism in fasting states.
Alanine inhibits pyruvate kinase, indicating that protein breakdown is occurring and energy needs are being met.
Insulin suppresses glycogen breakdown in the liver, promoting energy storage.
Glucagon and epinephrine promote glycogen phosphorylase activity, enhancing glucose mobilization.
Glucose acts as a regulator of glycogen phosphorylase in the liver by signaling sufficient glucose availability.
ATP and glucose 6-phosphate inhibit glycogen phosphorylase, reflecting energy status and storage signals.

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