Difference Between Oxidative and Nonoxidative Pentose Phosphate Pathway

The key difference between oxidative and nonoxidative pentose phosphate pathway is that oxidative pentose phosphate pathway generates nicotinamide adenine dinucleotide phosphate (NADPH). Meanwhile, nonoxidative pentose phosphate pathway generates pentose sugars.

The pentose phosphate pathway is a metabolic pathway that takes place parallel to glycolysis. It consists of two distinct pathways as oxidative pentose phosphate pathway and nonoxidative pentose phosphate pathway. NADPH is generated in the oxidative phase, while pentose sugars are generated through the non-oxidative phase. In addition to pentose sugars and NADPH, this pathway generates ribose 5-phosphate, which is a precursor for nucleotide synthesis.

CONTENTS

1. Overview and Key Difference
2. What is Oxidative Pentose Phosphate Pathway
3. What is Nonoxidative Pentose Phosphate Pathway
4. Similarities Between Oxidative and Nonoxidative Pentose Phosphate Pathway
5. Side by Side Comparison – Oxidative vs Nonoxidative Pentose Phosphate Pathway in Tabular Form
6. Summary

What is Oxidative Pentose Phosphate Pathway?

Oxidative pentose phosphate pathway is the first of two phases of the pentose phosphate pathway. The term “oxidative” has been given to this phase since oxidation takes place in this pathway, and at least one electron is removed in each reaction. The oxidative phase starts with the conversion of glucose 6 phosphate into 6-phosphogluconolactone by an enzyme called glucose 6-phosphate dehydrogenase.

During this conversion, one molecule of NADPH (reducing equivalents) is generated by taking the electron released. Then the 6-phosphogluconolactone transforms into 6-phosphogluconate by 6-phosphogluconolactonase. Finally, the 6-phosphogluconolactonase converts into ribulose 5-phosphate by an enzyme called 6-phosphogluconate dehydrogenase, producing another molecule of NADPH. Therefore, at the end of the oxidative phase, glucose 6 phosphate converts into Ribulose 5-phosphate, producing two molecules of NADPH. NADPH is used in reductive biosynthesis within cells, such as fatty acid synthesis.

What is Nonoxidative Pentose Phosphate Pathway?

Nonoxidative pentose phosphate pathway is the second phase of the PPT. Ribulose 5-phosphate (which is the final product of the oxidative phase) is the starting compound of nonoxidative pentose phosphate pathway. The nonoxidative phase starts with the transformation of ribulose 5 phosphate into ribose 5-phosphate by ribose-5-phosphate isomerase and ribulose 5 phosphate into xylulose 5-phosphate by ribulose 5-phosphate 3-epimerase. Then, both these products are converted into glyceraldehyde 3-phosphate  and sedoheptulose 7-phosphate by transketolase. Then the enzyme called transaldolase converts them into erythrose 4-phosphate and fructose 6-phosphate.

Finally, xylulose 5-phosphate and erythrose 4-phosphate are converted into glyceraldehyde 3-phosphate + fructose 6-phosphate by transketolase. The products of the nonoxidative pentose phosphate pathway are used in the synthesis of nucleotides and aromatic amino acids.

What are the Similarities Between Oxidative and Nonoxidative Pentose Phosphate Pathway?

• Oxidative and nonoxidative pentose phosphate pathway are two phases of PPT.
• They occur in the cytoplasm of cells.
• The oxidative phase is followed by a nonoxidative phase.
• ATP is not produced or used in both phases.
• The nonoxidative phase uses the product of the oxidative phase.

What is the Difference Between Oxidative and Nonoxidative Pentose Phosphate Pathway?

The oxidative phase is the first stage of the pentose phosphate pathway in which glucose 6 phosphate is converted into ribulose 5 phosphate by producing NADPH. The nonoxidative phase, meanwhile, is the second stage of pentose phosphate pathway in which ribulose 5 phosphate is converted into fructose-6-phosphate and glyceraldehyde-3-phosphate. So, this is the key difference between oxidative and nonoxidative pentose phosphate pathway. The product of the oxidative phase, the NADPH, helps to build other molecules while the product of nonoxidative phase, ribose-5-phosphate sugar, is used to make DNA and RNA.

Moreover, the overall reaction of oxidative phase is glucose 6-phosphate + 2 NADP⁺ + H₂O → ribulose 5-phosphate + 2 NADPH + 2 H⁺ + CO₂, while the overall reaction of nonoxidative phase is 3 ribulose-5-phosphate → 1 ribose-5-phosphate + 2 xylulose-5-phosphate → 2 fructose-6-phosphate + glyceraldehyde-3-phosphate. Therefore, this is also a difference between oxidative and nonoxidative pentose phosphate pathway.

The below infographic summarizes the difference between oxidative and nonoxidative pentose phosphate pathway.

Summary – Oxidative vs Nonoxidative Pentose Phosphate Pathway

Oxidative and nonoxidative are two distinct phases of the pentose phosphate pathway. Oxidative pathway is the first phase and it is followed by nonoxidative phase. NADPH is produced during the oxidative pentose phosphate pathway and the reactions are not reversible. In contrast, pentoses are produced during the nonoxidative pentose phosphate pathway and the reactions are reversible. Products of the pentose phosphate pathway are useful in different ways. In this regard, ribose-5-phosphate sugar used to make DNA and RNA while the NADPH molecules which help with building other molecules. So, this is the summary of the difference between oxidative and nonoxidative pentose phosphate pathway.

Reference:

1. Berg, Jeremy M. “20.3 The Pentose Phosphate Pathway Generates NADPH and Synthesizes Five-Carbon Sugars.” Biochemistry. 5th Edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here.
2. “Pentose Phosphate Pathway.” Wikipedia, Wikimedia Foundation, 25 Feb. 2020, Available here.

Image Courtesy:

1. “Ox Pentose phosphate pathway” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia
2. “Nichtox Pentosephosphatweg” By Yikrazuul – Own work (CC BY-SA 3.0) via Commons Wikimedia