Why do we need ATP from Gluconeogenesis? - USMLE Forums
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Old 04-08-2011
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Biochemistry Why do we need ATP from Gluconeogenesis?

Hi,

Why does ATP stimulate gluconeogenesis? Why do we need more ATP from gluconeogenesis since we have alrealdy got ATP?
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Old 04-08-2011
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I think there are a few things that are important to keep in mind:
  • The liver is not making glucose for itself, but for the rest of the body; the liver and the rest of the body have different dispositions in the fed/fasting states.
  • Gluconeogenesis is the opposite direction from glycolysis. Glycolysis and the ETC are more direct sources of ATP than gluconeogenesis, so a high-energy state will stimulate things to go in the direction of gluconeogenesis.
hope that helps!
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Old 04-08-2011
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Hi, Mondoshawan, thank you for your insight.

I agree with your comment on " the liver and the rest of the body have different dispositions in the fed/fasting states"

However, gluconeogenesis occurs after fasting for 12 hours. so it does not go opposite of glycolysis.

I am still puzzled why ATP stimulate gluconeogenesis.
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Old 04-08-2011
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Quote:
Originally Posted by usluipek View Post
... However, gluconeogenesis occurs after fasting for 12 hours, so it does not go opposite of glycolysis...
I think that, while it takes 12 hours for gluconeogenesis to approach the contribution of glycogenolysis, gluconeogenesis begins as soon as you run out of glucose (for glycolysis).
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Old 04-08-2011
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But the question is why ATP stimulates gluconeogenesis in a fast state?
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Old 04-08-2011
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I think you are focusing on the wrong thing - the main regulation of gluconeogenesis is glucagon. Glucagon binding -> cAMP etc. phosphorylates the bivalent enzyme PFK-2 and turns it in the direction of f1,6bisphosphatase. Glucagon also upregulates transcription of PEPCK...
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Old 04-08-2011
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Default The cell membrane cannot take ATP

It's about change of currency.
The cell membrane cannot just grab ATP, it needs glucose.
During fasting you have to translate those ATPs into glucose in order of the cell membrane to utilize the glucose and turn it back again into ATP inside the cell that can be then utilized in the ETC.
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Old 05-21-2011
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Glycolysis yields 2 ATP while gluconeogenesis expends 6 ATP. During fasting state, liver puts out more glucose to maintain blood glucose levels. If both glycolysis and gluconeogenesis occurred simultaneously, it would be futile as 4 high energy bonds would be wasted per cycle. To prevent waste of energy, the rate limiting enzymes of glycolysis and gluconeogenesis are reciprocally regulated.

During fast, blood glucose levels fall and there is a subsequent decline in insulin and increase in glucagon. The decreased insulin/glucagon ratio activates degradation of glycogen, protein, and triacylglycerols. This produces an abundance of ATP, which will be needed as gluconeogenesis is an energy intensive process. So, high ATP causes most biosynthetic pathways to slow down while stimulating gluconeogenesis.
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