Biochem 2 - glycogen metabolism

Question 1

What type of molecule is glycogen and how is it stored

Answer 1

A polysaccharide
Stored in granules in liver and muscle
Highly branched

Question 2

What is glycogen degraded to in the liver

Answer 2

To glucose-1-phosphate which can be converted to free glucose and exported into blood

Question 3

Is more glycogen stored in liver or muscle

Answer 3

Muscle, overall

Therefore liver needs gluconeogenesis within 24 hours

Question 4

How often is there an alpha 1,6 branch in glycogen

Answer 4

Every 8-14 glucose residues

Must be broken using debranching enzyme

Question 5

What enzyme does the liver have that muscle does not

Answer 5

Liver expresses the enzyme glucose-6-phosphatase - can convert glucose for export to other tissues
Therefore in muscle, G-1-P is converted to G-6-P and enters glycolysis

Question 6

How are alpha 1,4 linkages broken

Answer 6

By phosphorlysis, using glycogen phosphorylase
It removes single units from non-reducing ends of glycogen
Forms glucose-1-phosphate

Question 7

How are alpha 1,6 linkages broken

Answer 7

alpha-1,6 glucosidase - debranching enzyme
Forms free glucose by hydrolysis
10% of glucose mobilised from glycogen is free glucose

Question 8

Action of phosphorylase

Answer 8

Can break alpha 1,4 links up to within 4 glucose units from a branch point

Question 9

Action of transferase

Answer 9

Removes 3 residues from the branch and transfers them to the end of another chain in a alpha 1,4 linkage

Question 10

What happens to a single glucose left on a branch

Answer 10

Removed by debranching enzyme - has to break an alpha 1,6

This creates free glucose

Question 11

What is UDP glucose used for

Answer 11

For glycogen synthase to add glucose units in alpha 1,4 linkages onto a glycogen chain (with at least 4 glucoses)

Question 12

How does glycogen synthesis start

Answer 12

Glycogenin - priming function
UDP glucose donates the first glucosyl residue and attaches it to the tyrosine in the glycogenin
Glycogenic extends the glucose chain by up to 7 additional residues from UDP glucose via alpha 1,4 linkages

Question 13

What does branching enzyme do

Answer 13

Transfers a block of 7 residues from a growing chain to create a new branch with a alpha 1,6 linkage - has to be 4 residues away from a pre-existing branch

Question 14

Why is glycogen a good energy store

Answer 14

sensitive enzymes to regulation by hormones/stress or muscle contraction
Branched

Question 15

What activates/inhibits glycogen phosphorylase

Answer 15

AMP activates

ATP and G-6-P and glucose inhibit

Question 16

What molecule activates glycogen synthase

Answer 16

G-6-P activates

Question 17

In what state is glycogen phosphorylase active

Answer 17

When it has a phosphate group attached
Protein kinase activates it
And so does the cAMP cascade
This phosphorylation requires energy

Question 18

When is glycogen synthase active

Answer 18

When it is dephosphorylated
Done by protein phosphatase
The reaction that makes it active gives of a Pi - produces energy