2.9 Glycogen Metabolism

Question 1

Define glycogen

Answer 1

A soluble polymer of many glucose units

Question 2

Define glycogenesis

Answer 2

Intracellular synthesis of glycogen from glucose

- Occurs in cytosol of liver and muscle cells

Question 3

Define Glycogenolysis

Answer 3

Intracellular breakdown of glycogen to form glucose

- Breakdown of glycogen

Question 4

Describe liver glycogen

Answer 4

• Short term energy
• Liver stores and releases glucose from glycogenolysis in response to low blood glucose levels
• Puts glucose back in blood when brain needs it

Question 5

Describe muscle glycogen

Answer 5

• Main fuel substrate during high intensity exercise within muscle cells
• Depleted after prolonged exercise

Question 6

Why store glucose as glycogen?

Answer 6

A large amount of glucose can’t be in cells b/c accumulation of charged G-6-P has strong osmotic effect (cells swell)

Question 7

Is glycogen charged or uncharged?

Answer 7

Not charged

Question 8

A non-branched structure doesn’t allow for

Answer 8

Rapid synthesis and breakdown

Question 9

Glycogen core complexes consist of

Answer 9

Glycogenin protein

Question 10

Glycogen synthesis involves

Answer 10

Elongation of linear chains and formation of branch points

Question 11

Glycogen degradation involves

Answer 11

Sequential removal of glucose and the disassembly of branch points

Question 12

What are the enzymes of glycogenesis?

Answer 12

Glucokinase
Phosphoglucomutase
UDPG pyrophosphorylase
Glycogen synthase

Question 13

Glycogenesis is stimulated by

Answer 13

Insulin in the liver and muscle

Question 14

Enzymes involved in glycogenolysis

Answer 14

Phosphorylase
Phosphoglucomatuse
Glucose-6-phosphatase

Question 15

Glycogenolysis is stimulated by

Answer 15

Glucagon in the liver and epinephrine in muscle

Question 16

Most glycogen synthesis occurs via addition of glucose residues to a

Answer 16

preexisting glycogen molecule

Question 17

In the absence of a preexisting glycogen molecule, the protein ___ serves as a ___

Answer 17

glycogenin

primer

Question 18

Describe phosphoglucomutase (PGM)

Answer 18

Catalyzes G-1-P and G-6-P

• It’s required for both synthesis and degradation of glycogen
• Moves phosphate from position 6 to 1

Question 19

The first committed step in glycogen synthesis is the formation of

Answer 19

Uridine diphosphate (UDP) glucose

Question 20

Hydrolysis of PPi (when UDP glucose is the enzyme) drives glycogen synthesis and prevents a ___

Answer 20

Potential futile cycle

- Not reversible

Question 21

Describe glycogen synthase rxn

Answer 21

Glycogen chains are elongated 1 UDP-glucose unit at a time via glycogen synthase

Question 22

What bond is formed between UDP-glucose and a glycogen end

Answer 22

alpha-1,4-glycosidic bond

Question 23

What transfers 7 glucose units from the end of the chain to an internal position using a alpha-1,6-branch point?

Answer 23

glycogen branching enzyme (1,4-alpha-glucagon-branching enzyme)

• Does this once it reaches 11 glucose residues

Question 24

What is the first step in glycogenolysis?

Answer 24

Glycogen phosphorylase catalyzes glycogen to glucose-1-phosphate + glycogen

Question 25

Glycogen phosphorylase breaks the ___ bond, but NOT the ___

Answer 25

Breaks = 1,4 bond | DOESN'T break = 1,6 bond

Question 26

What does glycogen transferase do?

Answer 26

Removes 3 glucose units by breaking a 1,4 bond on one strand and forming a new 1,4 bond on another

Question 27

What enzyme breaks the 1,6 bond?

Answer 27

Debranching enzyme (alpha 1,6-glucosidase)

Question 28

Does the debranching enzyme remove a glucose or glucose-1-phosphate unit?

Answer 28

Glucose

Question 29

Complete degradation of glycogen releases ___% of glucose-1-phosphate and ___% glucose moleucles

Answer 29

90% | 10%

Question 30

What is the second enzyme used in glycogenolysis and what does it do?

Answer 30

Phosphoglucomutase G-1-P to G-6-P

Question 31

What is the third enzyme involved in glycogenolysis and what does it do?

Answer 31

Glucose-6-phosphatase G-6-P to glucose

Question 32

Where is glucose-6-phosphatase located and why?

Answer 32

Integral membrane protein of the ER because it separates reaction from glycolysis

Question 33

What's the process for dephosphorylation of G6P to glucose?

Answer 33

1. G-6-P is transported to ER lumen by a specific transporter (T1) 2. G-6-P is hydrolyzed by glucose-6-phosphatase 3. Glucose is transported to cytosol via transporter T2 and out of cell via GLUT2

Question 34

What activates glycogen phosphorylase in muscles?

Answer 34

Calcium

Question 35

What happens to glycogen phosphorylase in the liver?

Answer 35

It's inhibited

Question 36

Glycogen synthase and glycogen phosphorylase respond in opposite ways to phosphorylation, GS is ___ and GP is ___

Answer 36

``` GS = inactivated GP = activated ```

Question 37

Muscles don't respond to glucagon, so what triggers the same pathway?

Answer 37

epinephrine

Question 38

When going from G-1-P to glycogen, what enzymes are added to glycogen synthase?

Answer 38

Protein kinase A tries to add P from cAMP, but it negatively regulates it SO Protein phosphatase from insulin removes a P

Question 39

When going from glycogen to G-1-P, what enzymes are involved with P?

Answer 39

Protein phosphatase from insulin removes P SO Phosphorylase kinase from glucagon in liver, and epinephrine in muscle adds a P and activates GP to breakdown glycogen

Question 40

Fasting does what

Answer 40

Phosphorylates

Question 41

Insulin ___, leading to the activation of pyruvate kinase, pyruvate dehydrogenase and glycogen synthase

Answer 41

Dephosphorylates

Question 42

Glucagon leads to phosphorylation of what enzymes? Are they active or inactive?

Answer 42

``` Pyruvate Kinase (glycolysis) = INACTIVE Pyruvate dehydrogenase (Pyruvate to Acetyl Co-A) = INACTIVE Glycogen Synthase (glycogen synthesis) = INACTIVE Glycogen phosphorylase (glycogen breakdown) = ACTIVATION ```

Question 43

Insulin is required for storing ___

Answer 43

Glucose as glycogen

Question 44

Glucagon is required for releasing ___

Answer 44

Glucose from its stores in the liver

Question 45

What is glycogen storage disease (GSD)?

Answer 45

Defect in any one of the several enzymes required for either glycogen synthesis or degradation

Question 46

What 2 ways can GSDs be divided into?

Answer 46

- Hepatic involvement (hypoglycemia) | - Those with neuromuscular disease and weakness