Lecture (5) Glycogen Metabolism

Question 1

what are the Functions of glycogen?

Answer 1

1- The function of liver glycogen* is to maintain the blood glucose level, particularly during the early stages of fasting.
2- The function of muscle glycogen is to serve as a source of energy within the muscle itself during muscle contractions.

Question 2

Every 8-10 glucose units (glucosyl residues), there is a branch containing α 1,4-linkage
T or F?

Answer 2

F, α-1,6 linkages.

Question 3

……….is the source of all glucosyl residues that are added to the growing glycogen molecules.

Answer 3

UDP-glucose

Question 4

the Regulating enzyme responsible for Glycogenesis is………….

Answer 4

glycogen synthase

Question 5

Synthesis of UDP-glucose Requires……. for glucose phosphorylation &………… for glucose activation

Answer 5

ATP

UTP (uridine triphosphate)

Question 6

glycogen synthase responsible for…………

Answer 6

It forms α-1, 4 glycosidic bonds in glycogen

Question 7

Branching enzyme is responsible for…………

Answer 7

Formation of branches in glycogen (α-1, 6 linkage)

Question 8

write short notes about Glycogenesis steps.

Answer 8

(1) Synthesis of UDP-glucose:
Requires ATP for glucose phosphorylation &UTP (uridine triphosphate) for glucose activation.
(2) Synthesis of glycogen (formation of α-1→4 linkage):
• Needs glycogen synthase (Regulating enzyme) It forms α-1, 4 glycosidic bonds in glycogen.
(3) Formation of branches in glycogen (α-1, 6 linkage):
• By Branching enzyme.

Question 9

In absence of glycogen fragment, a specific protein called………….., can serve as an acceptor of glucose residues.

Answer 9

glycogenin

Question 10

………………characterized by the presence of tyrosine amino acid at its end which acts as an acceptor of glucose residue in the glycogenesis process.

Answer 10

glycogenin

Question 11

Transfer of the first molecule of glucose from UDP-glucose to glycogenin is catalyzed by…………..

Answer 11

glycogen initiator synthase.

Question 12

…………… is the key enzyme of glycogenolysis

Answer 12

Phosphorylase

Question 13

what is the mood of action of Phosphorylase?

Answer 13

acts on α-1,4 bonds, breaking it down by phosphorolysis (i.e breaking down by the addition of inorganic phosphate “Pi”). So, it removes glucose units in the form of glucose-1-phosphate

Question 14

what is the mood of action of the Debranching enzyme?

Answer 14

It causes hydrolysis of the glucosyl residue at α-1,6 branches (i.e breaking the bond by addition of H2O)

Question 15

what are the Products of Glycogenolysis?

Answer 15

are glucose-1-P(greater amount) & free glucose (small amount).

Question 16

write a short note about Glycogenolysis.

Answer 16

•It is not the reversal of glycogen synthesis.
•It occurs in cytosols of the liver & muscles.
(1) Phosphorylase (the key enzyme of glycogenolysis) acts on α-1,4 bonds, breaking it down by phosphorolysis (i.e breaking down by the addition of inorganic phosphate “Pi”). So, it removes glucose units in the form of glucose-1-phosphate.
(1) Debranching enzyme: It causes hydrolysis of the glucosyl residue at α-1,6 branches (i.e breaking the bond by addition of H2O).

Question 17

what is the Fate of glucosyl units released from glycogen?

Answer 17

a) In liver: Glycogen is degraded to maintain blood glucose level.
- Glucose 1-p → glucose-6- P by mutase
- Glucose 6- P→ glucose by Glucose-6-phosphatase
- Then glucose enters the blood.
b) In muscles: There is no glucose-6-phosphatase, so glucose-6-phosphate enters glycolysis.

Question 18

Muscle, although it stores glycogen, does not contribute glucose to blood, why?

Answer 18

due to the absence of a glucose-6-phosphatase enzyme in muscle.

Question 19

what is the mechanism of action of glucagon hormone in Glycogenesis & Glycogenolysis?

Answer 19

This hormone stimulates phosphorylation of both glycogen synthase (inhibiting it) and glycogen phosphorylase (activating it). Therefore, glycogenolysis will proceed to cause an increase in blood glucose. At the same time, glycogenesis will be inhibited.

Question 20

what is the mechanism of action of insulin hormone in Glycogenesis & Glycogenolysis?

Answer 20

Insulin stimulates the phosphatase enzyme, which removes phosphate from phosphorylase (inhibiting it) and glycogen synthase (stimulating it). Therefore, glycogenesis will proceed and glycogenolysis will be inhibited.

Question 21

Von Gierke’s disease occurs due to…………. deficiency, leads to……………

Answer 21

Glucose-6-phosphatase Deficiency

1. Due to decreased formation of glucose: children with this disorder are prone to severe fasting hypoglycemia since their only source of glucose is dietary carbohydrates.
2. Glycogen accumulates in the liver causing hepatomegaly.
3. The block in gluconeogenesis results in an accumulation of lactate leading to lactic acidosis.
4. Hyperlipidaemia results from increased fat synthesis.

Question 22

McArdle’s disease occurs due to…………. deficiency, leads to……………

Answer 22

(Muscle phosphorylase deficiency)
Muscle unable to break glycogen for energy production leading to Muscle cramps or weakness during physical activity. No hepatomegaly or hypoglycemia.

Question 23

Her’s disease occurs due to…………. deficiency, leads to……………

Answer 23

Liver phosphorylase deficiency Hepatomegaly and episodes of hypoglycemia when fasting; often no symptoms