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?

<p>UDP-glucose. (D)</p> Signup and view all the answers

Which of the following conditions stimulates the process of glycogenolysis?

<p>High glucagon levels. (D)</p> Signup and view all the answers

During which metabolic process is glycogen synthesized?

<p>Glycogenesis. (C)</p> Signup and view all the answers

What is the role of UDP-glucose in glycogenesis?

<p>It is the glucose derivative that reacts to elongate glycogen. (C)</p> Signup and view all the answers

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

<p>It may proceed down several metabolic pathways. (D)</p> Signup and view all the answers

What is the primary role of phosphoglucomutase in glycogen synthesis?

<p>To move the phosphate group from carbon 6 to carbon 1 (B)</p> Signup and view all the answers

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

<p>The phosphate group must be moved to carbon 1 (B)</p> Signup and view all the answers

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

<p>Glycosidic bond (B)</p> Signup and view all the answers

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

<p>Under physiological conditions (C)</p> Signup and view all the answers

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

<p>A uridylyl group (A)</p> Signup and view all the answers

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

<p>Carbon 1 (A)</p> Signup and view all the answers

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

<p>Anomeric carbon (C)</p> Signup and view all the answers

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

<p>The activation of glucose 6-phosphate (B)</p> 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?

<p>It activates glucose for further processing (A)</p> Signup and view all the answers

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

<p>It is where the first bond to glycogen forms (C)</p> Signup and view all the answers

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

<p>The movement of a phosphate group and transfer of a uridylyl group (B)</p> Signup and view all the answers

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

<p>Formation of UDP-glucose (D)</p> Signup and view all the answers

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

<p>To participate in glycosidic bonding (B)</p> Signup and view all the answers

How does the action of phosphoglucomutase affect glucose metabolism?

<p>It enables reversible conversion of glucose 6-phosphate (C)</p> Signup and view all the answers

Flashcards

Anabolic conditions

A state of energy abundance, often triggered by the hormone insulin.

Catabolic conditions

A state of energy depletion, often triggered by the hormone glucagon.

Glycogenesis

The process of synthesizing glycogen from glucose monomers, forming a highly branched polymer.

Glycogenolysis

The process of breaking down glycogen into glucose monomers, releasing glucose for energy.

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Hexokinase

The enzyme responsible for the phosphorylation of glucose to glucose 6-phosphate (G6P).

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UDP-glucose

The activated form of glucose that participates in glycogen elongation.

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Glycogen

Highly branched polymer of glucose units, serving as a storage form of energy.

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Glycogen storage

The liver and muscle cells store glucose in the form of this polymer.

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What is glucose 6-phosphate?

Glucose 6-phosphate is a molecule that is used in a number of metabolic processes. It is particularly important in the synthesis of glycogen, the storage form of glucose in animals.

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How is glucose 6-phosphate activated for glycogen synthesis?

For glycogen synthesis, the phosphate group on glucose 6-phosphate must be moved from carbon 6 to carbon 1, and a UDP group must be added. This makes the glucose molecule more reactive.

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What enzyme moves the phosphate group?

The enzyme phosphoglucomutase catalyzes the reversible conversion of glucose 6-phosphate to glucose 1-phosphate. This process is important for glycogen synthesis and breakdown.

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Is the conversion of glucose 6-phosphate to glucose 1-phosphate reversible?

The conversion of glucose 6-phosphate to glucose 1-phosphate is reversible, meaning it can occur in both directions. This allows for the dynamic regulation of glycogen synthesis and breakdown.

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What is the specific reaction catalyzed by phosphoglucomutase?

The reaction catalyzed by phosphoglucomutase involves the transfer of a phosphate group from carbon 6 to carbon 1 of glucose. This is important for glycogen synthesis.

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Which form of glucose is used by glycogen synthase?

Glucose 1-phosphate is the form of glucose required by glycogen synthase, the enzyme that adds glucose units to the growing glycogen chain.

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What is glycogen?

Glycogen is a branched polymer of glucose that serves as the primary storage form of glucose in animals. It is a readily accessible source of energy and plays a crucial role in maintaining blood glucose levels.

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What is the anomeric carbon?

The anomeric carbon is the carbon atom that is bonded to two oxygen atoms. In the case of glucose, this is carbon 1.

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Why is the anomeric carbon of glucose 1-phosphate activated?

The anomeric carbon of glucose 1-phosphate is activated because it has a phosphate group attached. This allows for the formation of a glycosidic bond with the glycogen chain.

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What is the reducing end?

The reducing end is the end of a sugar molecule that has a free anomeric carbon. This carbon can participate in a reaction with another molecule to form a new glycosidic bond.

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What is the nonreducing end?

The nonreducing end is the end of a sugar molecule that does not have a free anomeric carbon. It cannot participate in a reaction with another molecule to form a new glycosidic bond.

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What is a glycosidic bond?

A glycosidic bond is a type of covalent bond that links sugar molecules together. It is formed between the anomeric carbon of one sugar and a hydroxyl group of another sugar. These bonds are important for the formation of carbohydrates like glycogen.

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How does glycogen synthase build glycogen?

Glycogen synthase adds glucose units specifically to the nonreducing end of a growing glycogen molecule, forming a linear chain. Branching enzymes then create branches, providing a greater surface area for enzymatic activity. This maximizes the rate of glucose storage and release in the body.

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What is the overall significance of phosphoglucomutase and glycogen synthase?

The process regulated by phosphoglucomutase and glycogen synthase allows the body to efficiently store glucose and release it when energy is needed.

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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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