Glycogen Storage Diseases and Glycogenesis

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Questions and Answers

Which enzyme catalyzes the conversion of glucose to Glucose-6-Phosphate in muscle?

Hexokinase (correct)

Glucokinase

UDPGlc pyrophosphorylase

Phosphoglucomutase

What is the main function of UDPGlc pyrophosphorylase?

Catalyzing pyrophosphate hydrolysis

Catalyzing glucose phosphorylation

Catalyzing glucose isomerization

Catalyzing glycogen synthesis (correct)

Which organ mainly utilizes glucokinase for the conversion of glucose to Glucose-6-Phosphate?

Brain

Liver (correct)

Kidney

Muscle

What is the immediate precursor of UDPGlc in glycogenesis?

Glucose-1-Phosphate Signup and view all the answers

Which process involves the transfer of a phosphate group from the sixth carbon to the first carbon of the glucose molecule?

Isomerization to Glucose-1-Phosphate Signup and view all the answers

What is the final product formed by the catalytic action of UDPGlc pyrophosphorylase?

Uridine diphosphate glucose (UDPGlc) Signup and view all the answers

What is the main enzyme that catalyzes the rate-limiting step in glycogenolysis?

Glycogen phosphorylase Signup and view all the answers

What is the purpose of the debranching enzyme in glycogenolysis?

To transfer a trisaccharide unit from one branch to another, exposing the 1,6 branch point Signup and view all the answers

What is the role of phosphoglucomutase in glycogenolysis?

It catalyzes the conversion of glucose 1-phosphate to glucose-6-phosphate, and the reverse reaction is also possible Signup and view all the answers

Which of the following statements about the differences between glycogenolysis and glycogenesis is correct?

Glycogenolysis and glycogenesis are separate pathways that use different sets of enzymes Signup and view all the answers

What is the role of glucose-6-phosphatase in glycogenolysis in the liver?

It catalyzes the hydrolysis of glucose-6-phosphate to yield free glucose, which is then exported to increase blood glucose concentration Signup and view all the answers

What is the final outcome of the combined action of phosphorylase and the other enzymes involved in glycogenolysis?

The complete breakdown of glycogen to yield free glucose Signup and view all the answers

What is the role of 5' AMP in muscle phosphorylase?

It activates the inactive form (phosphorylase b) of the enzyme Signup and view all the answers

What is the primary signal that leads to increased cAMP formation in liver cells?

Glucagon secretion Signup and view all the answers

Which of the following is an allosteric inhibitor of active phosphorylase A in both liver and muscle tissues?

ATP Signup and view all the answers

What is the role of cAMP in the activation of glycogen phosphorylase?

It activates phosphorylase kinase a, which then activates phosphorylase b Signup and view all the answers

What is the primary signal that leads to increased cAMP formation in muscle cells?

Norepinephrine secretion Signup and view all the answers

Which of the following is an allosteric inhibitor of active phosphorylase A in liver, but not in muscle?

Free glucose Signup and view all the answers

What enzyme catalyzes the formation of a glycoside bond between C-1 of the glucose of UDPGlc and C-4 of a terminal glucose residue of glycogen?

Glycogen synthase Signup and view all the answers

At which end of the glycogen molecule does the addition of a glucose residue to a preexisting glycogen chain occur?

Nonreducing, outer end Signup and view all the answers

What is the minimum length of a growing glycogen chain before the branching enzyme can transfer a part of the 1 4-chain to a neighboring chain to form a 1 6 linkage?

11 glucose residues Signup and view all the answers

What is the role of insulin in the regulation of glycogen metabolism?

Insulin decreases the level of cAMP only after it has been raised by glucagon or epinephrine Signup and view all the answers

Which hormone is active in heart muscle but not in skeletal muscle?

Glucagon Signup and view all the answers

What is the relationship between the enzymes glucan transferase and debranching enzyme?

Glucan transferase and debranching enzyme appear to be two separate activities of the same enzyme Signup and view all the answers

How is phosphorylase A inactivated?

By protein phosphatase-1 Signup and view all the answers

What inhibits protein phosphatase-1?

Inhibitor-1 phosphorylated by cAMP-dependent protein kinase Signup and view all the answers

How does insulin reinforce the inactivation of phosphorylase b?

By increasing glucose uptake Signup and view all the answers

What happens to glycogen synthase a to make it active?

It gets dephosphorylated Signup and view all the answers

How many different protein kinases act on glycogen synthase?

Six Signup and view all the answers

What type of hormonal action allows the inhibition of glycogen synthesis along with the activation of glycogenolysis?

cAMP-mediated hormonal action Signup and view all the answers

Study Notes

Enzymatic Conversion and Function

Hexokinase catalyzes the conversion of glucose to glucose-6-phosphate in muscle.
UDP-glucose pyrophosphorylase's main function is to convert glucose-1-phosphate and UTP into UDP-glucose and pyrophosphate.
Glucokinase predominantly facilitates the conversion of glucose to glucose-6-phosphate in the liver.

Glycogenesis and Glycogenolysis

UDP-glucose serves as the immediate precursor in glycogenesis.
The transfer of a phosphate group from the sixth carbon to the first carbon of glucose involves the action of phosphoglucomutase.
UDP-glucose pyrophosphorylase's final product is UDP-glucose.
Glycogen phosphorylase is the primary enzyme that catalyzes the rate-limiting step in glycogenolysis.

Enzymatic Roles and Regulation

The debranching enzyme in glycogenolysis facilitates the removal of branches in glycogen, allowing for further breakdown.
Phosphoglucomutase aids in the conversion of glucose-1-phosphate to glucose-6-phosphate during glycogenolysis.
Key differences exist between glycogenolysis and glycogenesis, including their regulatory mechanisms and hormonal influences.
Glucose-6-phosphatase functions in the liver to convert glucose-6-phosphate to free glucose, allowing release into the bloodstream.

Signaling and Allosteric Regulation

The combined action of phosphorylase and other enzymes in glycogenolysis results in the breakdown of glycogen into glucose-1-phosphate.
AMP acts as an allosteric activator of phosphorylase in muscle, promoting glycogenolysis.
Increased cAMP formation in liver cells is primarily driven by hormones such as glucagon and epinephrine.
An allosteric inhibitor of active phosphorylase A in both liver and muscle tissues is glucose-6-phosphate.
cAMP serves as a secondary messenger that activates protein kinase A, which phosphorylates and activates glycogen phosphorylase.
Increased cAMP formation in muscle cells is also stimulated by epinephrine.

Structural Dynamics and Enzymatic Interactions

An allosteric inhibitor of active phosphorylase A in the liver, but not in muscle, is also glucose-6-phosphate.
Glycosyltransferase catalyzes the formation of a glycosidic bond between C-1 of glucose in UDP-glucose and C-4 of a terminal glucose in glycogen.
Glucose residues are added to the non-reducing end of the glycogen molecule.

Glycogen Chain and Hormonal Regulation

A minimum length of four glucose units is required for branching enzyme to transfer part of the 1-4 chain to create a 1-6 linkage.
Insulin plays a crucial role in regulating glycogen metabolism by promoting glycogenesis and inhibiting glycogenolysis.
Protein kinase A is active in heart muscle but not in skeletal muscle.
Glucan transferase and the debranching enzyme work together, with the former facilitating the transfer and the latter handling the cleavage of 1-6 bonds.
Phosphorylase A is inactivated through dephosphorylation by protein phosphatase-1.
Protein phosphatase-1 is inhibited by various factors like insulin signaling.
Insulin enhances the inactivation of phosphorylase b by promoting dephosphorylation.
Glycogen synthase becomes active when it is dephosphorylated.
Multiple protein kinases act on glycogen synthase, contributing to its regulation by phosphorylation.
Hormonal actions involving glucagon and epinephrine inhibit glycogen synthesis while stimulating glycogenolysis.

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Description

Learn about glycogen storage diseases, conditions caused by deficient mobilization or abnormal deposition of glycogen, leading to liver damage and muscle weakness. Explore the process of glycogenesis, involving UDP-Glucose and the conversion of glucose to Glucose-6-Phosphate (G6P) in muscle and liver.