HMP Shunt Overview

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Questions and Answers

What is the main end product of the oxidative phase of the HMP shunt?

Ribulose-5-P (correct)

Glucose-6-P

NADPH+H+ (correct)

Glyceraldehyde-3-P

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

To promote glycolysis

To convert glucose to pyruvate

To produce ATP

To synthesize fatty acids and steroid hormones (correct)

Which enzyme is key in the regulation of the HMP shunt?

Glucose-6-phosphate dehydrogenase (correct)

NADP+ synthase

Glutathione reductase

Malic enzyme

In the non-oxidative phase of the HMP shunt, how many molecules of glucose-6-P are generated?

2 Signup and view all the answers

What toxic compound is reduced by the action of reduced glutathione in red blood cells?

Hydrogen peroxide Signup and view all the answers

Glucuronic acid is synthesized primarily for which purpose?

Synthesis of glycosaminoglycans Signup and view all the answers

What effect does insulin have on the HMP shunt?

Stimulates the shunt Signup and view all the answers

Which of the following is NOT a product of the HMP shunt?

Glucose Signup and view all the answers

What is the primary function of glycogen synthase in glycogenesis?

To catalyze the formation of α1-4 glucosidic bonds Signup and view all the answers

During glycogenolysis, which enzyme acts on branches with more than 4 glucose units?

Phosphorylase enzyme Signup and view all the answers

What does glucan transferase do in the process of glycogenolysis?

It transfers 3 glucose units to another branch Signup and view all the answers

What is the conversion process that occurs with glucose-1-phosphate in the body?

It is converted to glucose-6-phosphate by phosphoglucomutase Signup and view all the answers

In which organ does glycogenolysis ultimately release glucose into the bloodstream?

Liver Signup and view all the answers

Which form of phosphorylase is considered the active form?

Phosphorylated form Signup and view all the answers

During fasting, which enzymes would be primarily active?

Phosphorylase and debranching enzyme Signup and view all the answers

What key regulatory feature differentiates the two forms of glycogen synthase?

Their activity based on phosphorylation status Signup and view all the answers

What is the primary reason humans cannot convert glucuronic acid into ascorbic acid?

Absence of L-gluconolactone oxidase Signup and view all the answers

Where is glycogen mainly stored in the human body?

Liver and muscles Signup and view all the answers

What is the role of glucagon in glycogen metabolism?

Stimulates glycogenolysis Signup and view all the answers

Which of the following substances serves as a primer for glycogen synthesis?

Glycogenin Signup and view all the answers

What is the main function of liver glycogen?

To maintain blood glucose levels during fasting Signup and view all the answers

During prolonged vigorous exercise, what happens to muscle glycogen?

It is depleted Signup and view all the answers

Which enzyme is responsible for the formation of UDP-glucose in muscle cells?

Hexokinase Signup and view all the answers

What type of bonds link glucose units in glycogen?

α1-4 and α1-6 glucosidic bonds Signup and view all the answers

Study Notes

HMP Shunt (Hexose Monophosphate Shunt)

Definition: An alternative pathway for glucose oxidation where ATP is neither produced nor used.
Location: Occurs in the cytoplasm of many tissues.
Phases: The process occurs in two phases: oxidative (irreversible) and non-oxidative (reversible).
Oxidative Phase: Three glucose molecules convert to three ribulose-5-phosphates, producing NADPH+H+ and CO2.
Non-Oxidative Phase: Three ribulose-5-phosphates convert to two glucose-6-phosphates and one glyceraldehyde-3-phosphate. These reactions are reversible.
Functions: Important for pentose (ribose-5-phosphate) synthesis (e.g., DNA, RNA), NADPH+H+, synthesis of fatty acids, steroid hormones, and non-essential amino acids. Additionally, NADPH+H+ is crucial for glutathione reduction, crucial to protect erythrocytes from oxidative damage
Regulation: Glucose-6-phosphate dehydrogenase is the key enzyme. It's stimulated by insulin and NADP+, inhibited by NADPH+H+ and acetyl-CoA.

Uronic Acid Pathway

Definition: An alternative pathway for glucose oxidation, converting glucose into glucuronic acid.
Location: Occurs in the cytoplasm of many tissues.
Importance: Produces UDP-glucuronic acid, a key substrate for various functions.
Synthesis of Substrates:
- Glycosaminoglycans (GAGs)
- Vitamin C (ascorbic acid) in other animals, not humans (humans lack L-gluconolactone oxidase).
Conjugation Reactions: Makes substances like bilirubin more water-soluble for excretion.
Detoxification Reactions: Modifies toxic compounds to make them less toxic.
Production of Pentose: The pathway also produces pentoses.

Glycogen Metabolism

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

Liver Glycogen

Function: Maintains blood glucose levels during fasting.
Concentration: Usually up to 6% of liver mass.
Depletion: Empties after approximately 12-18 hours of fasting.
Regulation: Glucagon stimulates glycogenolysis (break down) in liver cells.

Muscle Glycogen

Function: Acts as a quick source of glucose-6-phosphate for muscle glycolysis.
Concentration: Generally less than 1% of muscle mass.
Depletion: Empties during prolonged or vigorous exercise.
Regulation: Glucagon has no impact on muscle glycogen.

Glycogenesis

Definition: The synthesis of glycogen from glucose.
Location: Occurs in the cytoplasm of liver and muscle cells.
Sources of Glucose Units:
- Blood glucose
- Other hexoses (e.g., galactose, fructose)
- Non-carbohydrate sources generated by gluconeogenesis (e.g., lactate, glycerol)
For Muscle Glycogen: Blood glucose is the sole source for muscle glycogen synthesis.

Glycogenolysis

Definition: The breakdown of glycogen into glucose.
Location: Cytoplasm of liver and muscle cells.
Steps:
- Phosphorylase Enzyme: The key enzyme, breaks down α1-4 glycosidic bonds, producing glucose-1-phosphate.
- Glucan Transferase Enzyme: Moves 3 glucose units from one branch to another, leaving a single glucose unit on a 1-6 branch.
- Debranching Enzyme: Removes the single glucose unit on the 1-6 branch. Note: Glucose-1-P is then converted to glucose-6-phosphate.
Fate of Glucose-6-Phosphate:
- Liver: Contains glucose-6-phosphatase, converts G-6-P into free glucose released into the bloodstream.
- Muscle: Doesn't contain glucose-6-phosphatase, G-6-P remains within muscle cells, used for muscle metabolism.

Regulation of Glycogenesis and Glycogenolysis

Coordinated Regulation: Glycogenesis and glycogenolysis are regulated reciprocally; stimulation of one pathway usually inhibits the other.
Key Regulatory Enzymes:
- Glycogen Synthase: Exists in active dephosphorylated (a) and inactive phosphorylated (b) forms.
- Phosphorylase: Exists in active phosphorylated (a) and inactive dephosphorylated (b) forms.

Regulation During Fasting

Blood glucose decreases, causing secretion of epinephrine and glucagon.
Hormones bind to cell membrane receptors activating adenylate cyclase.
Adenylate cyclase produces cAMP from ATP.
cAMP activates cAMP-dependent protein kinase.
Protein kinases phosphorylate glycogen synthase, inhibiting glycogen synthesis, and phosphorylating phosphorylase kinase, activating glycogen degradation (glycogenolysis).

Regulation After Meals

Blood glucose levels increase, stimulating insulin release.
Insulin promotes glycogen synthesis and inhibits glycogenolysis.
Phosphodiesterase breaks down cAMP into 5' AMP, reducing protein kinase activation.
Phosphatases remove phosphate groups from glycogen synthase, activating it, and from phosphorylase, inhibiting glycogenolysis.

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Description

Explore the Hexose Monophosphate Shunt, an essential metabolic pathway in cellular respiration. This quiz covers its phases, functions, regulation, and significance in biosynthesis. Test your understanding of how this pathway contributes to NADPH production and cellular metabolism.