Pentose Phosphate Pathway: Steps and Functions

The primary breakdown of glucose-6-phosphate is the formation of pyruvate through glycolysis. The pyruvate enters the Krebs cycle to form ATP, the energy currency for the cell. Glucose-6-phosphate has other catabolic fates, which can produce other products required by the cell. 

In some tissues, the glucose-6-phosphate converts into pentose phosphate through the pentose phosphate pathway (PPP). Other names for this pathway are the phosphogluconate pathway or hexose monophosphate shunt. The pathway occurs parallel to glycolysis to produce NADPH (nicotinamide adenine dinucleotide phosphate hydrogen)and ribose-5-phosphate. 

It is an oxidative pathway in which NADP acts as an electron acceptor, forming NADPH. This pathway occurs in rapidly diving cells like bone marrow, intestinal mucosa, and skin, as these cells require pentoses to form RNA, DNA, and coenzymes like ATP (Adenosine Triphosphate), NADH (nicotinamide adenine dinucleotide + hydrogen), and FADH2 (Flavin adenine dinucleotide + two hydrogen).

Other tissues, such as the liver, adipose, and lactating mammary gland, require NADPH for extensive fatty acid synthesis and active synthesis of cholesterol and steroid hormones (in the liver, adrenal gland, and gonads). Besides reductive biosynthesis, NADPH is also necessary to counter the damaging effects of oxygen radicals in erythrocytes and the cells of the lens and cornea.     

Steps of Pentose Phosphate Pathway

The hexose monophosphate shunt has two phases: oxidative and non-oxidative. The oxidative phase generates the NADPH molecule and ribose-5-phosphate, whereas the non-oxidative phase produces intermediates like fructose-6-phosphate and glyceraldehyde-3-phosphate that can enter glycolysis or any nucleotide synthesis process. 

Oxidative Phase

1. Reaction by Glucose-6-dehydrogenase: In this reaction, the substrate glucose-6-phosphate converts into  6-Phosphoglucono-δ-lactone, an intramolecular ester in the presence of the enzyme glucose-6-dehydrogenase. A molecule of NADPH is produced as a by-product. 
2. Reaction by the enzyme 6-phosphogluconolactonase: The product of the first step, 6-Phosphoglucono-δ-lactone, transforms into 6-Phosphogluconate in the presence of the enzyme 6-Phosphogluconolactonase.
3. The reaction by 6-phosphogluconate dehydrogenase: Here, the  6-phosphogluconate is  undergoes oxidation and decarboxylation into ribulose-5-phosphate by the enzyme 6-phosphogluconate dehydrogenase. A molecule of NADPH and Co₂ is also produced in this step. 

Phosphopentose isomerase converts ribulose 5-phosphate to its aldose isomer, ribose 5-phosphate. This is final step in most tissues. 

Non-oxidative Phase

The non-oxidative phase recycles pentose Phosphates to Glucose 6-phosphate. In tissues that require NADPH, the pentose phosphates produced in the oxidative phase of the pathway recycle into glucose 6-phosphate. In this non-oxidative phase, ribulose 5-phosphate is first epimerized to xylulose 5-phosphate, which undergoes a series of rearrangements of the carbon skeletons.

1. Reaction of Phosphopentose Isomerase: The ribulose-5-phosphate from the oxidative phase converts into ribose-5-phosphate in the presence of the enzyme phosphopentose isomerase. 
2. Reaction of Phosphopentose Epimerase: The ribulose-5-phosphate from the oxidative phase of PPP is now epimerised into xylulose-5-phosphate in the presence of phosphopentose epimerase. 
3. First Reaction by Transketolase: Ribose-5-phosphate from the first step of this reaction and Xylulose-5-phosphate from the second reaction is the substrate for this step. These substrates rearrange into glyceraldehyde-3-phosphate and Sedoheptulose-7-phosphate. The enzyme used in this reaction requires thiamine pyrophosphate (TPP) as a cofactor.
4. Reaction of Transaldolase: Glyceraldehyde-3-phosphate and Sedoheptulose-7-phosphate act as the substrate for this reaction. Here, the formation of the fructose-6-phosphate and erythrose-4-phosphate occurs by the reaction of the enzyme transaldolase.   
5. Second Reaction by Transketolase:  The products of the previous step, Erythrose-4-phosphate and Xylulose-5-phosphate, convert into fructose-6-phosphate and glyceraldehyde-3-phosphate. The enzyme transketolase requires thiamine pyrophosphate (TPP) as a cofactor. 

End Products of Hexose Monophosphate Shunt

The end products of the PPP is distinct in the two different phases. The first phase produces NADPH, ribulose-5-phosphate, and carbon dioxide which are the primary products of pentose phosphate pathway. The non-oxidative phase produces metabolite intermediates that enter various metabolic processes. 

1. Products from Oxidative Phase
   • 2 NADPH molecule: It is released in first and third steps of this phase as a byproduct. 
   • CO₂: It is released as a byproduct of the oxidative decarboxylation of 6-phosphogluconate
   • Ribulose-5-phosphate:  It is produced as the final product of the oxidative phase. It can be converted into ribose-5-phosphate, a precursor for nucleotide and nucleic acid synthesis, or it can enter the non-oxidative phase to produce glycolytic intermediates.
2. Products from Non-oxidative phase
   • Ribose-5-phosphate: These are intermediates for the synthesis of nucleotide nucleic acids. 
   • Glyceraldehyde-3-phosphate is another glycolytic intermediate that is useful as a building block for biosynthetic pathways or energy production through glycolysis.
   • Fructose-6-phosphate is an intermediate in glycolysis and gluconeogenesis; it can be converted into glucose-6-phosphate or used to generate energy through glycolysis.
   • Intermediate products like Erythrose-4-Phosphate and Sedoheptulose-7-Phosphate: Erythrose-4-phosphate enters the Shikimate pathway in plants and microorganisms to synthesize aromatic amino acids. Sedoheptulose-7-phosphate participates in the non-oxidative reactions, particularly the transaldolase and transketolase reactions, to produce other intermediates.

End Products of Hexose Monophosphate Shunt

The end products of the PPP are distinct in its two different phases. The first phase produces NADPH, ribulose-5-phosphate, and carbon dioxide, which are the primary products of the pentose phosphate pathway. The non-oxidative phase produces metabolite intermediates that enter various metabolic processes. 

1. Products from the Oxidative Phase
   • 2 NADPH molecule: It is released in this phase’s first and third steps as a byproduct. 
   • CO2: It is released as a byproduct of the oxidative decarboxylation of 6-phosphogluconate.
   • Ribulose-5-phosphate: It is produced as the final product of the oxidative phase. It can be converted into ribose-5-phosphate, a precursor for nucleotide and nucleic acid synthesis, or it can enter the non-oxidative phase to produce glycolytic intermediates.
2. Products from the Non-oxidative phase
   • Ribose-5-phosphate: These are intermediates for the synthesis of nucleotide nucleic acids. 
   • Glyceraldehyde-3-phosphate is another glycolytic intermediate that is useful as a building block for biosynthetic pathways or energy production through glycolysis and Krebs Cycle.
   • Fructose-6-phosphate is an intermediate in glycolysis and gluconeogenesis; it can be converted into glucose-6-phosphate or used to generate energy through glycolysis.
   • Intermediate products like Erythrose-4-Phosphate and Sedoheptulose-7-Phosphate: Erythrose-4-phosphate enters the Shikimate pathway in plants and microorganisms to synthesize aromatic amino acids. Sedoheptulose-7-phosphate participates in the non-oxidative reactions, particularly the transaldolase and transketolase reactions, to produce other intermediates.

Function of Pentose Phosphate Pathway

The pentose phosphate pathway is integral to cellular metabolism, supporting biosynthesis, antioxidant defense, and energy production through its dual roles in producing NADPH and ribose-5-phosphate and generating intermediates for glycolysis. Here are the primary functions:

Production of NADPH

1. NADPH is a vital reducing agent in various anabolic (biosynthetic) processes.
2. Importance:
   • Fatty Acid Synthesis: NADPH reduces the power necessary for the biosynthesis of fatty acids.
   • Cholesterol Synthesis: Similarly, NADPH is crucial for the biosynthesis of cholesterol.
   • Detoxification: NADPH is used to reduce glutathione (GSH), which in turn helps detoxify reactive oxygen species (ROS) and prevents the cell from oxidative damage.
   • Biosynthesis of Nucleotides and Amino Acids: NADPH is required to synthesize certain nucleotides and amino acids.

Synthesis of Ribose-5-Phosphate

1. Ribose-5-phosphate is a precursor for synthesizing nucleotides and nucleic acids (DNA and RNA).
2. Importance:
   • Nucleotide Synthesis: Ribose-5-phosphate synthesizes ATP, NADH, FAD, and coenzyme A.
   • DNA and RNA Production: Essential for synthesizing the ribose backbone in DNA and RNA.

Generation of Metabolic Intermediates

1. Function: The non-oxidative phase of the PPP produces intermediates that can enter glycolysis and gluconeogenesis.
2. Importance:
   • Energy Production: Intermediates such as fructose-6-phosphate and glyceraldehyde-3-phosphate can generate ATP in glycolysis.
   • Flexibility in Metabolism: The pathway provides a means to interconvert sugars, allowing cells to adapt to varying metabolic needs.

Antioxidant Defense:

1. Function: NADPH produced in the PPP is crucial for maintaining the reduced state of glutathione.
2. Importance:
   • Reduction of Glutathione: Glutathione (GSH) is a major antioxidant that protects cells by neutralizing reactive oxygen species.
   • Protection Against Oxidative Stress: Helps protect cells from oxidative damage caused by free radicals.

Support for Rapidly Dividing Cells

1. Function: Provides both NADPH and ribose-5-phosphate for biosynthetic processes.
2. Importance: Rapid Growth and Division: Cells rapidly dividing, such as cancer cells or cells in growing tissues, have high demands for nucleotides and lipids, necessitating an active PPP.

References

1. Aziz H, Mohiuddin SS. Biochemistry, Hexose Monophosphate Pathway. [Updated 2023 May 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK551687/
2. Nelson, D., Lehninger, A., Cox, M., & Nelson, D. (2005). Lecture notebook for Lehninger principles of biochemistry, fourth edition. W.H. Freeman.