Cytidiini

Cytidine is a nucleoside consisting of the pyrimidine base cytosine attached to a ribose sugar by a beta-N1-glycosidic bond. In RNA, cytidine is one of the standard nucleosides; the corresponding deoxynucleoside in DNA is deoxycytidine, in which ribose is replaced by deoxyribose. The molecular formula is C9H13N3O5.

In cells, cytidine is phosphorylated to cytidine monophosphate (CMP), cytidine diphosphate (CDP), and cytidine triphosphate (CTP). CMP, CDP, and CTP participate in various metabolic and biosynthetic processes; CTP is essential for RNA synthesis and also serves in lipid metabolism as a donor in certain phospholipid pathways. Cytidine can be formed or replenished via salvage pathways and can be interconverted with other nucleosides, such as uridine, through enzymatic reactions.

Cytidine is deaminated by cytidine deaminase to uridine, a reaction that links cytidine metabolism to uridine pathways. In the cell, cytidine and its nucleotides act primarily as building blocks for RNA, with cytidine base pairing with guanine in base-paired structures.

Medical relevance: cytidine and its analogs are used in medicine. Cytidine itself has limited direct therapeutic use, but nucleoside analogs based on cytidine, such as cytarabine (ara-C) and gemcitabine, are important anticancer agents. These analogs are phosphorylated intracellularly and interfere with DNA synthesis, contributing to their anticancer activity.