Introduction

Pentose sugar is a component of nucleotides. Nucleotides are biological molecules that act as monomers and as subunits of DNA and RNA. These biological molecules carry energy packets to the cell in the form of nucleoside triphosphate (ATP, GTP, CTP, and UTP), which play a key role in metabolic processes.

Nucleotides are formed by combining a nitrogenous base and a phosphate group with pentose sugar. The main polymer of nucleotides is nucleic acid. This sugar is found only after the hydrolysis of nucleic acids.

Nucleic acids naturally act as molecules that carry primary information in cells. In nucleic acids, this sugar and phosphates are connected to each other in an alternative chain (sugar-phosphate backbone) by phosphodiester connection. So it is very important as a component of nucleotides. Below is a description of its structure and function (3) & (4).

Pentose sugar

Pentose sugar has five carbon atoms. The main basis of nucleic acid is sugar. It is known as pentose sugar because it is pentagonal in shape with a carbon atom at its five corners. Examples of these sugars are the deoxyribose sugar of DNA and the ribose sugar of RNA. Its chemical formula is C₅H₁₀O₅ (2).

Ribose and deoxyribose are in linear form when they are free but are present in furanose form when they are in the part of nucleic acid. In addition to ribose and deoxyribose, a few other examples of this type of sugar are D-Ribose, D-Arabinose, D-Xylose, D-Ribulose, D-Xylulose, etc. (1).

Brief description of the pentose sugar

There are two types of pentose sugar found in nucleic acid. These are ribose sugar and deoxyribose sugar. They have a five-membered ring in their structure. In addition to pentose sugar, a nucleic acid contains a nitrogenous base and phosphorus group. When the base is combined with this sugar, a nucleoside is formed, such as deoxyadenosine, adenosine guanosine, etc.

The nitrogenous bases that are present in the nucleic acid are purine and pyrimidine. In pyrimidine nucleoside, the nitrogen atom at position 1 of the pyrimidine ring is linked to the carbon atom at position 1’ of the pentose sugar.

On the other hand, in purine nucleoside, the link is between nitrogen atom 9 of purine and carbon atom 1’ of the pentose sugar. When a pentose sugar residue of a nucleoside is esterified with phosphoric acid then nucleotide is formed (1) & (2).

Properties of a pentose sugar

There are some properties of this sugar are discussed below

1. These are monosaccharides that contain 5-carbon atoms.

2. Ribose sugar, a type of pentose sugar, is very important for photosynthesis, respiration, and nucleic acid synthesis.

3. This sugar can form esters with a phosphate group.

4. It is located in the middle of the nucleotides.

5. These sugars are different in DNA and RNA. The difference between the pentose sugars of DNA and RNA is that ribonucleic acids contain one hydroxyl group, whereas deoxyribonucleic acid contains only one hydrogen atom.

6. It forms two groups, aldopentoses, and ketopentoses.

7. This sugar molecule determines whether the nucleotide forms part of a DNA molecule or RNA molecule.

8. The sugar in DNA and RNA is connected with nitrogenous bases. A nitrogenous base attached to pentose sugar is known as a nucleoside.

9. In pentose sugar, hemiacetals are formed within the same molecule between the alcohol groups and carbonyl groups.

10. Both ribose and deoxyribose sugar in pentose sugar are present in nucleic acid as the β-Furanoside ring structure (2).

Structure

Pentose sugar consists of five carbon atoms. It is also a type of monosaccharide. The monosaccharides are subdivided as trioses, tetroses, pentose, and hexoses, depending upon the number of carbon atoms. So the sugar belongs to the monosaccharides group.

These five carbon atoms are in the pentagonal structure. One carbon atom is present in each corner of this structure. The two main functional groups associated with this sugar are often mentioned for binding carbon atoms.

The phosphate group of a nucleotide is attached to the 5-carbon atom of this sugar. There is a hydroxyl group also present in it. This group is attached to the 3-no carbon atom of this sugar. The sugar present in DNA and RNA is mainly pentose sugar.

There are mainly two bases of nucleic acids. These are DNA and RNA. The pentose sugars that are present in DNA are deoxyribose. And RNA has ribose sugar, which is a pentose sugar. Ribose sugar is one more –OH group than deoxyribose. Phosphate residues are attached to a 5’ carbon hydroxyl group of sugar and a subsequent nucleotide to a 3’ carbon hydroxyl group of sugar, forming a 5’-3’ phosphodiester (1).

Two important pentose sugar in nucleotides

1. Deoxyribose sugar

Deoxyribose is a pentose sugar with 5 carbon atoms. Four of the five carbon atoms and a single atom of oxygen are combined together to form a five-membered ring. The fifth carbon atom is located on the outside of the ring and forms a -CH₂ group. Carbon atoms of -CH₂ are numbered 5’. Hydrogen atoms are attached to carbon atoms 1, 2, 3, and 4. And there are also three –OH groups in this sugar. These –OH groups are located in the carbon atoms 1, 3, and 5.

2. Ribose sugar

It is also a pentose sugar that is present in nucleotides. There is an –OH group instead of hydrogen on carbon atom 2’ (2) & (4).

Functions

1. The main function of this sugar is to help in the formation of DNA and RNA.
2. It is involved in protein synthesis as a regulator and catalyst of the process.
3. This sugar is used to store and produce energy in the body.
4. It has been revealed to modulate the production of oxygen-free radicals.
5. Sugar is very important for DNA because phosphate and pentose sugar combine together to form phosphodiester.
6. It combines with one molecule of the nitrogenous base to form a nucleoside.
7. This sugar generates esters when they are combined with phosphate. They are known as phosphodiesters. It forms B-D ribose with ring structure or B-D deoxyribose with ring structure nucleic acid (2) & (3).

Written By: Manisha Bharati

References

1. Ajoy Paul. Zoology Honours, Volume- 1, Books & Allied (P) Ltd. Chapter: Nucleic Acids. Page no- 794 to 795.

2. L. Dutta. Inorganic Chemistry: Chemical Elements and their Compounds. Part- II. The New Book Stall, Kolkata. Chapter: Chemistry of Nucleic Acids. Page No: 256 to 271.