Futile cycle

A Futile cycle is when two metabolic pathways run simultaneously in opposite directions and have no overall effect other than wasting energy. For example, if glycolysis and gluconeogenesis were to be active at the same time, glucose would be converted to pyruvate by glycolysis and then converted back to glucose by gluconeogenesis, with an overall consumption of ATP. Futile cycles may have a role in metabolic regulation, where a futile cycle would be a system oscillating between two states and very sensitive to small changes in the activity of any of the enzymes involved. The cycle does generate heat, and may be used to maintain homeostasis.

Glycolysis/Gluconeogenesis

This futile cycle is $ATP\; +\; H\_2\; O\; \{rightleftharpoons\}\; ADP\; +\; P\_i\; +\; heat$

example of futile cycle:

During glycolysis fructose-6-phosphate is converted to fructose-1,6-bisphosphate by the help of the enzyme phosphofructokinase(PFK).

ATP+fructose-6-phosphate---------->fructose-1,6-bisphosphate+ADP --------------(1)

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

fructose-1,6-bisphosphate+H2O---------->fructose-6-phosphate+Pi --------------(2)

the sum total of the two above reaction is-

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 been called a futile cycle.

References

Lehninger Principles of Biochemistry (Nelson, W[1]. H., Freeman, 4th Ed, 2004)

External links