The Hexose MonoPhosphate Shunt

The Hexose Monophosphate is also known as “Pentose phosphate Pathway” (PPP). This is alternative Glucose oxidation pathway. The hexose monophosphate pathway is used for  production of NADPH from  NADP. The NADPH is required  for  biosynthetic reactions  such  as fatty  acid synthesis, cholesterol  synthesis, drug  reduction, and as a  cofactor for  some  non-synthetic enzymatic reactions.  In addition, it  is used for  the  production of ribose for  nucleotide and nucleic  acid synthesis. The hexose monophosphate pathway also allows the entry  of some  carbohydrates  into the  glycolytic  pathway (especially ribose, but  also some others), and therefore acts as a connection route between different pathways. Steroidogenic  tissues,  red  blood  cells,  and the  liver  are  the  major sites  of hexose monophosphate  pathway.  Muscle  has  small  amounts  of some  of the  hexose monophosphate pathway enzymes,  because  it  has  little  need for  synthetic  reactions, and therefore,  little  need for  NADPH. The muscle, however, like all  tissues, needs to be able to synthesize ribose  in order to make nucleotides and nucleic acids.
Explanation:
A single passage of glucose-6-phosphate through the hexose monophosphate shunt oxidizes it to the C5-sugar ribulose-5-phosphate, releasing one molecule of CO2. In the process, two molecules of hydrogen are transferred to NADP+, yielding NADPH.
HMP pathway
The Ribulose-5-phosphate can be turned into ribose-5-phosphate and then used for the biosynthesis of nucleotides. Alternatively, it can be fully oxidized to yield more CO2 and NADPH. The hexose monophosphate shunt therefore provides a second means for complete degradation of glucose to CO2, apart from the the glycolysis / TCA pathway we have seen before. The purpose of this second oxidative pathway consists not in the regeneration of ATP but in the formation of NADPH. This coenzyme is required in many biosynthetic reactions, some of which we will consider below. Glycolysis and TCA don’t fill this need, because all hydrogen they abstract accumulates as NADH or FADH2.
Dehydrogenation of pyruvate and the TCA occur in the mitochondria, which is useful because the NADH generated is then fed into the respiratory chain. In contrast, the hexose monophosphate shunt occurs entirely in the cytoplasm. This is in keeping with the fact that most of the biosynthetic reactions involving NADPH also occur in the cytoplasm (or in the ER, which is still outside the mitochondrion). 
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