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One rate-limiting step of glycolysis is the conversion of Fructose-Phosphate (Fruc-P) to Fructose-Bisphosphate (Fruc-BP), catalysed by Phosphofructokinase (PFK). The reaction involves hydrolysing one ATP to ADP.

The reverse reaction of gluconeogenesis is catalysed by Fructose-Bisphosphatase (FBP). This reaction uses 1 H2O for hydrolysis and yields 1 phosphate (Pi).

One would expect the cell to be utilising only one of the two reactions at any one time, either to break down glucose or to generate it. However, the case is commonly that both reactions are taking place simultaneously in an equilibrium [ref tbc; I think this was in Voet & Voet Biochemistry], cycling Fruc-BP to no apparent benefit, at the expense of valuable ATP.

What is the purpose of such futile cycles?

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I'd like to note that I have known the reason before, and I'm almost certain I have it somewhere in my material from last year - I just realised that I couldn't explain it to someone right now and I believe this question would be a valuable addition to our site. I am currently busy, but will answer it myself once I am free (unless someone else does beforehand). –  Armatus May 2 '13 at 23:10
    
gluconeogenesis is not continuously happening and is tightly regulated at the step of FBP. Therefore there is no continuous futile cycle –  WYSIWYG May 3 '13 at 4:20
    
PFK is not rate-limiting, this has been shown many time. It can't be since it is regulated. –  rhody Oct 29 '13 at 1:23
    
In some organisms such as bubble bees, futile cyclying in muscle is thought to generate heat in flight muscles. Also a futile cycled system can act as a flux amplifier. –  rhody Oct 29 '13 at 1:24
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As far as I am aware there are various regulatory mechanisms (allosteric regulation, hormonal control) in place to minimise futile cycling by ensuring that phosphofructokinase (glycolysis) and fructose 1,6 bisphosphatase (gluconeogenesis) are not both active at the same time.

See this Wikipedia article for a starting point.

The precise regulation of PFK1 prevents glycolysis and gluconeogenesis from occurring simultaneously. However, there is substrate cycling between F6P and F-1,6-BP. Fructose-1,6-bisphosphatase (FBPase) catalyzes the hydrolysis of F-1,6-BP back to F6P, the reverse reaction catalyzed by PFK1. There is a small amount of FBPase activity during glycolysis and some PFK1 activity during gluconeogenesis. This cycle allows for the amplification of metabolic signals as well as the generation of heat by ATP hydrolysis.

The last two sentences of the quotation are probably what you remember reading about. There is a theory that having some cycling provides an element of regulatory responsiveness. See here for example. I'm not enough of a mathematician to explain this idea well.

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