Nucleoside

Nucleosides (including nucleosides ) are organic molecules that are composed of a nucleobase and a pentose. In a cell, various nucleosides occur, which differ in the base or sugar moiety. They contain, in contrast to the nucleotides making up the nucleic acids ( DNA or RNA), a phosphate residue.

• 2.1 pyrimidine nucleosides
• 2.2 purine nucleosides
• 2.3 Hyper Modified nucleosides and modified base backbone
• 2.4 Variations of the pentoses

• 4.1 azidothymidine
• 4.2 puromycin
• 4.3 nucleoside analogues with arabinose

Basic types

The five basic types of nucleosides consist of either a purine or pyrimidine base a. If they are the building blocks of an RNA pentose ( a simple sugar with five carbon atoms ), the ribose in DNA is a deoxyribose, pentose ago. Therefore they are called the building blocks of DNA precisely deoxynucleosides, while nucleosides are the building blocks of the various forms of RNA in the strict sense. Linking the base and the pentose is carried out at the purine base via the nitrogen atom is always in position 9, wherein the pyrimidine base via the nitrogen atom in 1-position and the C1 ' atom of the sugar.

Purine bases

Note: It is only shown one of the possible tautomeric structures.

Pyrimidine bases

Modifications of the basic forms

In addition to these basic forms, there are numerous modifications that are mainly found in the tRNAs and rRNAs. Some are also found in the DNA. These modified nucleosides usually arise only after the transcription and serve a fine adjustment of structure, activity and specificity of the molecules. Most modifications caused by methylation.

An overview of the possible modifications to give the following choice. For comparison, the basic forms are also listed:

Pyrimidine nucleosides

Purine nucleosides

Hyper Modified nucleosides and modified base backbone

Variations of the pentoses

Physiology

The hydroxy group of the C-5 atom of the pentose phosphate esterified with a nucleoside, creates the appropriate nucleotide. Depending on the number of phosphate residues is referred to as mono-, di-and triphosphates. From the central importance of nucleotides results in the same meaning for the corresponding nucleosides, since they can be converted as a component of the nucleotides in this.

The nucleosides are by elimination of the last phosphate group in nucleotides by hydrolysis, using nucleotidase enzymes in all living organisms are available. Furthermore, inosine can be synthesized from adenosine by AMP deaminase, or guanine deaminase. Xanthosine is accordingly available, but also from guanosine using the guanosine deaminase not only by hydrolysis of XMP.

The reduction will be Nukleosidasen to nucleobase and xanthine to uric acid in purines over pyrimidines or at the alanine or 2 -aminobutyric acid.

Nucleoside analogues

Nucleoside analogues play an important role especially in the anti -retroviral therapy (see AIDS). A number of modern antiviral drugs containing these substances. Best known is Zovirax ®, a drug that is commonly prescribed against the herpes simplex virus ( HSV) and acyclovir contains as active ingredient. Another widespread method is ganciclovir, which is exactly like aciclovir a guanosine analogue.

Other base analogs that are prescribed for viral infections are, for example zidovudine ( azidothymidine also short AZT), stavudine, zalcitabine, diadenosine, idoxuridine, Fluridin and ribavirin.

Azidothymidine

Azidothymidine (AZT, INN: zidovudine ) was the first effective drug against the HIV virus. Since instead of a hydroxyl group having an azido group at the 3'- carbon of the ribose, the chain can no longer continue to grow here in the synthesis of viral DNA and there is an inactive provirus.

However, some of these drugs also show significant side effects, so they are only suitable for long-term therapy.

Puromycin

Puromycin (also Purimycin ) is a alboniger from the bacterium Streptomyces derived nucleoside antibiotic that inhibits protein biosynthesis and against some tumors, amoebas, trypanosomes and worms is effective. However, since it is toxic to humans, it is used only in experiments in microbiology. Structurally, it is derived from adenosine.

Nucleoside analogues with arabinose

Nucleoside analogues with arabinose ( Arabinosylnukleoside ) are mostly used as cytostatics.

Vidarabine