HMP Shunt and Uronic Acid Pathway Overview

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Questions and Answers

What is the primary role of glycogen in the liver?

Facilitates the absorption of dietary fats

Acts as a source of glucose for muscle contraction

Stores excess protein from the diet

Maintains blood glucose levels during fasting (correct)

Which enzyme initiates the conversion of glucose to glucose-6-phosphate in muscle cells?

Glycogen phosphorylase

Phosphoglucomutase

Glucokinase

Hexokinase (correct)

What type of bond links α-D glucose units in glycogen?

β1-4 glycosidic bonds

β1-6 glucosidic bonds

α1-4 and α1-6 glycosidic bonds (correct)

α1-2 glycosidic bonds

Which of the following substances can be converted into glucose units for liver glycogen synthesis?

Fructose Signup and view all the answers

During glycogenesis, what is formed from glucose-1-phosphate?

UDP-glucose Signup and view all the answers

What is the primary source of glucose units for muscle glycogen?

Blood glucose Signup and view all the answers

What is the role of glucagon in glycogen metabolism?

Stimulates glycogenolysis in the liver Signup and view all the answers

What is the function of glycogenin in glycogenesis?

Acts as a primer for glycogen synthesis Signup and view all the answers

What type of bond does glycogen synthase form between glucose units?

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

What is the process called that breaks down glycogen into glucose units?

Glycogenolysis Signup and view all the answers

Which enzyme acts on branches of glycogen containing more than 4 glucose units during glycogenolysis?

Phosphorylase enzyme Signup and view all the answers

What is the fate of glucose-6-phosphate in muscle cells?

Ends as glucose-6-phosphate which cannot leave the muscle Signup and view all the answers

Which form of glycogen synthase is active?

Glycogen synthase a Signup and view all the answers

What role does the branching enzyme play in glycogen synthesis?

It transfers glucose units to form branches Signup and view all the answers

What is the action of debranching enzyme during glycogenolysis?

It releases glucose from α 1-6 linkages Signup and view all the answers

What is the relationship between glycogenesis and glycogenolysis?

Stimuli for glycogenesis inhibit glycogenolysis Signup and view all the answers

What is produced during the oxidative phase of the HMP shunt?

NADPH+H+ and CO2 Signup and view all the answers

Which enzyme is the key regulator of the HMP shunt?

Glucose-6-phosphate dehydrogenase Signup and view all the answers

What is the primary function of NADPH+H+ in the HMP shunt?

Reduction reactions in the cell Signup and view all the answers

What is one of the products of the non-oxidative phase of the HMP shunt?

Glucose-6-P Signup and view all the answers

Which of the following is a product of the uronic acid pathway?

UDP-glucuronic acid Signup and view all the answers

Reduced glutathione is crucial for which function in erythrocytes?

Removal of hydrogen peroxide Signup and view all the answers

Which compound inhibits glucose-6-phosphate dehydrogenase?

NADPH+H+ Signup and view all the answers

In which cellular location does the HMP shunt primarily occur?

Cytoplasm Signup and view all the answers

Study Notes

HMP Shunt (Hexose Monophosphate Shunt)

Definition: An alternative pathway for glucose oxidation, where ATP isn't produced or used.
Location: Cytoplasm of many tissues.
Phases: Two phases—oxidative (irreversible) and non-oxidative (reversible).
Oxidative Phase: Converts 3 glucose molecules into 3 ribulose-5-phosphates, producing NADPH+H+ and CO2.
Non-oxidative Phase: Converts 3 ribulose-5-phosphates into 2 glucose-6-phosphates and 1 glyceraldehyde-3-phosphate.

Functions of HMP Shunt

Pentose Production: Creates pentoses (ribose-5-phosphate) for DNA, RNA, ATP, GTP synthesis, etc.
NADPH+H+ Production: Essential for fatty acid and steroid hormone synthesis, non-essential amino acid synthesis, and malate production from pyruvate.
Reduced Glutathione Maintenance: Crucial for maintaining RBC integrity by removing hydrogen peroxide (toxic compound).

Uronic Acid Pathway

Definition: An alternative pathway for glucose oxidation that converts glucose into glucuronic acid.
Location: Cytoplasm of many tissues.
Importance: Producing UDP-glucuronic acid used in:
- Glycosaminoglycan synthesis.
- Vitamin C (L-ascorbic acid) production (only in some animals).
Conjugation reactions: Used with substances like bilirubin for more easily excreted water-soluble compounds
Detoxification reactions: Used to make toxic compounds less toxic

Glycogen Metabolism

Definition: Glycogen is the animal storage form of carbohydrates.
Structure: Formed from α-D glucose units linked by α1-4 glucosidic bonds and α1-6 glucosidic bonds at branch points.
Storage: Primarily stored in the liver and muscles.

Liver Glycogen

Amount: Up to 6% of liver mass.
Function: Maintains blood glucose levels during fasting.
Depletion: Depletes after 12-18 hours of fasting.
Regulation: Glucagon stimulates glycogenolysis.

Muscle Glycogen

Amount: Rarely exceeds 1% of muscle mass.
Function: Acts as a glucose-6-phosphate source for muscle glycolysis.
Depletion: Depletes after prolonged, vigorous exercise.
Regulation: No effect from glucagon.

Glycogenesis

Definition: The synthesis of glycogen from glucose.
Location: Cytoplasm of liver and muscle cells.
Sources of glucose units for liver: Blood glucose, other hexoses (galactose and fructose), non-carbohydrate sources (e.g., lactate, glycerol) from gluconeogenesis.
Sources of glucose units for muscle: Blood glucose only.

Steps of Glycogenesis

Formation of UDP-glucose: Glucose is converted to UDP-glucose by various enzymes (Hexokinase in muscle; glucokinase in liver).
Formation of glycogen from UDP-G units: Glycogen is formed from UDP-G units. Requires glycogen primer (few glucose molecules linked together).
Formation of Branches: Branching enzyme adds α1-6 linkages to create branches for increased glycogen storage capacity.

Glycogenolysis

Definition: The breakdown of glycogen into glucose.
Location: Cytoplasm of liver and muscle cells.
Steps: Involves phosphorylase enzyme, glucan transferase enzyme, and debranching enzyme.

Phosphorylase enzyme:

Breaks down α1-4 glucosidic bonds by phosphorylysis, creating glucose-1-phosphate.

Glucan Transferase enzyme:

Relinks glucose for further degradation.

Debranching enzyme:

Removes branches to create linear structures; breaking α1-6 bonds, creating glucose.
- Note: Glucose-1-phosphate is converted to glucose-6-phosphate by phosphoglucomutase

Fate of Glucose-6-Phosphate

Liver: Contains glucose-6-phosphatase; converts glucose-6-phosphate to glucose for release into the blood.
Muscle: Lacks glucose-6-phosphatase; glycogenolysis ends with glucose-6-phosphate within muscle cells

Regulation of Glycogenesis and Glycogenolysis

Coordinated Regulation: Glycogenesis and glycogenolysis are regulated in opposite manners.
Key enzymes:
- Glycogen synthase: Active (dephosphorylated) or inactive (phosphorylated) forms.
- Phosphorylase: Active (phosphorylated) or inactive (dephosphorylated) forms

Regulation in Fasting

Blood glucose levels decrease.
Epinephrine (muscle) and glucagon (liver) trigger cAMP production.
cAMP activates protein Kinase.
Inactivation of glycogen synthase and activation of phosphorylase for glycogenolysis.

Regulation after a Meal

Blood glucose levels increase, stimulating insulin secretion.
Insulin stimulates glycogenesis and inhibits glycogenolysis.
cAMP breakdown by phosphodiesterase.
Deactivation of phosphorylase and activation of glycogen synthase.

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Description

Explore the HMP Shunt and Uronic Acid Pathway, two critical glucose oxidation pathways. This quiz covers their definitions, phases, functions, and significance in cellular metabolism. Test your knowledge on how these pathways contribute to important biomolecule synthesis.