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Nucleosides are glycosylamines that can be thought of as nucleotides without a phosphate group. A nucleoside consists simply of a nucleobase (also termed a nitrogenous base) and a five-carbon sugar ( ribose or 2'-deoxyribose) whereas a nucleotide is composed of a nucleobase, a five-carbon sugar, and one or more phosphate groups.
Nucleosides are glycosylamines made by attaching a nucleobase to a ribose ring. Examples of these include cytidine, uridine, adenosine, guanosine, thymidine and inosine.
Synthesis of nucleosides involves the coupling of a nucleophilic, heterocyclic base with an electrophilic sugar. The silyl-Hilbert-Johnson (or Vorbrüggen) reaction, which employs silylated heterocyclic bases and electrophilic sugar derivatives in the presence of a Lewis acid, is the most common method for forming nucleosides in this manner.
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 as charged nucleotides cannot easily cross cell membranes. [citation needed] At least one set of new base pairs has been announced as of May 2014. [15]
Nucleotides are nucleosides covalently linked to one or more phosphate groups. [9] To provide information about the number of phosphates, nucleotides may instead be referred to as nucleoside (mono, di, or tri) phosphates. [10] Thus, nucleoside triphosphates are a type of nucleotide. [10]
Uridine (symbol U or Urd) is a glycosylated pyrimidine analog containing uracil attached to a ribose ring (or more specifically, a ribofuranose) via a β-N 1-glycosidic bond.The analog is one of the five standard nucleosides which make up nucleic acids, the others being adenosine, thymidine, cytidine and guanosine.
The antiviral drug aciclovir (bottom), a nucleoside analogue that functions by mimicking guanosine (top). Nucleoside analogues are structural analogues of a nucleoside, which normally contain a nucleobase and a sugar.
In other forms of life many other sugars are used and various donors are utilized for them. All five of the common nucleosides are used as a base for a nucleotide sugar donor somewhere in nature. As examples, CDP-glucose and TDP-glucose give rise to various other forms of CDP and TDP-sugar donor nucleotides. [9] [10]