UCK2 catalyzes the first committed, ATP-dependent phosphorylation step of pyrimidine ribonucleoside salvage, converting uridine to UMP and cytidine to CMP. This step is functionally rate-limiting for ribonucleoside salvage.

Substrate specificity is restricted to pyrimidine ribonucleosides: UCK2 does not phosphorylate purine ribonucleosides (adenosine/guanosine) or 2'-deoxyribonucleosides, and accepts ribonucleoside analogs but not sugar-modified analogs such as araC, consistent with a ribose-OH requirement.

UCK2 has lower Km and higher Vmax (and thus markedly higher catalytic efficiency) for uridine and cytidine than the paralog UCK1.

UCK2 is a ~29 kDa, 261-residue enzyme of the NMP kinase fold that functions as a homotetramer, with crystal structures showing ligand-free, substrate/ product-bound, and feedback-inhibited tetrameric states.

UCK2 is allosterically regulated: activated by ATP and feedback-inhibited by the downstream products UTP and CTP; a distinct intersubunit allosteric pocket has been described where noncompetitive inhibitors reduce kcat without changing Km.

Localization is described as cytosolic pyrimidine nucleoside salvage, but the retrieved evidence set does not include direct microscopy-based localization experiments, so cytosolic localization is treated as a pathway-context inference.

UCK2 is tissue-restricted in normal tissues (placenta/testis) but broadly upregulated across many cancers, motivating tumor-selective activation of cytotoxic nucleoside analogs (e.g., RX-3117) that depend on UCK2 phosphorylation.