Biochem 11.4

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Questions and Answers

What is the primary reason organisms store glucose as glycogen?

To prepare glucose for gluconeogenesis.

To enhance osmoregulation in the cell.

To provide cells with quick access to large amounts of glucose. (correct)

To increase blood glucose levels quickly.

What is the first step in the process of glycogenesis?

Activation of G6P to glycogen.

Phosphorylation of glucose to glucose 6-phosphate. (correct)

Elongation of glycogen polymers.

Formation of UDP-glucose.

Which enzyme is responsible for converting glucose to glucose 6-phosphate during glycogenesis?

Glycogen synthase.

Amylase.

Hexokinase. (correct)

Phosphorylase.

What molecule must glucose 6-phosphate be activated to in order to participate in glycogenesis?

UDP-glucose. Signup and view all the answers

Which of the following conditions stimulates the process of glycogenolysis?

High glucagon levels. Signup and view all the answers

During which metabolic process is glycogen synthesized?

Glycogenesis. Signup and view all the answers

What is the role of UDP-glucose in glycogenesis?

It is the glucose derivative that reacts to elongate glycogen. Signup and view all the answers

What happens to glucose 6-phosphate after it is produced in glycogenesis?

It may proceed down several metabolic pathways. Signup and view all the answers

What is the primary role of phosphoglucomutase in glycogen synthesis?

To move the phosphate group from carbon 6 to carbon 1 Signup and view all the answers

What must happen to glucose 6-phosphate to activate it for glycogen synthesis?

The phosphate group must be moved to carbon 1 Signup and view all the answers

What type of bond does the activated glucose form with the glycogen molecule?

Glycosidic bond Signup and view all the answers

Under what conditions is the conversion of glucose 6-phosphate to glucose 1-phosphate considered reversible?

Under physiological conditions Signup and view all the answers

What group must be transferred onto the phosphate during glucose activation?

A uridylyl group Signup and view all the answers

Which carbon of glucose 6-phosphate is phosphorylated to facilitate glycogen synthesis?

Carbon 1 Signup and view all the answers

What type of carbon does the activated glucose utilize in glycogen synthesis?

Anomeric carbon Signup and view all the answers

During the process of glycogen synthesis, what initiates the formation of a glycosidic bond?

The activation of glucose 6-phosphate Signup and view all the answers

What is the effect of transferring a phosphate group from carbon 6 to carbon 1 in glucose 6-phosphate?

It activates glucose for further processing Signup and view all the answers

What is the significance of the reducing end in glucose during glycogen synthesis?

It is where the first bond to glycogen forms Signup and view all the answers

What drives the activation of glucose 6-phosphate in the context of glycogen synthesis?

The movement of a phosphate group and transfer of a uridylyl group Signup and view all the answers

Which step in glycogen synthesis occurs after glucose 6-phosphate is activated?

Formation of UDP-glucose Signup and view all the answers

What is the primary function of the anomeric carbon in activated glucose during glycogen synthesis?

To participate in glycosidic bonding Signup and view all the answers

How does the action of phosphoglucomutase affect glucose metabolism?

It enables reversible conversion of glucose 6-phosphate Signup and view all the answers

Study Notes

Glycogen Metabolism Introduction

Glucose is the primary fuel source for cells, acquired from the diet.
Organisms need to maintain a glucose supply between meals.
Gluconeogenesis produces glucose but may not be fast enough in certain situations.
Glycogen is a storage form of glucose, allowing quick access without disrupting osmotic balance.

Glycogenesis

Glycogenesis involves the synthesis and elongation of glycogen molecules.
Glucose is first phosphorylated to glucose-6-phosphate (G6P).
G6P is converted to glucose-1-phosphate through phosphoglucomutase.
UDP-glucose is formed from glucose-1-phosphate and UTP.
Glycogen synthase adds UDP-glucose to an existing glycogen chain.
A-1,4-glycosidic bonds are formed.

Glycogen Elongation

Glycogen synthase catalyzes the transfer of glucose from UDP-glucose to the non-reducing end of a growing glycogen chain.
This forms a new a-1,4-glycosidic bond resulting in the release of UDP.

Glycogen Priming

Glycogen synthase can only elongate pre-existing glycogen chains.
Glycogenin, a protein, acts as a core for the initial glucose addition.
Glycogenin catalyzes the addition of the first few glucose units (using UDP-glucose) to its own tyrosine residues
These initial units act as a primer for glycogen synthase to continue elongation.

Glycogen Branching

Glycogen is highly branched.
Glycogen branching enzyme transfers an oligosaccharide from the non-reducing end of a linear chain to carbon 6 of a more internal glucose unit.
This creates a branch point (a-1,6 linkage) increasing the number of nonreducing ends.

Glycogenolysis

Glycogenolysis breaks down glycogen into glucose subunits.
Phosphorolysis (rather than hydrolysis) is primarily used (splitting with phosphate).
Glycogen phosphorylase removes glucose-1-phosphate from the non-reducing end of a glycogen chain.
a-1,4-glycosidic bonds are broken.

Debranching of Glycogen

Debranching enzyme is required because glycogen phosphorylase cannot cleave alpha-1,6-glycosidic linkages.
Debranching enzyme transfers a trisaccharide from a branch to the main chain.
a-1,6-glucosidase hydrolyzes the remaining branch-point glucose residue to release a free glucose molecule.

Regulation of Glycogen Metabolism

Glycogen phosphorylase, the key regulatory enzyme in glycogenolysis, is regulated by hormonal and allosteric mechanisms.
Glucagon and epinephrine promote glycogenolysis (through hormonal pathways leading to PKA activation followed by phosphorylase kinase activation leading to further activation of phosphorylase)
Insulin promotes glycogenesis.
Allosteric effectors like AMP and calcium and Ca2+ are tissue specific and regulate glycogenolysis.

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