WordNet

pass through a cycle; "This machine automatically cycles"
an interval during which a recurring sequence of events occurs; "the never-ending cycle of the seasons" (同)rhythm, round
recur in repeating sequences
a single complete execution of a periodically repeated phenomenon; "a year constitutes a cycle of the seasons" (同)oscillation
a periodically repeated sequence of events; "a cycle of reprisal and retaliation"
a series of poems or songs on the same theme; "Schuberts song cycles"
cause to go through a recurring sequence; "cycle the laundry in this washing program"
producing no result or effect; "a futile effort"; "the therapy was ineffectual"; "an otiose undertaking"; "an unavailing attempt" (同)ineffectual, otiose, unavailing
the sport of traveling on a bicycle or motorcycle

PrepTutorEJDIC

『周期』,循環期;一巡り / 周波,サイクル / (英雄・伝説などを扱った)一連の詩(物語) / 『自転車』;三輪車;オートバイ / 自転車に乗る / 循環する,反復する
(努力などが)むだな,むなしい / (人が)軽薄な,くだらない
自転車に乗ること,サイクリング

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/03/26 03:58:58」(JST)

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A futile cycle, also known as a substrate cycle, occurs when two metabolic pathways run simultaneously in opposite directions and have no overall effect other than to dissipate energy in the form of heat.^[1] 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.^[2] 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.^[3] The cycle does generate heat, and may be used to maintain thermal homeostasis, for example in the brown adipose tissue of young mammals, 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:

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 + P_i

Giving an overall reaction of:

ATP + H₂O ADP + P_i + 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.^[4]

References

^ Schwender J, Ohlrogge J, Shachar-Hill Y (2004). "Understanding flux in plant metabolic networks". Curr Opin Plant Biol 7 (3): 309–17. doi:10.1016/j.pbi.2004.03.016. PMID 15134752.
^ Boiteux A, Hess B (1981). "Design of glycolysis". Philos Trans R Soc Lond B Biol Sci 293 (1063): 5–22. doi:10.1098/rstb.1981.0056. PMID 6115423.
^ Samoilov M, Plyasunov S, Arkin A (2005). "Stochastic amplification and signaling in enzymatic futile cycles through noise-induced bistability with oscillations". Proc Natl Acad Sci USA 102 (7): 2310–5. doi:10.1073/pnas.0406841102. PMC 548975. PMID 15701703.
^ Nelson, D. L., Lehninger, A. L., & Cox, M. M. (2008). Lehninger principles of biochemistry (5th ed., pp. 582-583). New York: W.H. Freeman.

External links

Futile cycles at the US National Library of Medicine Medical Subject Headings (MeSH)

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