2. Carbohydrate metabolism

Pentose Phosphate Pathway

 Pentose Phosphate Pathway

 

PPP

The Pentose Phosphate Pathway (PPP) is primarily an anabolic process that takes place in the cytoplasm of cells. It runs parallel to the glycolysis pathway, branching  off at step 2, G6P. It can rejoin at F6P depending on cell needs.

G6P –> (PPP) –> Ribose-5-phosphate, reduces NADP+ to generate NADPH.

This is irreversible and is called the oxidative phase (OP). The R5P leads to PRPP, which leads to nucleotides.

The return path, the non-oxidative phase, is reversible. R5P converts back to F6P to rejoin the glycolytic pathway.

Regulation

  • High [NADPH] inhibits
  • High [NADP+} activates

Note that every carbon is conserved when converting from R5P to F6P. If there are N molecules of R5P, there will be (5/6)*N molecules of F6P. Integer outcomes only, of course.

image from the harvardX lecture slide, copyright is theirs.
image from the harvardX lecture slide, copyright is theirs.

Tissue-Specific Behaviors

Epithelial Cells: high rate of cell replication. Needs nucleotides, this R5P will be consumed more quickly.  Gluconeogenesis direction runs on the glycolytic pathway, allowing F6P to branch off toward R5P, while G6P will route down the oxidative phase to make even more R5P. PRPP > ATP

Adipocytes: Fatty acid synthesis needs NADPH. So it needs the oxidative phase to run. Once complete, it’ll veer off toward F6P, which is running in reverse (gluconeogenesis) to loop from G6P down the OP again. Loop. NADPH > ATP

Hepatocyte: Needs ATP and NADPH. Gly runs forward, PPP loops.

Muscle lacks the OP of the PPP.

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