[orgo814]

Hi,

I'm having trouble conceptualizing what a substract cycle does. Can both reactions run at the same side? I understand one of the opposing pathways is exergonic while the other (must be) endergonic and therefore coupled to a highly exergonic rxn such as ATP hydrolysis. But can both these opposing pathways. with two different enzymes, go at the same time or is one enzyme activated while the other is inhibited to have flux in one direction? And then vice versa.

[Babcock_Hall]

Both reactions in a substrate cycle are often thermodynamically favorable.  For example, phosphofructokinase-1 and fructose bisphosphatase-1 together catalyze a pair of reactions that constitute a substrate cycle.  The old party line was that substrate cycles are bad, and reciprocal regulation of the enzymes tended to minimize the amount of cycling that goes on.  The new party line is that there is utility to substrate cycling in some circumstances.  I am on the fence...

[orgo814]

But if one of the reactions was deltaG= -30 then wouldnt the other be +30 and therefore not favorable? Also, how do they run at same time? Or do they not? Still confused at the concepts of it

[Yggdrasil]

But if one of the reactions was deltaG= -30 then wouldnt the other be +30 and therefore not favorable? Also, how do they run at same time? Or do they not? Still confused at the concepts of it

The forward and backward reactions are not exactly the same.  In the phosphofructokinase/fructose bisphosphatase example, above, try writing out the full balanced chemical equations for both enzymes.  Where does phosphofructokinase get the phosphate to attach to fructose-6-phosphate?  Where does fructose bisphosphatase transfer the phosphate?

[orgo814]

Assuming phosphofructokinase catalyzes the transfer of a phosphoryl group from ATP. The other enzyme catalyzes the transfer of it to water

[orgo814]

Does the coupling of the high delta G reaction make the reaction thermodynamically favorable?

[Yggdrasil]

Yes.  f6p + ATP --> f-1,6-bp + ADP is thermodynamically favorable and f-1,6-bp + H2O --> f6p + Pi is also thermodynamically favorable, and this can happen because they are different reactions.  In fact, the net reaction from cycling the two reactions simultaneously is ATP + H2O --> ADP + Pi, which is a very thermodynamically favorable process.

[Babcock_Hall]

Butlerw,

AMP is an allosteric activator of phosphofructokinase-1 and an allosteric activator of fructose bisphosphatase-1.  How does this information help us understand a possible substrate cycle involving these two enzymes?