Lecture (5) Glycogen Metabolism Flashcards
what are the Functions of glycogen?
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.
Every 8-10 glucose units (glucosyl residues), there is a branch containing α 1,4-linkage
T or F?
F, α-1,6 linkages.
……….is the source of all glucosyl residues that are added to the growing glycogen molecules.
UDP-glucose
the Regulating enzyme responsible for Glycogenesis is………….
glycogen synthase
Synthesis of UDP-glucose Requires……. for glucose phosphorylation &………… for glucose activation
ATP
UTP (uridine triphosphate)
glycogen synthase responsible for…………
It forms α-1, 4 glycosidic bonds in glycogen
Branching enzyme is responsible for…………
Formation of branches in glycogen (α-1, 6 linkage)
write short notes about Glycogenesis steps.
(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.
In absence of glycogen fragment, a specific protein called………….., can serve as an acceptor of glucose residues.
glycogenin
………………characterized by the presence of tyrosine amino acid at its end which acts as an acceptor of glucose residue in the glycogenesis process.
glycogenin
Transfer of the first molecule of glucose from UDP-glucose to glycogenin is catalyzed by…………..
glycogen initiator synthase.
…………… is the key enzyme of glycogenolysis
Phosphorylase
what is the mood of action of Phosphorylase?
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
what is the mood of action of the Debranching enzyme?
It causes hydrolysis of the glucosyl residue at α-1,6 branches (i.e breaking the bond by addition of H2O)
what are the Products of Glycogenolysis?
are glucose-1-P(greater amount) & free glucose (small amount).
write a short note about Glycogenolysis.
•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).
what is the Fate of glucosyl units released from glycogen?
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.
Muscle, although it stores glycogen, does not contribute glucose to blood, why?
due to the absence of a glucose-6-phosphatase enzyme in muscle.
what is the mechanism of action of glucagon hormone in Glycogenesis & Glycogenolysis?
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.
what is the mechanism of action of insulin hormone in Glycogenesis & Glycogenolysis?
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.
Von Gierke’s disease occurs due to…………. deficiency, leads to……………
Glucose-6-phosphatase Deficiency
- 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.
- Glycogen accumulates in the liver causing hepatomegaly.
- The block in gluconeogenesis results in an accumulation of lactate leading to lactic acidosis.
- Hyperlipidaemia results from increased fat synthesis.
McArdle’s disease occurs due to…………. deficiency, leads to……………
(Muscle phosphorylase deficiency)
Muscle unable to break glycogen for energy production leading to Muscle cramps or weakness during physical activity. No hepatomegaly or hypoglycemia.
Her’s disease occurs due to…………. deficiency, leads to……………
Liver phosphorylase deficiency Hepatomegaly and episodes of hypoglycemia when fasting; often no symptoms
