Question

Which of the steps in glycolysis form substrate cycles in conjunction with gluconeogenesis? (substrate cycles are the same thing as futile cycles) O 2 0

Answer 1

Third step.

The simultaneous carrying out of glycolysis and gluconeogenesis is an example of a futile cycle, represented by the following equation:

ATP + H2O ? ADP + Pi + Heat

For example, during glycolysis, fructose-6-phosphate is converted to fructose-1,6-bisphosphate in a reaction catalysed by the enzyme phosphofructokinase 1 (PFK-1).

ATP + fructose-6-phosphate ? Fructose-1,6-bisphosphate + ADP

But during gluconeogenesis (i.e. synthesis of glucose from pyruvate and other compounds) the reverse reaction takes place, being catalyzed by fructose-1,6-bisphosphatase (FBPase-1).

Fructose-1,6-bisphosphate + H2O ? fructose-6-phosphate + Pi

Giving an overall reaction of:

ATP + H2O ? ADP + Pi + Heat

That is hydrolysis of ATP without any useful metabolic work being done. Clearly, if these two reactions were allowed to proceed simultaneously at a high rate in the same cell, a large amount of chemical energy would be dissipated as heat. This uneconomical process has therefore been called a futile cycle.