Introduction

Besides glycolysis, the carbohydrate can be metabolized by the pentose phosphate pathway. Different types of food items are digested in the nutrition system and turned into simple food. Simple food is absorbed by the small intestine flows through the bloodstream to different cells and becomes part of the protoplasm. The absorbed food reaches various cells and is used in various functions of the organism.

Metabolism is synthetic and disruptive chemical changes that occur in the protoplasm of living cells. Carbohydrates, proteins, fats, etc. contained in the cell are oxidized by metabolic processes to produce more energy, which is essential for survival. In cells, this process takes place in stages with the help of various enzymes.

The high-energy phosphate compounds are produced in which the usable energy of the animal is stored. All these reactions organized by metabolic processes always follow a metabolic path. That is, metabolism is completed through a metabolic pathway. One such metabolic pathway is the pentose phosphate pathway that completes carbohydrate metabolism (1) & (4).

Brief description

Glycolysis and the Krebs cycle are the main pathways of aerobic respiration. However, in some microorganisms, carbohydrate metabolism occurs in an alternative pathway called the pentose phosphate pathway. Glycolysis is the main process of carbohydrate metabolism in the lens of the eye, heart muscle, kidneys, and many other tissues. The liver, adrenal glands or cortex, adipose tissue, erythrocytes, testes, lactating mammary gland, ovary, etc. uses the pentose phosphate pathway to metabolize carbohydrate. This alternative method is known as the Pentose phosphate pathway or HMP shunt. The pathway occurs in the presence of oxygen.

In this pathway, the pentose sugars are synthesized from hexose sugars. During the metabolism of pentose sugars by this pathway, they are converted to 3-phosphoglyceraldehyde and enter the path of glycolysis. Besides, the reduction of NADP⁺ molecules produces NADPH + H⁺. In fact, it is important to note that in all tissues where reductive synthesis is performed, this pathway is even more important (1).

Definition

In addition to the glycolysis process, there is a method by which glucose molecules can be broken down. In this way, the glucose molecule forms glucose 6-phosphate similar to the first stage of glycolysis.

The glucose 6-phosphate is oxidized by NADP⁺ to form 6-phosphogluconic acid. This acid forms pentose sugars with 5 carbons like ribose 5-phosphate, ribulose 5-phosphate, xylulose 5-phosphate, etc. through many complex stages. The stage by which this process completes is called the pentose phosphate pathway (2).

Warburg, Lipmann, and Dickens were the first to confirm the existence of this pathway and to identify the reactions of this pathway. So according to their name, this pathway is called the Warburg-Lipmann-Dickens pathway.

The alternative pathway to carbohydrate metabolism is known by various names such as hexose monophosphate shunt direct oxidation pathway Warburg-Lipmann-Dickens pathway, or phosphogluconate pathway. But the most common name is the pentose phosphate pathway (3).

Why is pentose phosphate called hexose monophosphate shunt?

This pathway, deviating from the glycolysis process, is known as hexose monophosphate shunt. This is because in this process the glucose molecule first forms glucose 6-phosphate similar to the glycolysis process. The glucose 6-phosphate is a type of hexose monophosphate. Hence this pathway is known as HMP (hexose monophosphate shunt) shunt (2).

Properties of pentose phosphate pathway 

1. Carbohydrate molecules are directly oxidized with NADP⁺ at different stages of this pathway.

2. There are many pentose sugars (5 carbon atoms) produced in this process.

3. It has been observed that 5 molecules of glucose are regenerated when 6 molecules of glucose enter this pathway. As a result, the glucose molecule oxidizes and releases water, CO₂, and energy.

4. The 5 carbon-molecule sugars called ribose produced through this pathway are used to produce nucleic acids.

5. ATP is not produced by storing energy in this process.

6. The pentose phosphate pathway is particularly effective in cells, especially in the liver and adrenal cortex.

7. This pathway produces glyceraldehyde-3-phosphate which produces pyruvic acid in the process of glycolysis.

8. Besides pentose sugars, erythrose-4-phosphate with 4 carbon atoms, sedoheptulose-7-phosphate with 7 carbon atoms, etc. are produced in this pathway or shunt.

9. It is an important source of energy for many microorganisms (1) & (2).

Location of the pentose phosphate pathway  

It exists in both eukaryotic and prokaryotic cells. There is a jelly-type part in the cytoplasm of both eukaryotic and prokaryotic cells known as cytosol.

The cytosol is the part of the cell where this pathway occurs and all the enzymes of this pathway are present in the cytosol. The liver, adrenal glands, erythrocytes, mammary glands, etc. are highly active in the pentose phosphate pathway. These tissues participate in the biosynthesis of fatty acids and steroids that depend on NADPH supply (3).

Step of the pentose phosphate pathway  

The HMP shunt or pentose phosphate pathway is completed through the following processes.

The reaction sequence is divided into two stages.

Oxidative phase

1. The first stage of this pathway is done in the same way as the glycolysis process.
2. In this process, hexose sugar is phosphorylated by ATP to form glucose 6-phosphate.
3. This reaction is identical to that of the EMP pathway. The reaction is completed by the hexokinase enzyme.

Glucose + ATP → Glucose 6-phosphate + ADP

Next glucose 6-phosphate is oxidized to form 6 phosphogluconic acids (6 carbon atoms). The enzyme that participates in this reaction is glucose 6-phosphate dehydrogenase. Here NADP⁺ acts as an electron acceptor.

Glucose 6-phosphate + NADP⁺ → 6 phosphogluconic acid + NADPH + H⁺

6-phosphogluconic acid immediately undergoes oxidative decarboxylation to form ribulose 5-phosphate. In this reaction, one molecule of CO₂ is released. Here a second time, NADP is reduced to form NADPH + H⁺. This reaction occurs by 6- phosphogluconic dehydrogenase.

6-phosphogluconic acid + NADP → Ribulose 5-phosphate + CO₂ + NADPH + H⁺ (2) & (4).

Non-oxidative phase

1. The first stage of the Non-oxidative phase

This phase links the glycolytic pathway to the pentose phosphate pathway. In this step, non-oxidative synthesis of 5-carbon sugars occurs.

some ribulose 5-phosphate is isomerized into ribose 5-phosphate. The phosphoriboisomerase enzyme participates in this reaction. The remaining ribulose 5-phosphate is converted into xylulose 5-phosphate. The reaction is catalyzed by epimerase enzymes.

• Ribulose 5-phosphate → Ribose 5-phosphate (Phosphoribo isomerase enzyme)
• Ribulose 5-phosphate → Xylulose 5-phosphate (Epimerase enzyme)

2. In the second stage of the non-oxidative phase

the ribose 5-phosphate and xylulose 5-phosphate interact with each other to form sedoheptulose 7-phosphate and 3-phosphoglyceraldehyde. The process is completed through transketolase enzymes. The reaction is as follows

• Ribose 5-phosphate + Xylulose 5-phosphate → Sedoheptulose 7-phosphate + 3-phosphoglyceraldehyde (Transketolase enzyme)

3. Third stage of the non-oxidative phase

One molecule of each sedoheptulose 7-phosphate and 3-phosphoglyceraldehyde combine to produce fructose 6-phosphate and erythrose 4-phosphate. This process is completed in the presence of transketolase enzymes. Besides, fructose 6-phosphate is also formed by the reaction of erythrose 4-phosphate and xylulose 5-phosphate.

• Sedoheptulose 7-phosphate + 3-phosphoglyceraldehyde → Fructose 6-phosphate + Erythrose 4-phosphate
• Erythrose 4-phosphate + Xylulose 5-phosphate → Fructose 6-phosphate + 3-phosphoglyceraldehyde

Both reactions are catalyzed by transketolase enzymes.

4. In the last stage of the non-oxidative phase

Fructose 6-phosphate is isomerized to glucose 6-phosphate. The phosphohexoisomerase enzyme participates in this reaction. Two molecules of 3- phosphoglyceraldehyde is again utilized in this cycle. That is, two molecules of 3- phosphoglyceraldehyde formed by the interactions of these reactions may be converted into fructose-1-6-bisphosphate.

• Fructose 6-phosphate → Glucose 6-phosphate

The pentose phosphate pathway was then completed. Six pentose phosphate have been converted back to five hexose phosphate. All reactions in the non-oxidative phase of this pathway are immediately reversed and this provides a way to convert hexose phosphate to pentose phosphate (2) & (4).

The pentose phosphate pathway is a unique pathway. This pathway takes place in the cytoplasm of both eukaryotic cells and prokaryotic cells. In a plant cell, the pathway is complete in the mature cells.

Significance of pathway

One of the metabolic methods of carbohydrates is the pentose phosphate pathway. This pathway is very important in the case of carbohydrate metabolism. The importance of this cycle is described below.

• This process provides an alternative route for breaking down carbohydrates through respiration.
• This pathway is primarily involved in the interconnection of carbon compounds produced in the initial stage of photosynthesis. In most cases, the leaves of higher plants metabolize glucose in this way.
• The HMP Shunt or pentose phosphate pathway produces NADPH⁺ for the synthesis of various substances like amino acids.
• In plants, there is evidence that the prominent pathway of young tissues is glycolysis and the pentose phosphate pathway acquires more significance as the tissue matures. That is, the pentose phosphate pathway is predominant in mature plant cells.
• This reaction produces the ribose 5-phosphate (5 carbon atom sugar), which is used in the synthesis of nucleic acids.
• Erythrose 4-phosphate is also produced in this pathway. The erythrose 4-phosphate is used in the synthesis of lignin, anthocyanins, auxins, and several other substances.
• NADPH is produced in the oxidative phase of this pathway. The NADPH is not only useful for ATP synthesis but also functions in various biosynthetic processes in the cell.
• This pathway produces a non-oxidative, as well as an oxidative pathway for producing ribose and another pentose from the hexoses commonly encountered in metabolism.
• The pentose phosphate pathway provides the metabolism of hexose, pentose, heptoses, tetroses, etc.
• NADPH in this pathway in the red blood cell is used to inhibit the oxidation of hemoglobin and to regenerate hemoglobin by reduction of methemoglobin.
• NADPH produced in the pentose phosphate pathway in white blood cells participates in the production of bacterial superoxide radicals (1) & (4).

Q&A

1. Where does the pentose phosphate pathway occur?

The cytoplasm of both eukaryotic and prokaryotic cells contains a jelly-type part known as cytosol. The pentose phosphate pathway occurs in the cytosol of the cytoplasm of both eukaryotic and prokaryotic cells. In plant cells, this pathway occurs in the plastids.

2. Which molecule controls the rate of the pentose phosphate pathway?

Glucose 6-phosphate dehydrogenase is to control the rate.

3. What does the pentose phosphate pathway produce?

It produces ribose 5-phosphate and NADPH (nicotinamide adenine dinucleotide phosphate.

4. What is the purpose of the pentose phosphate pathway?

The purpose of this pathway is to provide NADPH for reduction biosynthesis and ribose 5-phosphate for nucleotide synthesis.

5. What is the pentose phosphate pathway?

The pentose phosphate pathway is an alternative metabolic pathway. The glucose molecule breaks down and forms glucose 6-phosphate. The glucose 6-phosphate is then oxidized by NADP⁺ to form 6-phosphogluconic acid. This acid forms various pentose sugars with 5 carbon atoms.

References

1. B Agarwal and V. K. Agarwal. Unified Botany, B.Sc. second Year. Shiva Lal Agarwal & Company Publications, Indore. Chapter: Respiration. Page no- 160 to 162.
2. B. Powar and G. R. Chatwal. Biochemistry, B. SC (General & honors course) and M. Sc. Himalaya publishing house, Chapter: Metabolism of carbohydrates. Page no- 404 to 407.
3. Biochemistry – E-Book – Google Books
4. Ajoy Paul. Zoology Honours, Volume- 2, Books & Allied (P) Ltd. Chapter: Carbohydrates metabolism. Page no- 302 to 307.