Pentose: Importance in pentose phosphate pathway

Pentose

Introduction

Monosaccharides are the simplest types of carbohydrates that cannot be hydrolyzed further into small units. Monosaccharides are classified according to the number of carbon atoms. Pentose is a type of monosaccharide that contains 5 carbon atoms in its structure.

For example, a molecule of three carbon atoms is trioses, four carbon atoms are tetrodes and five carbon atoms are pentose. That is simple sugar or monosaccharides have 3 to 7 carbon. And based on the number of these carbon atoms, different types of monosaccharides can be found in nature.

It is an important simple sugar or monosaccharide that is used in various metabolic processes. The pentose phosphate pathway or HMP pathway is a pathway of carbohydrates metabolism. It is an alternative pathway of carbohydrates metabolism. This simple sugar participates in this HMP pathway and plays a vital role in this metabolic process. Here is a discussion of its importance in the pentose phosphate pathway or HMP pathway (4) & (5).

Pentose

Pentose is a class of carbohydrates whose molecules contain five carbon atoms. The chemical formula of pentose is C₅H₁₀O₅. They have a five-membered ring in their structure. Carbohydrates are classified into four major categories. These are monosaccharides, disaccharides, polysaccharides, and oligosaccharides. It is a type of monosaccharide.

The five carbon atoms of monosaccharides are in the pentagonal structure. One carbon atom is present in each corner of this structure. So this monosaccharide molecule is known as pentose. It has two forms. These are open-chain or closed-chain. The open chain is linear and the closed chain is cyclic. Both chains are easily converted into each other in water solutions (3) & (6).

Example

Some examples of these monosaccharides are ribose, ribulose, xylose, deoxyribose, xylulose, etc. (3).

Features of pentose

1. They are monosaccharides with five carbon atoms.

2. It plays an important role in nucleic acid synthesis, photosynthesis, and respiration.

3. These compounds are the simple form of carbohydrates.

4. Its molar mass is 150.13 gm/mol.

5. These are found in RNA, DNA, ATP, FAD, NADP, etc.

6. Ribulose, which is a pentose, participates as a middle product of various metabolic pathways in the D-ribulose form.

7. Some of the compounds obtained from ribulose are one of the main mediators of the pentose phosphate pathway.

8. There is some pentose that plays a vital role in the plant body like, ribose, deoxyribose, and ribulose.

9. Xylulose molecules have five atoms in their structure. It is an intermediate in the uronic acid pathway in the form of D-Xylulose (4) & (2).

Types of pentose

There are two types of these simple sugar or monosaccharides, these are as follows

1. Aldopentose

It is a functional group. If the carboxyl group is at one end of the carbon chain of monosaccharide and the monosaccharide contains an aldehyde group, then it is called aldose.

When a pentose is combined with an aldehyde group it is known as aldopentose. That means, a carboxyl group is located at the end of its 5-carbon chain makes it an aldopentose.  Some examples of aldopentose are ribose, arabinose, lyxose, and xylose, etc. (6)

2. Ketopentose

Like aldopentose, it is also a functional group. When this function group is present in a pentose it is known as ketopentose. A monosaccharide containing a keto group is called a ketose. That is, if a carboxyl group is at any other position of the main chain of the monosaccharide is a ketone and called ketose.

A carboxyl group is located at the 2- position of its 5-carbon chain makes it a ketopentose. Some examples of ketopentose are ribulose, xylulose, etc. (2).

Function of pentose

  • It is a very important monosaccharide or simple sugar.
  • Ribose 5-phosphate is an important product of the pentose phosphate pathway. This simple sugar is used in the synthesis of nucleotides and nucleic acids.
  • They are also formed as intermediates during the cell’s metabolic processes.
  • It plays a significant role in various metabolic processes of carbohydrates.
  • Arabinose is a pentose. It is present in cherries. Arabinose is found in the glycoprotein of the body.
  • It forms an aldopentose that helps to bind the carbon dioxide in the Calvin cycle.
  • Ribose helps in the formation of DNA and RNA.
  • Xylose is a pentose that is derived from wood. It is used as a diagnostic agent and also as a diabetic sweetener in food.
  • It participates in protein synthesis as a regulator.
  • Ribose is a structural element of some coenzymes, like NAD, NADP, and flavoproteins.
  • Ribulose is used as an intermediate product of the HMP pathway or pentose phosphate pathway. It is used to break down glucose in the hexose monophosphate shunt.
  • Lyxose is used as the component of lyxoflavin of the human heart muscle (3) & (1).

Importance in Pentose phosphate pathway

Glycolysis and the Krebs cycle are the main pathways of aerobic respiration. In addition to Glycolysis and Krebs cycle, there is also an alternative pathway of carbohydrates metabolism. This is the pentose phosphate pathway. In the pathway, the pentose is synthesized from hexose. Warburg, Lipmann, and Dickens were the first to identify this pathway.

There are various products produced in this pathway. Pentose is one of them. It acts as an intermediate of various reactions in this pathway. Its importance is given below.

  • In this pathway, a hexose is converted into pentose (Ribose 5-phosphate). It is useful for the synthesis of nucleic acids. The primary components of nucleic acids are pentose.
  • It acts as an enzyme in both prokaryotic and eukaryotic metabolic pathways.
  • These monosaccharides are used as an intermediate product of pathways. It is used to break down glucose in this pathway.
  • It is activated by ribose-phosphate diphosphokinase to form phosphoribosyl pyrophosphate during nucleotide biosynthesis.
  • Adenosine triphosphate (ATP) and guanosine triphosphate (GTP) is synthesized from ribose 5-phosphate (pentose) in this pathway.
  • Ribose 5-phosphate, a pentose produced in this pathway, is used as the synthesis of various nucleotides, like, FAD, NADH, ATP, etc.
  • Various coenzymes are also synthesized in this pentose (1) & (6).

Q&A

1. Where does the pentose phosphate pathway occur?

It is an alternative pathway parallel to glycolysis. This process occurs in both eukaryotic and prokaryotic cells. The cytoplasm of both eukaryotic and prokaryotic cells contains a jelly-type part known as cytosol. This 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 of this pathway.

3. What does the pentose phosphate pathway produce?

Ribose 5-phosphate and NADPH (nicotinamide adenine dinucleotide phosphate).

4. Which of the following molecules is the pentose sugar found in RNA?

Ribose sugar.

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

It is a metabolic pathway. Mainly carbohydrate metabolism is completed by this pathway. So this pathway has few purposes. The primary purpose of this pathway is to provide NADPH for reduction biosynthesis and ribose 5-phosphate for nucleotide synthesis.

Written By: Manisha Bharati

References

1. Ajoy Paul. Zoology Honours, volume- 2, Books & Allied (P) Ltd. Chapter: Carbohydrates metabolism. Page no- 302 to 307.

2. Ajoy Paul. Zoology Honours, volume- 2, Books & Allied (P) Ltd. Chapter: Carbohydrates. Page no- 747 to 760.

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