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A '''futile cycle''', also known as a '''substrate cycle''', occurs when two [[metabolic pathway]]s run simultaneously in opposite directions and have no overall effect other than to [[dissipation|dissipate]] energy in the form of [[heat]].<ref>{{cite journal |author=Schwender J, Ohlrogge J, Shachar-Hill Y |title=Understanding flux in plant metabolic networks |journal=Curr Opin Plant Biol |volume=7 |issue=3 |pages=309–17 |year=2004 |pmid=15134752 |doi=10.1016/j.pbi.2004.03.016}}</ref> 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 [[adenosine triphosphate|ATP]].<ref>{{cite journal |author=Boiteux A, Hess B |title=Design of glycolysis |journal=Philos Trans R Soc Lond B Biol Sci |volume=293 |issue=1063 |pages=5–22 |year=1981 |pmid=6115423 |doi=10.1098/rstb.1981.0056}}</ref> 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 [[enzyme]]s involved.<ref>{{cite journal |author=Samoilov M, Plyasunov S, Arkin A |title=Stochastic amplification and signaling in enzymatic futile cycles through noise-induced bistability with oscillations |journal=Proc Natl Acad Sci USA |volume=102 |issue=7 |pages=2310–5 |year=2005 |pmid=15701703 |doi=10.1073/pnas.0406841102 |pmc=548975}}</ref> The cycle does generate heat, and may be used to maintain thermal [[homeostasis]], for example in the [[brown adipose tissue]] of young [[mammal]]s, or to generate heat rapidly, for example in [[insect]] flight muscles and in hibernating animals during periodical arousal from [[torpor]].
It has been reported that the glucose metabolism substrate cycle is not a futile cycle but a regulatory process. For example, when energy is suddenly needed, ATP is replaced by AMP, a much more reactive adenine.

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

<center><math>ATP + H_2 O {\rightleftharpoons} ADP + P_i + heat</math></center>

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

<center>ATP + fructose-6-phosphate [[Image:Biochem_reaction_arrow_forward_NNNN_horiz_med.png]] Fructose-1,6-bisphosphate + ADP</center>

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

<center>Fructose-1,6-bisphosphate + H2O [[Image:Biochem_reaction_arrow_forward_NNNN_horiz_med.png]] fructose-6-phosphate + Pi</center>

Giving an overall reaction of:

<center>ATP + H2O [[Image:Biochem_reaction_arrow_forward_NNNN_horiz_med.png]] ADP + Pi + Heat</center>

That is, [[hydrolysis]] of [[Adenosine triphosphate|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.{{Citation needed|reason=This is direct quote from textbook.|date=September 2011}}

==References==
{{reflist}}
Lehninger Principles of Biochemistry (Nelson, W[1]. H., Freeman, 4th Ed, 2004)

==External links==
* {{MeshName|Futile+cycles}}

[[Category:Metabolism]]
[[Category:Glycolysis]]

{{biochem-stub}}

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