Question of the day- Energy required for conversion of glycerol to glucose?

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?

A. 1

B. 2

C. 3

D. 4

E. 6

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.

 

Steps of glycolysis and gluconeogenesis

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-

Reaction of gluconeogenesis

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).

 Energetics of gluconeogenesis

Figure2- showing reactions and energetics of gluconeogenesis

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