NAM general Flashcards
(24 cards)
Q28.
What would be the most likely consequence of a deficiency of glucose-6-phosphatase?
A) Inability to store glycogen in the liver
B) Muscle weakness due to reduced glycogen use
C) Hypoglycaemia during fasting
D) Hyperglycaemia after meals
E) Excess glucose excretion in urine
Glucose-6-phosphatase is essential in the final step of glycogenolysis (and gluconeogenesis) in the liver.
It converts glucose-6-phosphate → free glucose, which can then enter the bloodstream.
Muscle lacks this enzyme, which is why it keeps glucose for itself.
So if glucose-6-phosphatase is deficient, the liver can’t release glucose during fasting, leading to:
👉 C) Hypoglycaemia during fasting
Which enzyme is present in the liver but not in muscle, and accounts for the fact that while liver can supply other organs/tissues with glucose, muscle cannot?
Glucose-6-phosphatase
✅ This enzyme is only found in the liver (and kidney), not in muscle.
✅ It converts glucose-6-phosphate → free glucose, which can be exported to the blood.
❌ Muscle lacks this enzyme, so glucose stays trapped as G6P and is used for local ATP production only.
Von Gierke’s disease
💡 Clinical Tip:
In a glucose-6-phosphatase deficiency (like Von Gierke’s disease):
BOTH gluconeogenesis and glycogenolysis happen…
…but they can’t finish → hypoglycaemia
glycogenolysis v gluconeogenesis
🧠 Helpful Visual Summary:
Glycogenolysis:
🔁 “Releasing pre-stored glucose”
🔑 Fast, low-energy, works until glycogen runs out
Gluconeogenesis:
🧪 “Making new glucose from scratch”
🔑 Slower, energy-demanding, but crucial once glycogen is gone
Q: Why is glycogen highly branched?
A: Increases solubility and provides many non-reducing ends for rapid synthesis/breakdown.
Q: What is the reducing end of glycogen?
A: The end with the free anomeric carbon; it’s usually buried and not involved in metabolism.
Q: Main function of liver glycogen?
A: Maintain blood glucose levels during fasting.
Q: Main function of muscle glycogen?
A: Provide local energy for muscle contraction.
Q: Why can’t muscle glycogen supply blood glucose?
A: Lacks glucose-6-phosphatase enzyme.
Q: What is the first step of glycogenesis?
A: Glucose → Glucose-6-phosphate (via hexokinase/glucokinase)
Q: What activates glucose for storage in glycogen?
A: G1P + UTP → UDP-glucose (by UDP-glucose pyrophosphorylase)
Q: What protein acts as the primer for glycogen synthesis?
Glycogenin
Q: Which enzyme adds glucose units to the growing chain?
A: Glycogen synthase
Q: What enzyme introduces branches into glycogen?
A: Branching enzyme (α-1,6-transglycosylase)
Q: What does glycogen phosphorylase do?
A: Cleaves α-1,4 bonds to release glucose-1-phosphate.
Q: What are the two activities of the debranching enzyme?
A: Transferase (moves 3 residues), and α-1,6-glucosidase (removes branch point glucose).
Q: What enzyme converts G1P to G6P?
A: Phosphoglucomutase
Q: What happens to G6P in the liver?
A: It is converted to glucose by glucose-6-phosphatase and released into the blood.
Q: When is glycogen synthase active?
A: When dephosphorylated (stimulated by insulin).
Q: When is glycogen phosphorylase active?
A: When phosphorylated (stimulated by glucagon/adrenaline).
Q: Which enzyme is present in liver but absent in muscle?
A: Glucose-6-phosphatase
Q: What allosterically activates glycogen phosphorylase in muscle?
A: AMP (signals low energy)
Q: What inhibits glycogen breakdown in liver when glucose is high?
A: Glucose binds and inactivates glycogen phosphorylase.
