Nucleoside- Definition, Types, Structure, Functions

What is Nucleoside?

A nucleoside is a molecule with a pentose sugar linked to a nitrogenous base or glycosylamines. A nucleoside can also be defined as a nucleotide without a phosphate group attached to it.

• The nucleosides present in DNA contain a 2` – deoxy – D- ribose sugar and nucleosides in RNA contain D-ribose sugar. 
• The main difference is seen at the second position of the pentose structure, in the case of 2` – deoxy -ribose there is an absence of an alcohol group/ oxy group/ -OH group at the second position, hence the name.
• In the case of D – ribose pentose the –OH group is present at the second position.
• In both types, the pentoses are present in their β-furanose form which is a close five-membered ring structure.

Types of Nucleoside

The nucleosides can be divided into two types based on the presence of the nitrogen base of the compound. 

1. Purine nucleosides 
2. Pyrimidine nucleosides

Purine nucleosides

These nucleosides are composed of two nitrogenous bases known as Adenine and Guanine.

In the case of RNA, the nucleosides are – Adenine and Guanine

In the case of DNA, the nucleosides are – Deoxyadenosine and Deoxyguanosine

Pyrimidine nucleosides

These nucleotides are composed of three nitrogenous bases known as Thymine, Cytosine, and Uracil.

In the case of RNA, the nucleosides are – Cytidine and Uridine 

In the case of DNA, the nucleosides are – Deoxycytidine and Thymidine or Deoxythymidine 

Structure of Nucleoside

The nucleosides consist of two main heterocyclic components –

Pentose sugar

It is a five-membered ring structure generally described as a puckered conformation. 

• In the case of DNA, the nucleosides lack –OH group at the second position of pentose ring and hence is called as 2` – deoxy – D – ribose. 
• In RNA it is a D – ribose pentose ring structure. 
• The pentoses are present in β- furanose form in both types of sugars.
• The pentose sugar is attached to the nitrogenous base at the primary carbon atom (1`). 
• It is linked by a glycosidic bond (N-β-glycosyl bond).

Nitrogenous base

It is a cyclic carbon structure containing nitrogen and has the properties of a base.

• The bases are derivatives of two major parent compounds – Purines and Pyrimidines. 
• In RNA the nitrogenous bases are – Adenine, Uracil, Cytosine, and Guanine.
• In DNA the nitrogenous bases are – Adenine, Thymine, Cytosine, and Guanine.
• The bases are joined covalently to the pentose sugar by N-β-glycosyl bond.
• In the case of purines the N-9 atom bonds to the pentose sugar.
• In the case of pyrimidines the N-1 atom bonds to the pentose sugar. 
• Although the majority of the bases are derived from purines and pyrimidines, some minor bases are also present in the DNA.
• Mostly these minor bases occur in methylated forms of purines and pyrimidines.
• In the case of some viral DNA, the bases might be hydroxyl-methylated or they might be glycosylated. 
• Some of the unusual bases found in DNA are 5-Methlycytidine, N⁶-Methlyadenosine, 7-Methlyguanosine, 4-Thiouridine.

Functions of Nucleosides

1. Nucleosides are an integral part of nucleotides. They are precursors for nucleotides. When a phosphate group is attached to the nucleoside it forms the nucleotide which is the backbone of DNA.
2. Nucleoside molecules also function as signaling molecules.
3. Minor bases or altered nitrogen bases in nucleosides have a role in regulating or protecting genetic information. 
4. Many nucleoside analogs have been used in the treatment of malignancies, tumors, and viral infections. This is achieved by some modifications in the purine and pyrimidine bases. 
5. Cytarabine (cytosine arabinoside) was the first drug approved by the US FDA for treatment against acute myeloid leukemia.
6. Some nucleoside analogs are also used in treatments of Human Immunodeficiency Virus (HIV) disease. Lamivudine is being used in this treatment. 
7. Nucleoside transporters play a vital role in the transportation of nucleosides across membranes. 
8. There are two types of nucleoside transporters – Concentrative and Equilibrative. These transporters play an important role in the transport of antiviral and anticancer drugs across membranes. 

References

1. Mirza A. Z. (2019). Advancement in the development of heterocyclic nucleosides for the treatment of cancer – A review. Nucleosides, nucleotides & nucleic acids, 38(11), 836–857. https://doi.org/10.1080/15257770.2019.1615623
2. https://www.frontiersin.org/articles/10.3389/fphar.2018.00606/full 
3. Lehninger Principles of Biochemistry – 6th ed- c2013.