Lecture 22 - Glycogen Metabolism I

Question 1

What bonds between glucose are found in glycogen?

L22 S5

Answer 1

Linear polymer:
-α-1,4 glycosidic bond

Branch points:
-α-1,6 glycosidic bond

Question 2

Compare the reducing end and non reducing end of glycogen.

L22 S5

Answer 2

Reducing end:
-attached to protein called glycogenin

Non-reducing end:

• free 4’ -OH
• where degradation and extension occurs

Question 3

What is the first step of glycogenesis?

L22 S12-18

Answer 3

Trapping/activation of glucose:
-phosphorylation of glucose and addition of UDP

• hexokinase/glucokinase phosphorylate glucose to glucose-6-phosphate
• phosphogluomutase converts glucose-6-phosphate to glucose-1-phosphate
• UDP-glucose pyrophosphorylase generates UDP-glucose

Question 4

What is the rate limiting step of glycogenesis?

L22 S14

Answer 4

Glycogen synthase

Question 5

What is the second step of glycogenesis?

L22 S14

Answer 5

Elongation of glycogen primer:

-glycogen synthase adds UDP-glucose to glycogen primer

Question 6

What is the third step of glycogenesis?

L22 S16

Answer 6

Branching:

• when a glycogen fragment gets too long a portion of it breaks off and reattached as a branch via α-1,6 glycosyl bond
• glucosyl (4:6) transferase

Question 7

What are the potential fates of glycogen that undergoes glycogenolysis?

L22 S19

Answer 7

Broken down into glucose for the blood

Broken down into glucose for glycolysis in brain or muscle

Goes through pentose phosphate pathway

Question 8

What are the 4 enzymes of glyogenolysis and what do they do?
Which is the rate limiting enzyme?

L22 S23,25-26

Answer 8

Glycogen phosphorylase (rate limiting):

• adds orthophosphate using vitamin B6
• release glucose as glucose-1-phosphate

Phosphoglucomutase:
-converts glucose-1-phosphate to glucose-6-phosphate

Transferase/debranching enzyme:
-takes 3 of 4 remaining glucose from a brand and debranches it

α-1,6-glucosidase:
-removes branching glucose by cleaving α-1,6 glycosidic bond