HMP Shunt Quiz

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Questions and Answers

What is the primary function of the hexose monophosphate shunt?

Conversion of glucose to glucuronic acid

Generation of NADPH and pentoses (correct)

Production of ATP

Oxidation of fatty acids

Which of the following is produced during the oxidative phase of the HMP shunt?

Ribulose-5-phosphate (correct)

Glucose-6-phosphate

NADH

Ribose-5-phosphate

Which enzyme is crucial for regulating the HMP shunt?

Aldolase

Glutathione peroxidase

Glucose-6-phosphate dehydrogenase (correct)

Hexokinase

What does increased NADPH indicate regarding the HMP shunt regulation?

Inhibition of shunt activity Signup and view all the answers

Which product is a key outcome of the non-oxidative phase of the HMP shunt?

Glucose-6-phosphate Signup and view all the answers

What metabolic product does the uronic acid pathway primarily produce?

Glucuronic acid Signup and view all the answers

In which tissues does the HMP shunt primarily occur?

Cytoplasm of many tissues Signup and view all the answers

What role does reduced glutathione play in red blood cells?

Protects against oxidative damage Signup and view all the answers

What is the primary function of liver glycogen?

Maintains blood glucose during fasting Signup and view all the answers

Which substance is involved in the synthesis of glycogen in muscle cells?

Blood glucose Signup and view all the answers

What initiates the formation of UDP-glucose from glucose-1-phosphate?

Uridine triphosphate (UTP) Signup and view all the answers

Which of the following statements is true regarding glycogenolysis in liver and muscle?

Glucagon stimulates glycogenolysis in both liver and muscle Signup and view all the answers

What type of bonds link the glucose units in glycogen?

Alpha-1,4 and alpha-1,6 glucosidic bonds Signup and view all the answers

Which of the following is NOT a source of glucose for liver glycogen synthesis?

Tyrosine residues Signup and view all the answers

What is the role of glycogenin in glycogen synthesis?

To serve as a primer for glycogen formation Signup and view all the answers

What happens to muscle glycogen during prolonged vigorous exercise?

It depletes significantly Signup and view all the answers

What is the function of glycogen synthase in glycogenesis?

It catalyzes the formation of α 1-4 glucosidic bonds. Signup and view all the answers

Which enzyme is responsible for breaking down α 1-4 glycosidic bonds during glycogenolysis?

Phosphorylase Signup and view all the answers

What role does the branching enzyme play in glycogen structure?

It creates α 1-6 linkage by forming branches. Signup and view all the answers

What happens to glucose-6-phosphate in muscle cells?

It cannot leave the muscle cells. Signup and view all the answers

During what condition is glycogen synthesis promoted?

High levels of glucose-6-phosphate Signup and view all the answers

Which enzyme's active form is phosphorylated in glycogen metabolism?

Phosphorylase Signup and view all the answers

What triggers the conversion of glucose-1-phosphate to glucose-6-phosphate?

Enzymatic action of phosphoglucomutase Signup and view all the answers

How does muscle glycogenolysis differ from liver glycogenolysis?

Muscle glycogenolysis ends with glucose-6-phosphate, while liver converts it to glucose. Signup and view all the answers

Study Notes

HMP Shunt (Hexose Monophosphate Shunt)

Definition: An alternative pathway of glucose oxidation where ATP isn't produced or used
Location: Cytoplasm of many tissues
Phases: The reactions occur in two phases
- Oxidative (irreversible): Converts 3 glucose molecules into 3 ribulose-5-P, producing NADPH+H+ and CO2
- Non-oxidative (reversible): Converts 3 ribulose-5-P into 2 glucose-6-P and 1 glyceraldehyde-3-P

Functions of HMP Shunt

Pentoses (ribose-5-P): Used for synthesis of DNA, RNA, NAD, FAD, ATP, and GTP
NADPH+H+: Important for the synthesis of fatty acids, steroid hormones, and non-essential amino acids
Maintenance of reduced glutathione in erythrocytes by removal of hydrogen peroxide (H2O2) which is toxic to cells. This process is done by enzymes called glutathione peroxidase and glutathione reductase. Glutathione reduction is crucial for maintaining the integrity of RBCs

Regulation of HMP Shunt

Key enzyme is glucose-6-phosphate dehydrogenase (G6PD)
Stimulated by insulin and NADP+
Inhibited by NADPH+H+ and acetyl CoA

Uronic Acid Pathway

Definition: An alternative glucose oxidation pathway that converts glucose to glucuronic acid
Location: Cytoplasm of many tissues
Importance:
- Production of UDP-glucuronic acid for use in:
  - Synthesis of glycosaminoglycans
  - Synthesis of vitamin C (L-ascorbic acid) in animals (Humans cannot convert glucuronic acid into ascorbic acid due to the absence of L-gluconolactone oxidase.)
- Conjugation reactions: Making substances like bilirubin more water-soluble for excretion
- Detoxification reactions: Making toxic compounds less toxic

Glycogen Metabolism

Definition: The storage form of carbohydrates in animals
Composition: α-D glucose units linked together by α1-4 glucosidic bonds and by α1-6 bonds at the branch points
Storage location: Primarily in the liver and muscles
Liver glycogen:
- Makes up to 6% of liver mass
- Maintains blood glucose during fasting
- Depletes after 12-18 hours of fasting
- Glucagon stimulates glycogenolysis
Muscle glycogen:
- Rarely exceeds 1% of muscle mass
- Acts as a glucose source for muscle glycolysis only
- Depletes after prolonged vigorous exercise

Glycogenesis (Formation of Glycogen)

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

Glycogenolysis (Breakdown of Glycogen)

Definition: The breakdown of glycogen into glucose
Location: Cytoplasm of liver and muscle cells
Glycogenolysis steps:
- Phosphorylase: Breaks down α1-4 glycosidic bonds. Acts on branches with more than 4 glucose units
- Glucan transferase: Transfers 3 glucose units to another branch, leaving one glucose unit linked via an α1-6 bond
- Debranching enzyme: Removes the glucose unit linked via an α1-6 bond by hydrolysis, forming free glucose
Fate of glucose-6-phosphate:
- In liver: Glucose-6-phosphatase converts glucose-6-P into glucose, which is released into the blood
- In muscle: No glucose-6-phosphatase, so glycogenolysis ends with glucose-6-P in the muscle

Regulation of Glycogenesis and Glycogenolysis

Coordinated regulation: Glycogenesis and glycogenolysis are regulated in an opposite manner
Key enzymes:
- Glycogen synthase (active as dephosphorylated form; inactive as phosphorylated form)
- Phosphorylase (active as phosphorylated form; inactive as dephosphorylated form)

Regulation of Glycogen Metabolism During Fasting

Blood glucose decreases, causing hormone release (epinephrine in muscle, glucagon in liver)
Hormones bind to beta receptors, activating adenylate cyclase
Adenylate cyclase produces cAMP from ATP
cAMP activates cAMP-dependent protein kinase
Protein kinase phosphorylates and inactivates glycogen synthase, inhibiting glycogenesis
Protein kinase phosphorylates and activates phosphorylase, promoting glycogenolysis

Regulation of Glycogen Metabolism After Meals

Blood glucose increases, causing insulin release
Insulin stimulates glycogen synthesis and inhibits glycogenolysis
Phosphodiesterase breaks down cAMP into 5'AMP
Phosphatase removes phosphate groups from enzymes, further regulating synthesis and breakdown of glycogen

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Description

Test your knowledge on the Hexose Monophosphate Shunt, its functions, phases, and regulation. This quiz will cover key concepts including ATP production, the importance of NADPH, and the role of the HMP shunt in cellular processes.