L9 Glycogen Metabolism

Question 1

what is glycogen?

Answer 1

branched polysaccharide
homopolymer of glucose (large)
insoluble
carbohydrate energy store

Question 2

what bonds does glycogen have?

Answer 2

alpha-1,4 (units next to each other) (93%)

alpha-1,6 (branches to other glucose chains) (7%)

Question 3

What are the benefits of glycogen having branches?

Answer 3

• improved solubility
• more sites available for synthesis and degradation interactions
• rapid synthesis and break down

NB - “reducing end” blocked, “non-reducing end” open.

Question 4

where is glycogen stored?

Answer 4

LIVER

• glycogen reservoir
• glucose directly into blood by hepatic glycogenolyis

SKELETAL MUSCLE

• anaerobic glycolysis
• glycogen can not be broken down in skeletal muscle bc lacks glucose-6-phosphatase

Question 5

what is glycogenolysis?

Answer 5

break up of glycogen to G-1-P

Question 6

what is glycogenesis

Answer 6

building new glycogen from G-1-P

Question 7

what is UDP-Glucose?

Answer 7

• activated form of glucose

- G-1-P + Uridine diphosphate

Question 8

what 3 enzymes are required in glycogenesis?

Answer 8

1. UDP-glucose pyrophosphorylase (G-1-P to UDPglu)
2. Glycogen Synthase (alpha 1,4)
3. Branching enzyme/4,6 transferase (alpha 1,6)

Question 9

how are glycogen branches formed?

Answer 9

Glycogen synthase extends chain (alpha 1,4)

Branching enzyme breaks 1,4 (after 10 units) reattaches chain somewhere else (alpha 1,6)

Question 10

Glycogen synthase can only extend existing chains, what primer is used for new chains?

Answer 10

glycogenin

Question 11

glycogen phosphorylase?

Answer 11

• releases glucose molecules one at a time
• glucose released as G-1-P
• breaks alpha 1,4
• from non-reducing end pf glycogen chain (end with free 4’-OH group)

2 types:
phosphorylase b: active form

phosphorylase a: not active

• deactivated by glucose
• not affected by ATP AMP, or G6P

Question 12

Glycogen Phosphorylase can only removed glucose residues from 5 away from branch point - what happens to remaining residues?

Answer 12

remaining residues are added to an existing chain (alpha 1,4 glycosidic bond) catalysed by transferase

• residue at alpha 1,6 branching point is removed with glycogen-debranching enzyme

Question 13

Which 3 enzymes are required for glycogenolysis?

Answer 13

1. Glycogen phosphorylase
2. Transferase
3. Glycogen-debranching enzyme