Pentose Phosphate Pathway: Definition, Importance, Steps.

Biochemistry

Pentose Phosphate Pathway: Definition, Importance, Steps.

Pentose phosphate pathway is a type of metabolic pathway where NADPH and Ribose 6 phosphate is generated from glucose 6 phosphate with...

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Pentose Phosphate Pathway Definition

  • Pentose phosphate pathway is a type of metabolic pathway where NADPH and Ribose 5 phosphate is generated from glucose 6 phosphate with a series of reactions.
  • The Pentose Phosphate Pathway is known as the pentose phosphate shunt or phosphogluconate pathway or hexose monophosphate pathway.
  • In this pathway, NADP functions as an electron acceptor and generates NADPH.
  • The generated NADPH functions as an electron donor, needed for reductive biosynthesis or to reduce the damaging effects of oxygen radicals. NADPH helps in the conversation of oxidized glutathione to glutathione, it is a process which contributes to cellular antioxidant defenses.
  • The Pentose phosphate pathway occurs in the cytosol of many microorganisms, because it is the primary location of many associated enzymes. In some plants, parasites, protozoa, and in other organisms this reaction occurs in different organelles such as glycosomes, the endoplasmic reticulum, and plastids.

Importance of Pentose Phosphate Pathway

  • NADPH and glutathione protects the cell from free radicals and hydrogen Peroxide. Reduced glutathione (GSH) destroys the hydrogen peroxide and hydroxyl free radicals and regenerates from its oxidized form (GSSG). For its regeneration requires the NADPH which is produced from the glucose 6-phosphate dehydrogenase reaction.
Effect of NADPH in Pentose Phosphate Pathway
Use of NADPH
  • The product Ribose 5 phosphate is used to make RNA, DNA, and such coenzymes as ATP, NADH, FADH2, and coenzyme A.

Pentose Phosphate Pathway Mechanisms

Pentose Phosphate Pathway: Definition, Importance, Steps.
Pentose Phosphate Pathway

There are two-phase of Pentose Phosphate Pathway such as oxidative phase and non-oxidative phase. The oxidative phase is mainly found in eukaryotic cells and it produces ribulose 5-phosphate as well as NADPH, CO2 from Glucose 6 phosphate.

The non-oxidative phase is ubiquitous. In this phase Glucose 6 phosphate is regenerated from the remaining Ribose 5 phosphate (Product of Pentose Phosphate Pathway) and from the other intermediates of glycolysis ( fructose 6-phosphate and glyceraldehyde 3-phosphate) to continued the Pentose Phosphate Pathway.

Oxidative Phase

Step: 1

In the first step, the Glucose 6 phosphate is converted into 6-phosphoglucono 𝛿 lactone with the help of the enzyme Glucose 6 phosphate dehydrogenase. 

In this reaction, NADP functions as an electron acceptor and convert into NADPH. Mg2+ ion is required for the proper function of the enzyme Glucose 6 phosphate dehydrogenase.

Step: 2

In 2nd step, lactone is released from the 6-phosphoglucono 𝛿 lactone by the enzyme lactonase and convert into 6-phosphogluconate with the addition of one molecule H2o. In this reaction, Mg2+ is required.

Step: 3

The 6-phosphogluconate is decarboxylated with the help of enzyme 6-phosphogluconate

Dehydrogenase and generate Ribulose 5 phosphate. In this reaction, NADP+ accept two released electrons and form NADPH.

Step: 4

The enzyme phosphopentose isomerase converts the  Ribulose 5 phosphate into its isomer termed Ribose 5 phosphate. This reaction is also known as the isomerization reaction.

Oxidative Phase of Pentose Phosphate Pathway
Oxidative Phase of Pentose Phosphate Pathway | Image Source: https://www.microscopemaster.com/pentose-phosphate-pathway.html

Non-oxidative Phase

Step: 1

In the first step, the ribose 5 phosphate is converted into ribulose 5 phosphate with the help of enzyme phosphopentose isomerase. Next, the Ribulose 5 phosphate is converted into Xylulose 5 phosphate with the help of the enzyme Ribose 5 phosphate epimerase.

Non-oxidative Phase of Pentose Phosphate Pathway
Non-oxidative Phase of Pentose Phosphate Pathway (Step 1)

Step: 2

In the 2nd step, xylulose 5 phosphate combined with ribose 5 phosphate and formed sedoheptulose 7 phosphate and Glyceraldehyde 3-phosphate with the help of enzyme transketolase and co-factor thiamine pyrophosphate (TPP).

The enzyme transketolase transfers the C-1 and C-2 from xylulose 5 phosphate to ribose 5 phosphate and thus Sedoheptulose 7 phosphate is formed.

Non-oxidative Phase of Pentose Phosphate Pathway

Step: 3

Sedoheptulose 7 phosphate and Glyceraldehyde 3-phosphate combined with each other and formed Fructose 6 phosphate and erythrose 4 phosphate with the help of enzyme transaldolase and TPP.

The enzyme transaldolase transfers 3 carbon groups from Sedoheptulose 7 phosphate to Glyceraldehyde 3-phosphate as a result Sedoheptulose 7 phosphate converted into erythrose 4-phosphate and Glyceraldehyde 3-phosphate into fructose 6-phosphate. 

Non-oxidative Phase of PPP

Step: 4

In the final step, the erythrose 4-phosphate combined with xylulose 6 phosphate to generate Fructose 6 phosphate and Glyceraldehyde 3-phosphate with the help of enzyme transketolase.

In this reaction, the enzyme transketolase transfers the two carbon groups from xylulose 6 phosphate to erythrose 4-phosphate. 

Non-oxidative Phase of PPP

Two Glyceraldehyde 3-phosphate molecules further undergo the gluconeogenesis process and formed fructose 1,6 bisphosphate. Now fructose 1,6 bisphosphate converts into fructose 6 phosphate with the help of enzyme fructose 1,6 bisphosphatase. 

Next, it converted into Glucose 6 phosphate with the help of an enzyme phosphopeptide isomerase. Now this Glucose 6 phosphate can renters into Pentose Phosphate Pathway and continued the formation of NADPH and Ribose 5 phosphate.

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