The Pentose Phosphate Pathway

In this post, we'll discuss the function and regulation of the pentose phosphate pathway as well as the roles of NADPH and the major synthetic processes that require NADPH. Enjoy! :)

The Pentose Phosphate Pathway

The Pentose Phosphate Pathway (PPP) is also known as the hexose monophosphate shunt and the phosphogluconate pathway. It starts from glucose but is not cental to glucose homeostasis. There are two main functions of the PPP:

1. An oxidative pathway that produces NADPH for reductive synthesis. eg. for fatty acid synthesis or steroid hormone synthesis.
2. A non-oxidative pathway that produces ribose 5-phosphate for synthesis of nucleotides. 

Any unused intermediates are returned to glycolysis at the fructose-6-phosphate or glyceraldehyde-3-phosphate stage. 

All cells require NADPH for reductive detoxification and most cells require ribose 5-phosphate for nucleotide synthesis. Thus, the pathway is found in the cytosol of all cells. A higher concentration of NADPH exists in the liver and adipose tissue.

Regulation:

This pathway is quite complex and for this unit we only need to know the general outline. The PPP has two distinct parts, an oxidative phase and a non-oxidative phase. The oxidative phase involves the conversion of glucose-6-phosphate to ribulose-5-phosphate. One of the enzymes invloved in this part is glucose-6-phosphate dehydrogenase which is the rate limiting step of the PPP. NADPH is an allosteric inhibitor of this enzyme so the amount of NADPH present in the cytosol regulates the PPP. 

The non-oxidative phase of the PPP involves the conversion of ribulose-5-phosphate to fructose-6-phosphate over a series of steps which involves several enzymes.

That's all we need to know for this post. See you next time :)