Relationship between PPP and glycolysis

[laczlacylaci]

Can we assume that as the pentose phosphate pathway occurs, it stimulates/turns on glycolysis (and turns off gluconeogenesis)?

My reasoning is because the products of the PPP (G3P and F6P) can go back into glycolysis.
Is there a time where the PPP is shut off?

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[wizzed101]

No. The primary products are R5P and NADPH.
From glucose -> Ru5P, all involved reactions are irreversible.

Ru5P can become R5P to make nucleotides, or can become R5P and Xu5P in 1:2 ratio to recycle back to glycolysis. However, the set of reactions for the latter process are all reversible and, in a sense, is NOT a component of the PPP. If demands for R5P is high, the glycolysis metabolites are actually funneled into making R5P without going through the first stage of the PPP.

 

[theonlytycrane]

In addition to what @wizzed101 says above, think of p3 as separate from glycolysis / gluconeogenesis. The oxidative phase of p3 happens to make r5p and NADPH. This is a much different function than metabolizing glucose in glycolysis or building up glucose in gluconeogenesis. The pathways have a connection point through intermediates such as g6p, f6p, and g3p, but they serve different purposes and won't always be regulated together or reciprocally like other pathways.