Glycogen Breakdown

Question 1

What is the composition of glycogen?

Answer 1

Alpha (1-4) linked D-glucose with alpha(1-6) linked branches every 8-14 residues.

Question 2

What type of shape does a glycogen molecule take?

Answer 2

Spheroidal

Question 3

In what types of cells is glycogen most prominent?

Answer 3

Muscle and liver

Question 4

Where do the enzymes that catalyze glycogen synthesis and breakdown come from?

Answer 4

They are contained in glycogen.

Question 5

From which ends of glycogen is glucose removed from?

Answer 5

Non-reducing ends

Question 6

True or false: Glycogen contains many reducing ends, but only one non-reducing end.

Answer 6

False; glycogen contains many non-reducing ends, but only one reducing end.

Question 7

What is glycogenolysis?

Answer 7

The breakdown of glycogen into glucose

Question 8

The breakdown of glucose

Answer 8

glycogenolysis

Question 9

What pathway converts glucose to pyruvate?

Answer 9

glycolysis

Question 10

What pathway converts pyruvate to glucose?

Answer 10

gluconeogenesis

Question 11

What is the end of a glycogen molecule with a free carbon #1 called? How many reducing ends does each molecule of glycogen have?

Answer 11

A reducing end; one

Question 12

What does glycogen phosphorylase do?

Answer 12

Cleaves glycogen bonds by substituting a Pi to form glycogen and glucose-1-phosphate (G1P).

Question 13

Which enzyme breaks glycogen branch bonds in the second step of glycogenolysis?

Answer 13

Glycogen debranching enzyme

Question 14

Which enzyme converts G1P to G6P?

Answer 14

Phosphoglucomutase

Question 15

What is the RDS of glycogenolysis?

Answer 15

The first step, glycogen phosphorylase

Question 16

What controls glycogen phosphorylase?

Answer 16

1) Allosteric interactions (inhibitors=ATP, G6P, and glucose, activators=AMP)
2) Covalent modification (phosphorylation and dephosphorylation)

Question 17

What is the phosphorylated form of glycogen phosphorylase?

Answer 17

Phosphorylase a

Question 18

What is the dephosphorylated form of glycogen phosphorylase?

Answer 18

Phosphorylase b

Question 19

Why can glycogen phosphorylase only get within 5 units from a branch?

Answer 19

The crevice on its surface that connects the glycogen binding site to the active site is too small.

Question 20

True or false: Several glucose units can be cleaved by glycogen phosphorylase without unbinding/rebinding glycogen between catalytic cycles?

Answer 20

True

Question 21

What is the cofactor for glycogen phosphorylase? What is it derived from?

Answer 21

pyridoxal-5-phosphate (PLP); from vitamin B6

Question 22

True or false: Glycogen phosporylase undergoes conformational changes from T to R and vice versa.

Answer 22

True

Question 23

What activator promotes a conformation change in glycogen phosphorylase from T to R?

Answer 23

AMP

Question 24

True or false: The T site is buried in glycogen phosphorylase and has low substrate affinity.

Answer 24

True

Question 25

What substrate binds at the T state of glycogen phosphorylase, inhibiting the shift from the T to the R state?

Answer 25

ATP

Question 26

Which form of glycogen phosphorylase requires AMP for activity?

Answer 26

Phosphorylase b

Question 27

Which form of glycogen phosphorylase is active without AMP?

Answer 27

Phosphorylase a

Question 28

Which glycolysis enzyme works to remove branches?

Answer 28

Glycogen debranching enzyme

Question 29

How does the glycogen debranching enzyme work?

Answer 29

It removes a trisaccharide from a branch to a non-reducing end, then hydrolyzes the remaining alpha (1-6) bond, yielding glucose and debranched glycogen.

Question 30

True or false: The debranching enzyme phosphorylizes the alpha(1-6) glucose bond.

Answer 30

False; it hydrolyzes it

Question 31

True or false: The glucose that is being removed by a debranching enzyme is G1P.

Answer 31

False; it is glucose

Question 32

True or false: The debranching enzyme works much faster than glycogen phosphorylase.

Answer 32

False; glycogen phosphorylase is much faster.

Question 33

Which branches of a glycogen molecule are degraded most quickly by a debranching enzyme?

Answer 33

The outer banches (in just a few seconds); the slower debranching occurs.

Question 34

Why does maximum muscle contraction last only a few seconds?

Answer 34

In part, because the debranching of inner branches of glycogen is slow.

Question 35

True or false: The debranching enzyme has only one active site.

Answer 35

False; it has two

Question 36

Which enzyme converts G1P to G6P in glycogenolysis?

Answer 36

Phosphoglucomutase

Question 37

True or false: Near equilibrium, the reaction of G1P to G1,6P to G6P by phosphoglucomutase is irreversible.

Answer 37

False

Question 38

What can happen to the G6P from glycogenolysis?

Answer 38

It can enter glycolysis or enter the Hexose Monophosphate Pathway (Pentose Phosphate Pathway).

Question 39

In the liver, what must happen to G6P for it to be put back into circulation from the cell?

Answer 39

It must be de-phosphorylated to glucose.

Question 40

What enzyme transports G6P to the ER to be be de-phosphorylated?

Answer 40

G6P translocase

Question 41

What enzyme dephosphorylates G6P to glucose in the ER?

Answer 41

G6Pase (Glucose-6-phosphatase)

Question 42

What transporter transports glucose out of the liver?

Answer 42

GLUT2

Question 43

Why do muscles and other tissues besides the liver not release glucose back into circulation?

Answer 43

They lack GTPase to break G16 back down to glucose.

Question 44

What is the result of any defect in the G6P hydrolysis system in liver/

Answer 44

Type 1 Glycogen Storage Disease

Question 45

What are the symptoms of Type 1 Glycogen Storage Disease?

Answer 45

1) Increased [G6P] and liver and kidney[glucose] 2) Inability to increase blood glucose concentration 3) Hepatomegaly and hypoglycemia 4) General failure to thrive