How many steps in gluconeogenesis are not the exact reversal...

Name: Problem 66, Chapter 22: Fundamentals of General, Organic, and Biological Chemistry
Uploaded: 2022-01-21T09:52:10-08:00
Duration: 1 min
Channel: John Nicolle
Description: How many steps in gluconeogenesis are not the exact reversal of the steps in glycolysis? What kind of conversion of substrate to product does each involve? What is the common theme in each of these reactions?

How many steps in gluconeogenesis are not the exact reversal of the steps in glycolysis? What kind of conversion of substrate to product does each involve? What is the common theme in each of these reactions?

Key Concepts

Common Mechanistic Theme in the Bypass Reactions

A recurring theme in these bypass reactions is the conversion of a highly phosphorylated or carboxylated intermediate to a lower energy product through effective removal of energy-rich groups, such as phosphate groups. They often involve dephosphorylation or combined decarboxylation and dephosphorylation steps to overcome the kinetic and thermodynamic barriers imposed by the irreversible glycolytic reactions.

Bypass Reactions in Gluconeogenesis

Because the irreversible steps of glycolysis cannot simply be reversed, gluconeogenesis employs bypass reactions. These include alternative enzyme-mediated reactions such as the conversion of pyruvate to phosphoenolpyruvate via an initial carboxylation followed by decarboxylation, as well as dephosphorylation reactions that bypass the phosphoryl transfer steps of glycolysis. These reactions are critical for maintaining proper metabolic flux in the reverse direction.

Gluconeogenesis

Gluconeogenesis is the metabolic pathway that synthesizes glucose from non?carbohydrate precursors. It is essentially the reverse of glycolysis in that it generates glucose, but because several glycolytic steps are irreversible, the pathway has its own unique enzymes and reactions to bypass those steps.

Irreversible Reactions in Glycolysis

In glycolysis, there are key steps that are energetically irreversible under cellular conditions—namely, the conversion of glucose to glucose-6-phosphate, the conversion of fructose 6-phosphate to fructose 1,6-bisphosphate, and the conversion of phosphoenolpyruvate to pyruvate. These irreversible reactions set the stage for the need for distinct bypass steps during gluconeogenesis.

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