Glycogen Metabolsim

Question 1

How are the liver and muscle involved in glycogen metabolism?

Answer 1

Liver exports as glucose and the muscle runs through glycolysis.

Question 2

What are the two enzymes involved in structural formation of glycogen?

Answer 2

Glycogen synthase - a-1,4 linkage between two glucose units (lots of these)

Glycogen-branching enzyme - a-1,6 linkage between two glucose units (few of these)

Question 3

What is the advantage to the way glycogen is synthesized? (Structure)

Answer 3

More compact and easier to break apart when needed.

Question 4

Glycogen synthesis relies on what?

Answer 4

Sugar nucleotides. UDP-glucose formed and stuck onto glucose then allowing glycogen synthase to lock on.

Question 5

Glucose-6-phosphate has many fates. What dictates it’s fate for glycogen synthesis?

Answer 5

The formation of UDP-glucose.

Question 6

What protein is required for glycogen synthase? What purpose does it serve?

Answer 6

Glycogenin serves as a means of glucose attachment.

It is both the primer on which new chains are assembled & the enzyme that catalyzes assembly.

Limits the number of glycogen molecules “out there”
Limits the size of glycogen molecule.

Question 7

What are the steps involving glycogenin and glycogen synthase?

Answer 7

Complex formed
Attachment of another glucose to tyrosine residue
Growth continues
End of this glycogen molecule being synthesized

Question 8

T or F: Glycogen synthase can initiate a new glycogen chain de novo.

Answer 8

F

Question 9

What are the requirements of glycogen-branching enzyme? Why is this significant?

Answer 9

11 glucose molecules
Branch point must be at least 4 residues away from non-reducing end

Significant because this opens up more access points for both break-down and build-up.

Question 10

What is the key enzyme involved in glycogenolysis? Why is this the key enzyme?

Answer 10

Glycogen phosphorylase because it is the rate limiting enzyme.

Question 11

What is the other name for glycogen phosphorylase in plants?

Answer 11

Starch phosphorylase

Question 12

Why does glycogen phosphorylase use an inorganic phosphate?

Answer 12

Because this spares having to use ATP.

Question 13

What is the cofactor involved in glycogen phosphorylase? What is it’s function?

Answer 13

Pyridoxal phosphate. This acts as an acid/base catalyst.

Question 14

Describe the mechanism of action of glycogen phosphorylase?

Answer 14

Cleaves the a-1,4 glycosidic bonds in glycogen. The enzyme needs to be in the active (phosphorylated) form. The muscles use Ca2+/calmodulin binding to “speed up” this conversion and the liver responds to glucagon levels.

Question 15

What are three uses of sugar nucleotides other than glycogen synthesis?

Answer 15

Aminohexoses (nucleotide binding)
Vitamin C synthesis
Detoxification

Question 16

What are the two major types of attachment for glucose molecules in glycogen?

Answer 16

a-1,4 and a-1,6 linkages. The a-1,4 linkages create the “chain” and the a-1,6 linkages create the “branching.”

Question 17

What two organs are primarily affected by glycogen storage diseases?

Answer 17

Liver and muscle

Question 18

What are the two functions of the debranching enzyme during glycogenolysis?

Answer 18

Note: glycogen phosphorylase is incapable of removing glucose residues that lie within four residues of a branch point. This is where debranching enzyme helps.

Glucanotransferase activity - shift 3 of the a-1,4 residues from 1,6 branch to another 1,4 chain, leaving behind a single glucose unit in a-1,6 glucosidic linkage.

Glucosidase activity - hydrolyzes the a-1,6 glucosidic bond, releasing free glucose and producing a long, unbranched a-1,4 glycosidic chain that is a suitable substrate for continued glycogenolysis by glycogen phosphorylase.

Question 19

What are the two major forms of phosphorylase (glycogen phosphorylase) in glycogenolysis? What are the differences?

Answer 19

Phosphorylase A and phosphorylase B.

A is the phosphorylated, more active form of the enzyme
B is the unphosphorylated, less active form.

Question 20

Where are glycogen granules located within the cell?

Answer 20

Cytosol

Question 21

Describe glycogen granules.

Answer 21

Linear chains of glucose in a a-1,4 glycosidic linkage, with a-1,6 glycosidic linkages forming branches after approximately every 8 to 10 glucose residues.

Question 22

Why is the glucose moiety in the core of glycogen nonreactive?

Answer 22

Because it is covalently bound to glycogenin.

Question 23

Glucose derived from liver glycogen is used to…

Answer 23

maintain the concentration of glucose in the blood.

Question 24

Glucose derived from muscle glycogen is used to…

Answer 24

provide energy for muscle work. It does not leave the muscle cell.

Question 25

What hormone is primarily responsible for glycogenolysis in the liver?

Answer 25

Glucagon. Glycogenolysis in liver occurs in the fasted state.

Question 26

When is glycogen synthesis in muscle most rapid?

When does glycogenolysis in muscle occur?

Answer 26

When blood glucose levels are high.

During muscle contraction.

Question 27

What is the fate of glucose 1-phosphate in muscle cells?

Answer 27

Transfer of the phosphate group from C1 to C6 by phosphoglucomutase, yielding glucose 6-phosphate which then enters the glycolytic pathway and catabolized to provide energy for muscle contraction.

Question 28

What is the role of phophoglucomutase? How is it regulated?

Answer 28

Catalyze the reversible reaction between glucose 1-phosphate and glucose 6-phosphate. The direction of the reaction is determined by the relative concentrations of substrate and produce.

Question 29

What is the fate of glucose 1-phosphate in liver cells?

Answer 29

Transfer of the phosphate group from C1 to C6 by phosphoglucomutase, yielding glucose 6-phosphate, which is released from the cell as free glucose.

Question 30

What is the activated form of glucose that is used for glycogen synthesis?

Answer 30

UDP-1-glucose

Question 31

Glucose is taken up by cells and phosphorylated by hexokinase (in muscle) or glucokinase (in liver) to generate glucose 6-phosphate. What is the next enzyme involved if glucose 6-phosphate is to undergo glycogen synthesis?

Answer 31

Phosphoglucomutase. This enzyme catalyzes the reversible reaction of glucose 6-phosphate to glucose 1-phosphate.

Question 32

Which enzyme transfers the activated UDP-1-glucose to the 4-hydroxyl group at one of the nonreducing ends of the branched glycogen molecule, ultimately forming a new a-1,4 linkage and extending the carbohydrate chain by one glucose unit?

Answer 32

Glycogen synthase