Glycogen Metabolism and Blood Glucose Regulation

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Questions and Answers

What is the product of the reaction catalyzed by nucleoside diphosphate kinase?

UTP (correct)

UDP

ADP

ATP

What is the average distance between branches in glycogen?

Eight glucosyl residues (correct)

Twelve glucosyl residues

Four glucosyl residues

Sixteen glucosyl residues

What is the result of branching in glycogen?

A decrease in the number of nonreducing ends

A linear, unbranched molecule

No effect on the number of nonreducing ends

An increase in the number of nonreducing ends (correct)

What is the function of the branching enzyme?

To break α(1→4) bonds and form α(1→6) linkages Signup and view all the answers

What is the primary product of glycogen degradation in liver and skeletal muscle?

Glucose 1-phosphate Signup and view all the answers

How are additional branches synthesized in glycogen?

By removing terminal glucosyl residues and attaching them to non-terminal residues Signup and view all the answers

What type of linkage is formed by the branching enzyme?

α(1→6) linkage Signup and view all the answers

What is the fate of free glucose in glycogen degradation?

It is released from each α(1→6)-linked glucosyl residue Signup and view all the answers

What percentage of glycogen is continuously degraded by the lysosomal enzyme, α(1→4)-glucosidase?

1-3% Signup and view all the answers

What is the purpose of the lysosomal degradation of glycogen?

Unknown. Signup and view all the answers

What is the result of a deficiency of the lysosomal enzyme, α(1→4)-glucosidase?

Accumulation of glycogen in vacuoles in the lysosomes. Signup and view all the answers

What is the result of the binding of glucagon or epinephrine to their specific hepatocyte GPCR?

Activation of cAMP-dependent protein kinase A (PKA) Signup and view all the answers

What type of disease is caused by the deficiency of the lysosomal enzyme, α(1→4)-glucosidase?

Glycogen storage disease Type II: Pompe disease. Signup and view all the answers

What is the effect of cAMP on PKA?

It binds to the regulatory subunit, releasing individual catalytic subunits Signup and view all the answers

What is the effect of PKA on phosphorylase kinase?

It phosphorylates the inactive 'b' form of phosphorylase kinase, producing the active 'a' form Signup and view all the answers

When does glycogenolysis accelerate in the liver?

During periods of fasting. Signup and view all the answers

What is the primary function of glycogen synthesis and degradation in the liver?

To maintain blood glucose levels. Signup and view all the answers

What is the effect of active phosphorylase kinase on glycogen phosphorylase?

It phosphorylates the inactive 'b' form of glycogen phosphorylase, producing the active 'a' form Signup and view all the answers

What percentage of the fresh weight of a well-fed adult liver is made up of glycogen?

10% Signup and view all the answers

What is the effect of insulin on cAMP levels?

It activates the phosphodiesterase that degrades cAMP Signup and view all the answers

How are glycogen synthesis and degradation regulated in the liver?

Through both hormonal regulation and allosteric control. Signup and view all the answers

What is the result of the binding of hormones, such as glucagon or epinephrine, to plasma membrane G protein-coupled receptors?

Activation of glycogen degradation. Signup and view all the answers

What is the effect of protein phosphatase-1 on phosphorylase a?

It dephosphorylates phosphorylase a, re-converting it to phosphorylase b Signup and view all the answers

What is the primary source of carbons for the gluconeogenic pathway when glycogen stores are depleted?

Amino acids from the body’s proteins Signup and view all the answers

Why is the body's reliance on dietary glucose alone not sufficient for maintaining blood glucose levels?

Dietary glucose is not always a reliable source due to sporadic intake Signup and view all the answers

What is the role of inhibitor proteins in regulating glycogen degradation?

They bind to and inhibit protein phosphatase-1 Signup and view all the answers

What is the primary function of muscle glycogen?

To serve as a fuel reserve for the synthesis of ATP during muscle contraction Signup and view all the answers

What is the net result of the cascade of reactions involving PKA, phosphorylase kinase, and glycogen phosphorylase?

Glycogenolysis Signup and view all the answers

What is the approximate amount of glycogen that makes up the fresh weight of resting muscle?

400 g Signup and view all the answers

What is the primary source of glucose for the brain during fasting or starvation?

Glycogen degradation Signup and view all the answers

What is the primary function of liver glycogen?

To maintain blood glucose concentration Signup and view all the answers

What is the reason for the body's development of mechanisms for storing glucose in a rapidly mobilizable form, such as glycogen?

To ensure a constant supply of glucose for the brain Signup and view all the answers

What is the product of glycogen phosphorylase activity?

Glucose 1-phosphate Signup and view all the answers

What is the resulting structure after glycogen phosphorylase activity?

Limit dextrin Signup and view all the answers

What is the enzyme responsible for removing branches in glycogen degradation?

Debranching enzyme Signup and view all the answers

What is the function of the debranching enzyme?

Breaks α(1→4) bonds and forms α(1→4) bonds Signup and view all the answers

What is the enzyme responsible for converting glucose 1-phosphate to glucose 6-phosphate?

Phosphoglucomutase Signup and view all the answers

What is the intermediate formed during the conversion of glucose 1-phosphate to glucose 6-phosphate?

Glucose 1,6-bisphosphate Signup and view all the answers

What is the enzyme responsible for dephosphorylating glucose 6-phosphate in the liver?

Glucose 6-phosphatase Signup and view all the answers

Why can't glucose 6-phosphate be dephosphorylated in muscle?

Lack of glucose 6-phosphatase Signup and view all the answers

Study Notes

Importance of Glycogen

A constant source of blood glucose is essential for human life, particularly for the brain and exercising muscle.
Blood glucose can be obtained from three primary sources: dietary intake, degradation of glycogen, and gluconeogenesis.

Structure and Function of Glycogen

Glycogen is a rapidly mobilizable form of glucose storage.
Muscle glycogen serves as a fuel reserve for the synthesis of adenosine triphosphate (ATP) during muscle contraction.
Liver glycogen maintains blood glucose concentration, particularly during the early stages of fasting.

Amounts of Liver and Muscle Glycogen

400 g of glycogen make up 1–2% of the fresh weight of resting muscle.
100 g of glycogen make up to 10% of the fresh weight of a well-fed adult liver.

Formation of Branches in Glycogen

Branching increases the number of nonreducing ends to which new glucosyl residues can be added, accelerating glycogen synthesis and increasing the size of the molecule.
Branches are made by the action of the branching enzyme, amylo-α(1→4) → α(1→6)-transglucosidase.

Synthesis of Additional Branches

After elongation, terminal six to eight glucosyl residues can be removed and used to make additional branches.

Degradation of Glycogen (Glycogenolysis)

Glycogen phosphorylase sequentially cleaves α(1→4) glycosidic bonds, producing glucose 1-phosphate.
Branches are removed by the debranching enzyme, α(1→4) bond is broken, and a new α(1→4) bond is made.

Shortening of Chains

Glycogen phosphorylase cleaves α(1→4) glycosidic bonds until four glucosyl units remain on each chain before a branch point, resulting in a limit dextrin.

Removal of Branches

The debranching enzyme breaks an α(1→4) bond and makes a new α(1→4) bond, and then hydrolytically removes the remaining single glucose residue.

Conversion of Glucose 1-Phosphate to Glucose 6-Phosphate

Glucose 1-phosphate is converted to glucose 6-phosphate by phosphoglucomutase in the cytosol.
In the liver, glucose 6-phosphate is transported into the endoplasmic reticulum, where it is converted to glucose by glucose 6-phosphatase.

Lysosomal Degradation of Glycogen

A small amount (1–3%) of glycogen is continuously degraded by the lysosomal enzyme, α(1→4)-glucosidase (acid maltase).

Regulation of Glycogen Synthesis and Degradation

Regulation is accomplished on two levels: hormonal regulation to meet the needs of the body as a whole, and allosteric control to meet the needs of a particular tissue.

Activation of Glycogen Degradation by cAMP-Directed Pathway

Hormones such as glucagon or epinephrine bind to G protein-coupled receptors, signaling the need for glycogen to be degraded.
cAMP-dependent protein kinase A (PKA) is activated, phosphorylating and activating enzymes of glycogen metabolism.

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Description

This quiz covers the importance of maintaining a constant source of blood glucose in the human body and the three primary sources of blood glucose, including dietary intake, glycogen degradation, and gluconeogenesis.