Lecture 22, Glyc Metabolism (Zaidi)

Question 1

Describe the structure of glycogen.

Answer 1

Long chain homopolymer of glucose molecules with branches

Question 2

What are the glucose molecules within a chain of glycogen linked together by?

Answer 2

alpha-1,4-glycosidic bonds

Question 3

What kind of bonds are the branch points of glycogen formed by?

Answer 3

alpha-1,6-glycosidic

Question 4

What are the 2 kinds of ends of a glycogen molecule?

Answer 4

Non-reducing and reducing

Question 5

Describe the non-reducing end of a molecule of glycogen.

Answer 5

Contain a terminal glucose with a free hydroxyl group at C4

Question 6

Describe the reducing end of a molecule of glycogen.

Answer 6

Consists of a glucose monomer connected to glycogenin

Question 7

Why is glycogenin important?

Answer 7

Creates a short glycogen polymer on itself and serves as a primer for glycogen synthesis

Question 8

From which end is glycogen degraded and extended?

Answer 8

Non-reducing end

Question 9

Where is glycogen stored?

Answer 9

Liver, muscle, and other tissues

Question 10

How is glycogen stored?

Answer 10

As granules, but the granules also contain enzymes needed for glycogen metabolism

Question 11

T or F: Defects in the enzymes contained in the glycogen granules can lead to disorders.

Answer 11

True.

Question 12

What is the function of liver glycogen?

Answer 12

Regulates blood glucose levels

Question 13

What is the function of muscle glycogen?

Answer 13

Provides reservoir of fuel (glucose) for physical activity

Question 14

What is an overview of glycogen metabolism?

Answer 14

Regulated storage and release of glucose

Question 15

T or F: The synthesis and degradation of glycogen involve different pathways, but are regulated by the same mechanism.

Answer 15

False. The synthesis and degradation of glycogen involve different pathways and are regulated independently.

Question 16

How is glycogen metabolism regulated?

Answer 16

Allosteric control, covalent modification through reversible phosphorylation of key enzymes, hormonal control

Question 17

There are 3 key steps in glycogenesis. What is the purpose of the first step?

Answer 17

Trapping and activation of glucose

Question 18

In glycogenesis, what traps glucose in the hepatocytes and muscle cells?

Answer 18

Glucokinase/hexokinase catalyze phosphorylation of glucose to glucose 6-phosphate

Question 19

What enzyme reversibly isomerizes glucose 6-phosphate to glucose 1-phosphate?

Answer 19

Phosphoglucomutase

Question 20

What is the role of uridine diphosphate (UDP) glucose pyrophosphorylase? What is generated?

Answer 20

Transfers the glucose 1-phosphate to UTP; UDP-glucose is generated

Question 21

How does UDP glucose pyrophosphorylase generate energy?

Answer 21

Breakdown of pyrophosphate to Pi generates energy

Question 22

What is the formation of UDP glucose catalyzed by?

Answer 22

UDP-glucose pyrophosphorylase

Question 23

There are 3 key steps in glycogenesis. What is the purpose of the second step?

Answer 23

Elongation of a glycogen primer

Question 24

What is the rate-limiting enzyme of glycogenesis?

Answer 24

Glycogen synthase

Question 25

What is the enzyme that adds glucose to the preexisting glycogen polymer?

Answer 25

Glycogen synthase

Question 26

The peexisting glycogen polymer serves as a ___ to which glucose units are added in the elongation of glycogen.

Answer 26

Primer

Question 27

What is the mechanism of glycogen synthase? What kind of bond is formed?

Answer 27

Catalyzes transfer of glucose from UDP-glucose to non-reducing end of glycogen chain; Forms alpha-1,4-glycosidic bond between glucose molecules

Question 28

There are 3 key steps in glycogenesis. What is the purpose of the third step?

Answer 28

Branching of glycogen chains

Question 29

When a glycogen chain reaches about 11 residues long, what happens?

Answer 29

A fragment of the chain (about 7 residues) is broken off at an alpha-1,4-link and reattached elsewhere through an alpha-1,6- link

Question 30

What is the enzyme that allows the branching of glycogen?

Answer 30

Glucosyl (4:6) transferase

Question 31

How far away must a new branch point be from a preexisting branch point?

Answer 31

At least 4 residues away

Question 32

Why is the branching of glycogen molecules important?

Answer 32

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

Question 33

What does the breakdown of glycogen release?

Answer 33

Glucose 1-phosphate

Question 34

What is broken down glycogen (glucose 1-phosphate) called?

Answer 34

Glycogen remnant (n-1)

Question 35

After glycogen breakdown, what is remodeled to permit further degradation?

Answer 35

Glycogen remnant

Question 36

What is glucose 1-phosphate converted to? Where can this be used?

Answer 36

Glucose 6-phosphate; Glycolysis, free glucose for release into blood stream, PPP (NADPH and ribose derivative)

Question 37

In glycogenolysis, what are the roles of the 4 key enzymes?

Answer 37

1. Degrade glycogen (chain shortening) 2 and 3. Remodel glycogen remnants 4. Convert glycogen breakdown product suitable for further metabolism

Question 38

What catalyzes the cleavage of glycogen?

Answer 38

Glycogen phosphorylase (GP)

Question 39

What is the rate-limiting enzyme of glycogenolysis?

Answer 39

Glycogen phosphorylase (GP)

Question 40

Where does chain shortening occur in glycogenolysis?

Answer 40

Non-reducing end of polymer

Question 41

What is the mechanism of GP?

Answer 41

Adds orthophosphate and releases glucose residue as G1P; Uses pyridoxal phosphate (vitamin B6) as a cofactor

Question 42

How long does the phosphorolysis of glucose residues continue?

Answer 42

Until the GP gets within 4 residues of the alpha-1,6-linkage of a branch point

Question 43

What is the role of phosphoglucomutase?

Answer 43

Converts G1P to G6P

Question 44

What is the mechanism of phosphoglucomutase?

Answer 44

Phosphoryl group is transferred form enzyme to the substrate, and a different phosphoryl group is transferred back to restore the enzyme to its initial state

Question 45

What is the role of transferase in glycogenolysis?

Answer 45

Transfers a block of 3 of the remaining 4 glucose to the non-reducing end of the main chain forming an alpha-1,4 bond

Question 46

What is the role of the debranching enzyme (alpha 1,6 glucosidase)?

Answer 46

Cleaves alpha-1,6 bond of single remaining glucose residue to release the free glucose

Question 47

In glycogenolysis, what is glucose phosphorylated by?

Answer 47

Hexokinase

Question 48

What enzymes convert branched glycogen into a linear structure for further action by GP?

Answer 48

alpha-1,6-glucosidase and transferase

Question 49

T or F: G6P cannot get out of the cell.

Answer 49

True.

Question 50

Which is the only organ that has glucose 6 phosphatase?

Answer 50

Liver

Question 51

What does the liver do with glucose 6 phosphatase?

Answer 51

Converts G6P to glucose, which can be released in the bloodstream