Green - Glycogen Metabolism

Question 1

How is glycogen stored?

Answer 1

Large cytostolic granules

Complex contains enzymes that synthesize, degrade, and regulate

Question 2

What major enzymes or hormones control glycogen metabolism?

Answer 2

Insulin and Glucagon

Question 3

What type of linkages are found in glycogen?

Structure

Answer 3

1-4 linked glucose with 1-6 branches

Structure:

Reducing and Non-Reducing Ends

Question 4

Where does addition/remobal occur in glycogen?

Answer 4

Addition/Removal of glucose residues occur at non-reducing ends

Question 5

What are three major enzymes of synthesis and breakdown of glycogen?

Answer 5

1. Glycogen Phosphorylase
2. Glycogen Debranching Enzyme
3. Phosphoglucomutase

Question 6

Glycogen Phosphorylase

Answer 6

Removes glucose from non-reducing ends

Requires pyridoxal phosphate as co-factor

Enzyme stops when reaches four glucose molecules from (α1-6) branch point

Further degradation requires glycogen debranching enzyme

Question 7

Glycogen Debranching Enzyme

Answer 7

Removes branches–two major functions

1. Transferase activity moves 3 glucose molecules from branch to non-reducing end
2. Glucosidase activity removes remaining glucose molecule

Question 8

Phosphoglucomutase

Answer 8

Converts G1P to G6P

Question 9

Glucose 6-phosphatase

Where is it expressed?

Answer 9

Conversion of G6P to Glucose

Expressed in liver/kidney, NOT muscle and adipose tissue

Question 10

Where does glycogen synthesis and breakdown occur?

Answer 10

Liver and skeletal muscle

Question 11

What is the starting point for glycogen synthesis?

Answer 11

Starts w/glucose 6-phosphate; converted to glucose 1-phosphate by phosphoglucomutase

Question 12

UDP-glucose Pyrophosphorylase

Answer 12

G1P converted to UDP-glucose

Question 13

Glycogen Synthase

Answer 13

Adds glucose residues to non-reducing end of glycogen

Question 14

Glycogen Branching Enzyme

Answer 14

Adds (α1-6) branches (opposite of debranching enzyme)

Question 15

What does initiation require?

Answer 15

Requires primer

• usually preformed (α1-4) polyglucose chain or branch having at least 8 glucose residues

- Glycogenin Protein acts as both primer and enzyme

Question 16

Glycogenin Protein

Answer 16

Acts as primer and enzyme in glycogen synthesis initiation

Question 17

Regulation of Glycogen Phosphorylase?

Answer 17

Two subunits

a = catalytically active

b = less active

Regulated by phosphorylation

active = phosphorylated

inactive = not phosphorylated

Question 18

Phosphorylase B Kinase

Phosphoprotein Phosphatase 1 (PP1)

Answer 18

Phosphorylase B Kinase: Phosphorylates glycogen phosphorylase to ACTIVATE

PP1: Dephosphorylates glycogen phosphorylase to INACTIVATE

Question 19

Activators of Glycogen Phosphorylase

Inhibitors of Glycogen Phosphorylase

Answer 19

Activators: Epinephrine, Ca²⁺, AMP, Glucagon

Inhibitors: ATP, G6P, Insulin

Question 20

What can act as glucose sensor in liver?

Answer 20

Glycogen Phosphorylase

Glucose Levels Low = Glycogen Phosphorylase A

Glucose Levels High = Glycogen Phosphorylase B

Question 21

Glycogen Synthase

Answer 21

Two forms:

a = active

b = inactive

Regulation by phosphorylation

active = unphosphorylated

inactive = phosphorylated

Question 22

Glycogen Synthase regulation

Answer 22

Activators: Glucose, G6P, Insulin

Inhibitors: Epinephrine, Glucagon

Question 23

High and Low blood glucose:

Glycogen Breakdown / Glycogen Synthesis / Glycolysis

Answer 23

High Blood Glucose:

Decrease Glycogen Breakdown, Increase Glycogen Synthesis, Increase Glycolysis

Low Blood Glucose:

Increase Glycogen Breakdown, Decrease Glycogen Synthesis, Decrease Glycolysis

Question 24

How is carbohydrate metabolism regulated in muscle?

Answer 24

Muscle uses glycogen only for own use

When active, requires ATP generated by glycolysis–muscle lacks enzymatic machinery for gluconeogenesis

Question 25

What type of receptors do muscle cells lack?

Answer 25

Muscle cells lack glucagon receptors

Muscle isozyme of pyruvate kinase NOT phosphorylated by PKA

Muscle cells not inhibited by high cAMP

Question 26

Hypoglycemia

Answer 26

No glycogen

Question 27

Hyperglycemia

Answer 27

Too much glycogen, builds up in tissues