Pentose phosphate pathway

• Pentose phosphate pathway is also known as hexose monophosphate pathway, oxidative phosphate pathway, Warburg-Limpam-Dickens cycle and phosphogluconate shunt.
• It is an alternative method of oxidation of glucose that occurs inside the cytoplasm.
• This pathway takes place in the presence of NADP in the cytoplasm and is mostly seen in mature cells.
• Out of six molecules of glucose only one is completely oxidized while the other five are regenerated through this pathway.
• Oxidation of glucose causes the formation of NADPH2 from NADP+.
• One molecule of glucose is completely oxidized to form 12 NADPH2 which is equivalent to 36 ATP molecules.
• Glycolysis and Pentose phosphate shunt are two major pathways for the catabolism of glucose but the two pathways have very less in common.
• Formation of glucose-6-phosphate is common to both pathway
• In the pentose phosphate pathway, NADP+ is utilized in place of NAD+ which is used in glycolysis.
• Also ATP is not formed in the pentose phosphate pathway through substrate level phosphorylation, whereas ATP is produced in glycolysis.
• A number of tetroses and pentoses are produced in this pathway which can be used for the synthesis of nucleosides, nucleotides, nucleic acids, lignin, anthocyanins, indole 3-acetic acid and many other compounds.
• RBCs depend on HMP shunt for NADPH2, which is required to maintain glutathione in the reduced state and hence helps in maintaining the integrity of the RBC membrane.
• The deficiency of enzyme glucose-6-phosphate dehydrogenase can cause hemolytic anemia in RBCs as they are not able to maintain reduced glutathione levels.
• Pentose phosphate pathway is mostly seen in RBCs, liver, mammary glands, adipose tissue and adrenal cortex.

Steps of pentose phosphate pathway:

• Pentose phosphate pathway starts with glucose-6-phosphate and is divided into following steps:

• Step 1: Glucose-6-phosphate is oxidized to form 6-phosphogluconate.
• Step 2: Decarboxylation takes place to form ribulose-5-phosphate.
• Step 3: Ribulose-6-phosphate is converted into two different forms of 5 carbon compounds, xylulose-5-phosphate and ribose-5-phosphate and these two compounds are interconvertible.
• Step 4: Two ribose-5-phosphate can combine to form a 10 carbon compound which breaks to form a 3 carbon molecule (glyceraldehyde-3-phosphate) and 7 carbon compound (Sedoheptulose-7-phosphate).
• Step 5: The 3 carbon and the 7 carbon compounds are interconvertible and form 4 carbon erythrose-4-phosphate and 6 carbon fructose-6-phosphate. Erythrose-4-phosphate gives rise to amino acid and fructose-6-phosphate enters into glycolysis.

Frequently asked questions (FAQ’s)

Q1. What are the other names for the pentose phosphate pathway?
Ans :

• Pentose phosphate pathway is also known as hexose monophosphate pathway, oxidative phosphate pathway, Warburg-Limpam-Dickens cycle and phosphogluconate shunt.

Q2. How many ATP molecules are formed through substrate level phosphorylation in the pentose phosphate pathway?
Ans :

• No ATP molecules are formed through substrate level phosphorylation in the pentose phosphate pathway.

Q3. What is the location for the pentose phosphate pathway in the cell?
Ans :

• Pentose phosphate pathway takes place in the cytoplasm of the cell.

Q4. At which places does the pentose pathway commonly occur?
Ans :

• Pentose phosphate pathway mostly occurs in RBCs, liver, mammary glands, adipose tissue and adrenal cortex.

Q5. How many NADPH2 are formed by complete oxidation of 1 glucose through the pentose phosphate pathway?
Ans :

• One molecule of glucose is completely oxidized to form 12 NADPH2 molecules.