Glycogenin Overview

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Questions and Answers

What role does glycogenin play in glycogen metabolism?

It catalyzes the synthesis of glucose from pyruvate.

It is responsible for the breakdown of glycogen into glucose.

It acts as a regulator for glucose storage.

It serves as the initial acceptor of glucose residues for glycogen synthesis. (correct)

Which of the following accurately describes the attachment site for the first glucosyl unit in glycogenin?

The hydroxyl group of a specific tyrosine side chain. (correct)

The amine group of a lysine side chain.

The carboxyl group of a glutamic acid side chain.

The hydroxyl group of a specific serine side chain.

What type of linkage does glycogenin use to transfer the first few glucose molecules?

β-1,6 linkages

α-1,4 linkages (correct)

β-1,4 linkages

α-1,3 linkages

Where is glycogenin primarily located within the glycogen molecule?

At the center of the completed glycogen molecule. Signup and view all the answers

What type of reaction does UDP-glucose pyrophosphorylase catalyze?

A reversible reaction. Signup and view all the answers

Which statement best describes glycogenin's function after glycogen synthesis?

It stays associated with the completed glycogen molecule. Signup and view all the answers

What initiates the process of glycogen synthesis?

The transfer of the initial UDP-glucosyl unit to glycogenin. Signup and view all the answers

Glycogenin's function in glycogen synthesis primarily revolves around which of the following?

Serving as a template for glucose polymerization. Signup and view all the answers

What is the relationship between glycogenin and finished glycogen molecules?

Glycogenin remains associated with completed glycogen. Signup and view all the answers

What is the primary function of Glucosyl 4:6 Transferase in glycogen metabolism?

It forms α-1,6 linkages to create branches in glycogen. Signup and view all the answers

During glycogen synthesis, which of the following is required as a coenzyme for the branching enzyme?

Pyridoxal phosphate Signup and view all the answers

What is the significance of the nonreducing end of the glycogen chain in branching?

It can be elongated by glycogen synthase after branching. Signup and view all the answers

What does the branching enzyme provide in the context of glycogen structure?

It introduces additional nonreducing ends for rapid mobilization. Signup and view all the answers

How does the action of the branching enzyme affect glycogenolysis?

It increases the efficiency of glucose-1-phosphate release. Signup and view all the answers

What type of bond does the Glucosyl 4:6 Transferase form during the branching process?

α-1,6 linkage Signup and view all the answers

What limitation does glycogenolysis face due to the structure introduced by the branching enzyme?

It can only hydrolyze α-1,4 linkages until a branch point. Signup and view all the answers

What initiates the branching process in glycogen synthesis?

Formation of new α-1,6 linkages. Signup and view all the answers

Where do the cleaved glucosyl residues go after the action of the branching enzyme?

They extend new branches in the glycogen molecule. Signup and view all the answers

What is the primary component of glycogen metabolism?

Glucose Signup and view all the answers

Which edition of Harper's Illustrated Biochemistry is referenced?

31st Edition Signup and view all the answers

Which institution published the 5th edition of Biochemistry, Molecular Biology & Genetics?

Lippincott Williams & Wilkins Signup and view all the answers

Who are the authors of the 5th edition of Biochemistry, Molecular Biology & Genetics?

Swanson, Kim, and Glucksman Signup and view all the answers

What is an essential sugar involved in human metabolism that is derived from glycogen degradation?

Glucose Signup and view all the answers

Which of the following best describes the role of glycogen in the human body?

A short-term storage form of glucose Signup and view all the answers

What is the primary role of pyrophosphatase in glycogen metabolism?

It hydrolyzes inorganic pyrophosphate, making other reactions irreversible. Signup and view all the answers

What does phosphoglucomutase primarily convert in glycogen metabolism?

Glucose 1-phosphate to glucose 6-phosphate. Signup and view all the answers

During glycogen synthesis, what type of bond does glycogen synthase form?

α-1,4 glycosidic bond. Signup and view all the answers

What is the mechanism by which glycogen is degraded?

Phosphorolysis using a phosphate ion as a nucleophile. Signup and view all the answers

What happens to UDP after a new α-1,4 linkage is formed during glycogen synthesis?

It is released and recycled back to UTP. Signup and view all the answers

Which two enzymes are primarily involved in the degradation of glycogen?

Glycogen phosphorylase and the debranching enzyme. Signup and view all the answers

How does branching of glycogen contribute to its function?

It enhances the solubility and sites for synthesis and degradation. Signup and view all the answers

What is a key characteristic of how glycogen phosphorylase functions?

It utilizes a phosphate ion to break glycosidic bonds. Signup and view all the answers

Which of the following best describes the reaction UTP + ATP ↔ UTP + ADP?

The regeneration of UTP from ATP using a phosphate. Signup and view all the answers

What is the result of glycogen branching on metabolic processes?

Allows faster mobilization of glucose when needed. Signup and view all the answers

What is the primary genetic defect associated with Type VI Glycogen Storage Disease?

Genetic deficiency of liver phosphorylase Signup and view all the answers

Which of the following is NOT a common manifestation of Type VI Glycogen Storage Disease?

Extreme muscle wasting Signup and view all the answers

What typical liver condition is associated with Type VI Glycogen Storage Disease?

Hepatomegaly Signup and view all the answers

What is the average number of residues from which the new branch points are formed in glycogen?

7 to 11 residues Signup and view all the answers

Which enzyme activities are performed by the bifunctional debranching enzyme?

Glucosyl 4:4 transferase and α-(1,6)-glucosidase Signup and view all the answers

Which metabolic condition is the least likely to be observed in patients with Type VI Glycogen Storage Disease?

Hyperglycemia Signup and view all the answers

What primarily causes growth retardation in Type VI Glycogen Storage Disease?

Nutritional deficiencies from hypoglycemia Signup and view all the answers

What is a key step in glycogenolysis regarding the removal of branches in glycogen?

Debranching by a bifunctional protein Signup and view all the answers

What is the consequence of the partial deficiency of liver phosphorylase in Type VI Glycogen Storage Disease?

Impaired glycogen mobilization Signup and view all the answers

Which condition is least likely to directly result from Type VI Glycogen Storage Disease?

Severe acidosis Signup and view all the answers

Study Notes

Glycogenin

Serves as the protein anchor for glycogen.
Functions as a glucose residue acceptor in the absence of glycogen fragments.
Initial attachment site for glucosyl units is the hydroxyl group of a specific tyrosine side chain.
Catalyzes the transfer of the first few glucose molecules using α-1,4 linkages.
Remains associated with the completed glycogen molecule at its center.

Glycogen Synthesis Enzymes

UDP-Glucose Pyrophosphorylase: Reversible reaction for synthesizing UDP-glucose.
Pyrophosphatase: Hydrolyzes inorganic pyrophosphate, making glycogen synthesis irreversible.
Phosphoglucomutase: Converts glucose 6-phosphate to glucose 1-phosphate necessary for glycogen synthesis.
Glycogen Synthase: Transfers glucose from UDP-glucose to the growing glycogen chain, forming new glycosidic bonds.

Glycogen Degradation

Glycogen breakdown occurs via phosphorolysis, using a phosphate ion to break bonds.
Enzymes involved in degradation include glycogen phosphorylase and the debranching enzyme.
UDP released during the formation of new α-1,4 linkages is converted back to UTP by nucleoside diphosphate kinase.

Branch Formation

Glucosyl 4:6 Transferase: Enzyme responsible for creating branches by forming α-1,6 linkages.
Breaks an α-1,4 bond from 6-8 glucosyl residues and attaches them to a non-terminal glucosyl residue via an α-1,6 bond.
Resulting branches enhance the molecule’s synthesis sites and solubility, allowing continued elongation by glycogen synthase.

Type VI Glycogen Storage Disease (Hers' Disease)

Caused by a genetic deficiency of liver phosphorylase.
Patients exhibit partial deficiency in glycogen metabolism.
Manifestations:
- Hepatomegaly (enlarged liver).
- Moderate hypoglycemia.
- Mild acidosis.
- Growth retardation.

Branch Removal

Branches in glycogen are removed by a bifunctional protein, the debranching enzyme, possessing both glucosyl 4:4 transferase and α-1,6-glucosidase activities.
New branch points maintain a minimum of 4 residues, usually 7 to 11 residues from existing branches.

Key Notes on Glycogenolysis

Glycogen degradation is distinct from synthesis, utilizing a different set of enzymes.
Glycogen phosphorylase plays a vital role in the rate-limiting step during glycogenolysis.
Continues hydrolyzing α-1,4 linkages until approaching a branch point.

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Description

This quiz explores the role of glycogenin in glycogen storage. It discusses how glycogenin acts as a protein to which glycogen is attached and its function as an acceptor of glucose residues in the absence of glycogen fragments. Test your understanding of this essential protein in carbohydrate metabolism.