cytidin

Cytidin, also known as cytidine, is a nucleoside consisting of the pyrimidine base cytosine attached to a ribose sugar by a β-N-glycosidic bond. It is the ribonucleoside form of cytosine and is one of the four standard nucleosides found in RNA. The deoxyribonucleoside counterpart is deoxycytidine, which is found in DNA.

In cells, cytidin participates in nucleotide metabolism and RNA synthesis. It can be formed through salvage pathways in which cytosine is combined with ribose, or by phosphorylation of cytidine to cytidine monophosphate (CMP), then to CDP and CTP, which are substrates for RNA polymerases. Cytidine can be deaminated by cytidine deaminase to form uridine, linking cytidine metabolism to uridine pools.

Cytidin-containing nucleotides play essential roles in RNA biology, including transcription and various RNA processing events. In RNA, cytidine pairs with guanine via standard Watson-Crey base pairing, contributing to the structure and function of messenger RNA, ribosomal RNA, and transfer RNA. Post-transcriptional modifications of cytidine residues also contribute to RNA stability and function in cells.

Cytidin and its derivatives have notable clinical relevance. Chemotherapeutic and antiviral nucleoside analogs based on cytidine scaffolds—such as cytarabine (ara-C) and other cytidine analogs like azacitidine and gemcitabine—are used in cancer therapy and antiviral treatments. These compounds disrupt nucleic acid synthesis or function by mimicking natural cytidine.