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Biochemistry Exam 2 > Glycogen Metabolism I > Flashcards

Glycogen Metabolism I Flashcards

1
Q

What is the structure of glycogen?

A

A long chain homopolymer of glucose molecules with branches.

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2
Q

How are glucose molecules that compose glycogen linked together?

A

Linear chains are connected via alpha- 1,4 glycosidic bonds

Branched chains are connected via alpha- 1,6 glycosidic bonds

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3
Q

Describe the ends of glycogen chains

A

Made up of non-reducing ends that contain terminal glucose with an exposed hydroxyl group at Carbon 4

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4
Q

What is the purpose of glycogenin?

A

The reducing ends connect to this

Serves as a primer for glycogen synthesis

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5
Q

Glycogen is degraded and extended from the ____ ____ _____

A

Non-reducing end

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6
Q

Where is glycogen stored?

A

In the liver and muscles

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7
Q

What is glycogen stored as?

A

Granules

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8
Q

What are the 2 functions of glycogen?

A

Liver glycogen- regulates blood glucose levels

Muscle glycogen- provides a reservoir of fuel (glucose) for physical activity

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9
Q

What is the purpose of glycogen metabolism?

A

To regulate storage and release of glucose

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10
Q

What are the 3 regulation mechanisms of glycogen?

A

Allosteric control
Covalent modification- through reversible phosphorylation of key enzymes
Hormonal control

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11
Q

What are the 3 key steps of glycogenesis?

Explain the first key step.

A
  1. Trapping and activating glucose
  2. Elongation of glycogen primer
  3. Branching of glycogen chains
  4. Trapping and activation of glucose- Glucokinase/Hexokinase catalyzes glucose to G6P
    - Phosphoglucomutase then reversibly isomerizes G6P to G1P
    - G1P is converted into UDP-Glucose via Uridine diphosphate (UDP)-Glucose Phosphorylase. Note UDP-Glucose is the active form of Glucose
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12
Q

What enzyme catalyzes G1P to UDP-Glucose?

A

UDP-Glucose Phosphorylase

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13
Q

What is the rate-limiting enzyme in glycogenesis?

A

Glycogen Synthase- catalyzes the transfer of glucose from UDP- glucose to the non-reducing end of the glycogen chain. Forms alpha- 1,4 glycosidic bonds between glucose molecules

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14
Q

What enzyme catalyzes the branching of glycogen?

A

Glucosyl (4:6) Transferase

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15
Q

Describe the process of glycogen branching.

A

When the glycogen chain reaches 11 residues, a fragment of the chain (about 7 residues long) is broken off at an alpha- 1,4 link and reattached elsewhere through an alpha- 1,6 link by Glucosyl (4:6) Transferase

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16
Q

The new branch point must be at least ___ residues away from the preexisting branch.

A

4

17
Q

What is the purpose of glycogen branching?

A

Increases solubility of glycogen
Increases the number of terminal non-reducing ends
Increases rate at which glycogen can be synthesized and degraded

18
Q

What are the enzymes that catalyze the branching of glycogen?

A

alpha- 1,4 linkage adds glucose polymers by UDP-glucose and Glycogen Synthase (1 non-reducing ends)

Long glucose polymer branches via Glucosyl (4:6) Transferase to catalyze the alpha (1,6)- linkage (2 non-reducing ends)

19
Q

Describe the steps of glycogenesis.

A

Glucose to G6P via Hexokinase/Glucokinase

G6P to G1P via Phosphoglucomutase (PGM)

G1P to UDP-Glucose via UDP-glucose pyrophosphorylase

UDP-Glucose to linear Glycogen via Glycogen Synthase (GS)- rate limiting steps

Linear Glycogen to branched glycogen via Glucosyl (4:6) Transferase

20
Q

What is the first step of glycogenolysis?

A

Glycogen break down to release glucose-1 Phosphate

21
Q

What are the 4 key enzymes of glycogenolysis and their functions?

A

Glycogen Phosphorylase -Catalyzes the cleavage of glycogen (causes linear chain shortening at the non-reducing end)

Phosphoglucomutase -Two to Remodel glycogen remnants (one for the liver and one for muscle)

Glucose 6 Phosphatase -Converts glycogen breakdown product suitable for metabolism

22
Q

What is the rate-limiting enzyme of glycogenolysis?

A

Glycogen Phosphorylase

23
Q

Glycogen phosphorylase add an ________ and releases a glucose residue as glucose-1 phosphate

A

Orthophosphate (PO4^3-)

24
Q

Glycogen phosphorylase uses _____ ____ (AKA ______ ___) as a cofactor

A

Pyridoxal Phosphate

Vitamin B6

25
Q

Phosphorolysis of glucose residues continues till the Glycogen Phosphorylase gets within _ ____ of the _____ _____ of a branch point.

A

4 residues

Alpha- 1,6 linkage

26
Q

Once G1P is released from the linear chain glycogen what catalyzes G1P to G6P?

A

Phosphoglucomutase

27
Q

Glycogen at this point has 4 glucose molecules attached/branched to a long glucose polymer underneath it. What enzyme catalyzes the release of 3/4 of the glucose molecules on the top strand and adds them to the bottom strand in a linear fashion?

A

Transferase

28
Q

What enzyme catalyzes the release of the last glucose molecule attached to the linear chain in a branched fashion?

A

Alpha- 1,6 Glucosidase

29
Q

What is the overall yield of debranching of glycogen?

A

10 G1P for every 1 glucose

the 1 glucose is always the last glucose monomer in the branched chain to be released

30
Q

Can G6P leave the cell?

A

No! Glucose 6 Phosphate cannot leave the cell. It must first be converted to glucose and then leave.

T1- Pulls G6P into the cell
T2- Rips phosphate off G6P via G6P phosphatase
T3- Pushes glucose out of the cell

31
Q

Which one, the liver or muscle cells, has Glucose 6 Phosphatase?

A

Liver, converts it to glucose which can be released into the bloodstream

Muscle cells do not have glucose 6 phosphatase so G6P stays in the cells

32
Q

Describe lysosomal glycogenolysis. What enzyme is used to catalyze this reaction?

A

A small amount of glycogenolysis occurs via lysosomal alpha 1,6 glucosidase (AKA acid maltase).

Shortens branches of glycogen but does not debranch them.

Causes the release of free glucose

33
Q

What is Pompe Disease?

A

When you do not have sufficient levels of alpha- glucosidase (AKA acid maltase)

34
Q

What is Von Gierke Disease?

A

Deficiency in Glucose 6 Phosphatase levels, therefore leading to the inability to form free glucose from G6P in the liver

35
Q

During glycogenolysis, G6P is found in both the liver and muscle, but what is the difference when they are in their respective locations?

A

G6P in the liver is able to become free glucose via glucose 6 phosphatase

G6P in muscle can be directly used in glycolysis to form energy for active muscles

36
Q

During glycogenolysis, G1P cannot enter what 2 tissue types and must be converted into what to enter these tissues?

A

Liver and Muscle

G6P via Phosphoglucomutase

37
Q

What is Hers Disease?

A

The inability to form G1P (for the liver) from Glycogen due to a deficiency in Glycogen Phosphorylase

38
Q

What is McArdle Disease?

A

The inability to form G1P (for muscles) from Glycogen due to a deficiency in Glycogen Phosphorylase

39
Q

What is Cori Disease?

A

The inability to release the last glucose molecule attached in a branched fashion to the linear chain below it.

Due to the deficiency of the debranching enzyme alpha 1,6- Glucosidase

This causes a buildup of glycogen to form.

Biochemistry Exam 2 (13 decks)

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