Nucleoside

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Nitrogenous base Ribonucleoside Deoxyribonucleoside
Chemical structure of adenine
Adenine
Chemical structure of adenosine
Adenosine
A
Chemical structure of deoxyadenosine
Deoxyadenosine
dA
Chemical structure of guanine
Guanine
Chemical structure of guanosine
Guanosine
G
Chemical structure of deoxyguanosine
Deoxyguanosine
dG
Chemical structure of thymine
Thymine
Chemical structure of 5-methyluridine
5-Methyluridine
m5U
Chemical structure of thymidine
Thymidine
dT
Chemical structure of uracil
Uracil
Chemical structure of uridine
Uridine
U
Chemical structure of deoxyuridine
Deoxyuridine
dU
Chemical structure of cytosine
Cytosine
Chemical structure of cytidine
Cytidine
C
Chemical structure of deoxycytidine
Deoxycytidine
dC

Nucleosides are glycosylamines that can be thought of as nucleotides without a phosphate group. A nucleotide is composed of a nucleobase (also termed a nitrogenous base), a five-carbon sugar (either ribose or deoxyribose), and one or more phosphate groups while a nucleoside consists simply of a nucleobase and a 5-carbon sugar. In a nucleoside, the base is bound to either ribose or deoxyribose via a beta-glycosidic linkage. Examples of nucleosides include cytidine, uridine, adenosine, guanosine, thymidine and inosine.

Biological function[edit]

While a nucleoside is a nucleobase linked to a sugar, a nucleotide is composed of a nucleoside and one or more phosphate groups. Thus, nucleosides can be phosphorylated by specific kinases in the cell on the sugar's primary alcohol group (-CH2-OH) to produce nucleotides. Nucleotides are the molecular building-blocks of DNA and RNA.

Nucleosides can be produced by de novo synthesis pathways, in particular in the liver, but they are more abundantly supplied via ingestion and digestion of nucleic acids in the diet, whereby nucleotidases break down nucleotides (such as the thymidine monophosphate) into nucleosides (such as thymidine) and phosphate. The nucleosides, in turn, are subsequently broken down:

  • in the lumen of the digestive system by nucleosidases into nucleobases and ribose or deoxyribose.

In addition, nucleotides can be broken down:

Use in medicine and technology[edit]

In medicine several nucleoside analogues are used as antiviral or anticancer agents. The viral polymerase incorporates these compounds with non-canonical bases. These compounds are activated in the cells by being converted into nucleotides. They are administered as nucleosides since charged nucleotides cannot easily cross cell membranes.

In molecular biology, several analogues of the sugar backbone exist. Due to the low stability of RNA, which is prone to hydrolysis, several more stable alternative nucleoside/nucleotide analogues that correctly bind to RNA are used. This is achieved by using a different backbone sugar. These analogues include LNA, morpholino, PNA.

In sequencing, dideoxynucleotides are used. These nucleotides possess the non-canonical sugar dideoxyribose, which lacks 3' hydroxyl group (which accepts the phosphate). It therefore cannot bond with the next base and terminates the chain, as DNA polymerases cannot distinguish between it and a regular deoxyribonucleotide.

The structure elements of the nucleosides and the phosphate group bearing nucleotides

See also[edit]