Glycolysis and pentosephosphate pathway paradox

[NghiaFromVN]

In glycolysis, I learned that the reaction Fructose6P to Fructose1,6BP is irreversible, yet in the non oxidative phase PPP, somehow F1,6P is converted to F6P so that Glyceraldehyde 3P become Glucose6P. How is this possible? If the reaction is reversible ,will the regulation of the enzyme be pointless? I will add an image for this. As you can see, G3p is transformed to F6P which is only possible via F16Bp

Edited September 7, 2016 by NghiaFromVN

[CharonY]

If I understood you correctly, I think there may be two potential points of confusion. The first is that an irreversible reaction only means that with a particular enzyme (under physiological conditions) the reaction cannot be reveresed. In this case the phosphofructokinase reaction. A direct reversal would be catalyzed by the fructose-1,6 biphosphatase which catalyzes the reaction from F1-6 PP to F6P, This is an irreversible step of gluconeogenesis.

In the pentose phosphate pathway F6P is formed from glycerylaldehyde-3P via a transaldolase reaction, requiring sedoheptulose 7P, if memory serves.

[NghiaFromVN]

Thanks a million! But I think F6p is actually formed from g3p and DHAP (dihidroxyacetone phosphate), which is the reversed step of glycolysis 4th reaction

[NghiaFromVN]

By the way, does that mean the enzyme of reversible reactions catalyze both ways?

[CharonY]

The aldolase reaction from DHAB is found in glycolysis but not in the pentose phosphate pathway (note that there are intersecting products/substrates so combined pathways can be confusing to read if they do not provide the specific reaction). In the pentose phosphate pathway specifically look at the non-oxidative branch, you will find a transaldolase reaction there.

 

And yes typically the enzymes catalyze both directions, with the few exceptions that are considered to be irreversible.

[BabcockHall]

Fructose 6-phosphate and glyceraldehyde 3-phosphate may also be formed in one of the reactions catalyzed by transketolase.