cytidine

Cytidine is a nucleoside consisting of the pyrimidine base cytosine attached to a ribose sugar (beta-D-ribofuranose) via a β-N1 glycosidic bond. In most biological contexts, the term refers to the ribonucleoside found in RNA; the deoxyribonucleoside form is deoxycytidine.

In RNA, cytidine pairs with guanine through hydrogen bonds, contributing to RNA structure and function. It is one of the four standard nucleosides in RNA, alongside adenosine, guanosine, and uridine. Cytidine is not used to denote a DNA component, though a corresponding deoxyribonucleoside, deoxycytidine, exists in DNA.

Metabolism: Cytidine is phosphorylated by ribonucleoside kinases to cytidine monophosphate (CMP), then to cytidine diphosphate (CDP) and cytidine triphosphate (CTP). CTP serves in RNA synthesis and as a substrate in various biosynthetic pathways. Cytidine can be deaminated to uridine by cytidine deaminase, linking pyrimidine metabolism to uridine pools. Salvage pathways help recycle cytidine from degradation products.

Clinical and research relevance: Cytidine and its nucleotides participate in cellular metabolism and signaling. Cytarabine, a cytidine analog with arabinose in place of ribose, is used as chemotherapy. Other derivatives, such as CMP and CTP, are essential cofactors in nucleotide biosynthesis and RNA transcription. Cytidine is a fundamental component of RNA and of nucleotide metabolism in general.