A 45-year-old obese male presents for a routine examination. He has a sedentary life style and eats a high-fat diet. His triglyceride level is abnormally elevated. When glycerol derived from adipose triglycerides is converted to glucose, how many moles of high- energy phosphates are required?
Answer- The correct answer is B- 2 ATP.
Glycerol is phosphorylated at the expense of ATP to form Glycerol-3-P; the reaction is catalyzed by glycerol Kinase. Glycerol-3-P enters the main pathway of Gluconeogenesis at the level of Dihydroxy acetone-P (Figure-1). Since glycerol is a three carbon compound, hence two glycerol and two ATP molecules (one ATP per glycerol) are required for their conversion to glucose.
Figure-1- Glycerol -3-P the product of glycerol kinase reaction is converted to Dihydroxy acetone-P before its entry in to the main pathway of gluconeogenesis.
Energetics of gluconeogenesis
The overall reaction of gluconeogenesis is-
Six nucleotide triphosphate molecules are hydrolyzed to synthesize glucose from pyruvate in gluconeogenesis (Figure-2)
But in case of Glycerol only 2 ATP are hydrolyzed to synthesize glucose. DHAP (Dihydroxy acetone-P) condenses with glyceraldehyde-3-P forming fructose 1, 6 bisphosphate and there is no energy expenditure onwards (Figure-2).
Figure2- showing reactions and energetics of gluconeogenesisPlease help Biochemistry for Medics by "CLICKING ON THE ADVERTISEMENTS" every time you visit us. Thank you!