Glycogenolysis and glycogenesis

Question 1

Glycogenesis

Answer 1

Synthesis of glycogen from glucose

Question 2

Glycogenolysis

Answer 2

Breakdown of glycogen

Question 3

Where is glycogen stored

Answer 3

Liver : for maintenance of blood glucose levels

Muscle : for glycolysis during prolonged muslce contraction

Question 4

What is glucose stored as in the body

Answer 4

Glucose is stored as glycogen

Question 5

The 2 types of glycogen synthesized

Answer 5

• Linear chained glycogen

- Branched chain glycogen

Question 6

Step 1a synthesis of linear chain glycogen

Activation

Answer 6

Glucose is converted into glucose 6 phosphate via hexokinase, while converting ATP to ADP.

Question 7

Step 1b synthesis of linear chain glycogen

Activation, phospho-gluco-mutase

Answer 7

Glucose 6 phosphate is isomerized into glucose 1 phosphate via phospho-gluco-mutase.

Question 8

Step 1c synthesis of linear chain glycogen

UDP glucose pyro-phosphorylase Activation

Answer 8

Glucose 1 phosphate is converted into UDP glucose via UDP glucose pyro-phosphorylase. UTP becomes PI in the process.

Question 9

Step 2 synthesis of linear chain glycogen. Initiation step. Current and new glycogen chain.

Answer 9

Current chain - Activated glucose aka UDP glucose will add on to glycogen via glycogen synthase, producing glycogen and UDP. UDP glucose can add on to the glycogen chain one by one.

New chain - UDP glucose can add to glycogenin to make a new chain of glycogen, forming a new chain of glycogen. Glycogenin is the starting point of glycogenolysis where UDP glucose can anchor on to it.

Question 10

How is glucose added to a new current chain?

Answer 10

Glucose (from UDP-glucose) is then transferred to the 4-OH at non-reducing end of glycogen core by glycogen synthase.

Alpha 1,4 glycosidic bond is formed, for linear chain
UDP is attached to carbon 1 of UDP glucose

Question 11

Branching enzymes

Answer 11

alpha 1,6 branch linkage, alpha 1,6 glycosidic bond
Cleave the linear chain and form branches from there.

Glycogen synthase is still needed

Question 12

Glycogenolysis

Answer 12

Breakdown of glycogen by phosphorolytic cleavage.. Sequential removal of glucose residues from glycogen.

Question 13

Step 1 Glycogenolysis

Answer 13

Glycogen + inorganic phosphate via glycogen phosphorylase release glucose from glycogen chain.

Glycogen (n-1) + glucose-1-phosphate is formed.

alpha 1,4 Glycosidic bond is broken to release 1 glucose molecule.

Question 14

Step 2 Glycogenolysis

Answer 14

Glucose-1-phosphate from step 1 becomes glucose 6 phosphate via phosphoglucomutase

Question 15

Step 3 Glycogenolysis

Answer 15

Glucose 6 phosphate can enter liver to become glucose via gluconeogenesis to diffuse into the blood.

Glucose 6 phosphate can enter muscles to synthesize ATP via glycolysis.

Question 16

Glycogen phosphorylase

Answer 16

• Uses inorganic phosphate to cleave a glucose residue from glycogen.
• Breaks alpha-1-4 glycosidic bond.
• Results in the release of glucose-1-phosphate.

Glycogen (n-1) + Glucose-1-phosphate is formed.

Question 17

Debranching enzymes

Answer 17

to debranch the glycogen chain

Question 18

Where do glycogenolysis occur

Answer 18

Liver and muscles
Liver - maintain blood glucose levels. In the liver, there is glucose 6 phosphatase as an extra step to form glucose.

Muscles - dont require glucose 6 phosphatase and enter glycolysis directly

Question 19

Glycogenolysis in muscles

Answer 19

Glycogenolysis depends on availability or (demand for) ATP for muscle function.

Small amount of free glucose is produced, bulk is Glucose-1-phosphate.

G-1-P converted to G-6-P, able to undergo glycolysis to produce pyruvate, pyruvate can undergo lactic acid fermentation for energy.

Absence of glucose-6-phosphatase in muscle. Therefore, not converted to blood glucose.

Question 20

Regulation of muscle glycogen storage during rest

Answer 20

Insulin decrease, glycogen synthesis decrease and glucose transport decrease

Question 21

Regulation of muscle glycogen storage after meal and resting

Answer 21

Insulin increase, glycogen synthesis increase and glucose transport increase

Question 22

Regulation of muscle glycogen storage during exercise

Answer 22

Epinephrine increase, glycogen synthesis decrease, glycogen degradation increase and glycolysis increase.

Question 23

Regulation of liver glycogen storage during rest

Answer 23

Glucagon increase, insulin decrease. Glycogen degradation increase, glycogen synthesis decrease

Question 24

Regulation of liver glycogen storage after carbohydrate meal

Answer 24

Glucagon decrease, insulin increase, glucose increase.

Glycogen degradation decrease, glycogen synthesis increase

Question 25

Regulation of liver glycogen storage after exercise and stress

Answer 25

Epinephrine increase. Glycogen degradation increase, glycogen synthesis decrease