| Functional aspect | Key findings | Key sources with year and DOI/URL |
|---|---|---|
| Reaction | Human UCK2 (UniProt Q9BZX2; EC 2.7.1.48) catalyzes ATP-dependent phosphorylation of the pyrimidine ribonucleosides uridine and cytidine to UMP and CMP, respectively, constituting the first/rate-limiting step of pyrimidine ribonucleoside salvage in many settings. Foundational structural and biochemical studies agree on this core activity. (pqac-00000000, pqac-00000004, pqac-00000007) | Suzuki et al., 2004, Structure, doi:10.1016/j.str.2004.02.038, https://doi.org/10.1016/j.str.2004.02.038; Fu et al., 2022, Front Oncol, doi:10.3389/fonc.2022.904887, https://doi.org/10.3389/fonc.2022.904887; Okesli-Armlovich et al., 2019, Bioorg Med Chem Lett, doi:10.1016/j.bmcl.2019.08.010, https://doi.org/10.1016/j.bmcl.2019.08.010 |
| Substrate specificity | UCK2 is selective for pyrimidine ribonucleosides and does not phosphorylate purine ribonucleosides or 2′-deoxyribonucleosides. Compared with UCK1, UCK2 has lower Km and higher Vmax for uridine/cytidine; reported catalytic efficiencies were ~26 × 10^3 s^-1 M^-1 for uridine and ~37 × 10^3 s^-1 M^-1 for cytidine in one recombinant assay, versus ~1.2 × 10^3 and ~2.0 × 10^3 for UCK1. Multiple analogs are accepted, including 6-azauridine, 5-fluorouridine, 4-thiouridine, 5-bromouridine, 5-fluorocytidine, 2-thiocytidine, 5-methylcytidine, and N4-substituted cytidines. (pqac-00000015, pqac-00000000, pqac-00000002) | Van Rompay et al., 2001, Mol Pharmacol, doi:10.1124/mol.59.5.1181, https://doi.org/10.1124/mol.59.5.1181; Suzuki et al., 2004, https://doi.org/10.1016/j.str.2004.02.038; Fu et al., 2022, https://doi.org/10.3389/fonc.2022.904887 |
| Regulation / feedback inhibition | UCK2 is activated by ATP and feedback-inhibited by UTP and CTP. Structural studies showed inhibitor-bound states and conformational changes associated with ligand binding, while recent medicinal chemistry identified a distinct intersubunit allosteric pocket where noncompetitive inhibitors reduce kcat without materially changing Km. (pqac-00000000, pqac-00000005, pqac-00000006, pqac-00000017) | Suzuki et al., 2004, Structure, doi:10.1016/j.str.2004.02.038, https://doi.org/10.1016/j.str.2004.02.038; Mashayekh et al., 2022, Biochemistry, doi:10.1021/acs.biochem.2c00451, https://doi.org/10.1021/acs.biochem.2c00451 |
| Structure / oligomerization | UCK2 is a ~29 kDa, 261-aa enzyme in the NMP kinase-fold family that forms a homotetramer. Crystal structures revealed ligand-free, substrate/product-bound, and feedback-inhibited tetrameric states, with induced-fit remodeling of the acceptor site; key residues highlighted across studies include Asp62 (catalytic base), Tyr112/His117 (base recognition), Asp84/Arg166 (ribose OH recognition), and Mg2+-coordinating residues. (pqac-00000000, pqac-00000012, pqac-00000002, pqac-00000017) | Suzuki et al., 2004, Structure, doi:10.1016/j.str.2004.02.038, https://doi.org/10.1016/j.str.2004.02.038; Fu et al., 2022, Front Oncol, doi:10.3389/fonc.2022.904887, https://doi.org/10.3389/fonc.2022.904887 |
| Expression patterns | Foundational work detected UCK2 mRNA much more restrictively than UCK1, with placenta-specific detection in one early normal-tissue panel; later translational studies and reviews describe expression in placenta/testis and broad upregulation across many cancers. A 2023 mutagenesis study further noted that UCK2 expression is low in blood and liver relative to many cultured cells/tissues, potentially relevant to tissue-specific drug effects. (pqac-00000015, pqac-00000003, pqac-00000013, pqac-00000010) | Van Rompay et al., 2001, https://doi.org/10.1124/mol.59.5.1181; El Hassouni et al., 2019, AntiCancer Res, doi:10.21873/anticanres.13508, https://doi.org/10.21873/anticanres.13508; Xu et al., 2023, Nucleic Acids Res, doi:10.1093/nar/gkad1002, https://doi.org/10.1093/nar/gkad1002; Li et al., 2024, J Inflamm Res, doi:10.2147/JIR.S440295, https://doi.org/10.2147/jir.s440295 |
| Cancer roles | UCK2 is repeatedly linked to malignant proliferation and metastasis, with both metabolic and reported non-metabolic signaling roles. Recent studies show overexpression in HTLV-1/ATL T cells supporting proliferation, and in intrahepatic cholangiocarcinoma promoting growth, migration/invasion, poorer survival, and cisplatin resistance via PI3K/AKT/mTOR-autophagy; pan-cancer analyses also report amplification/upregulation associations. (pqac-00000008, pqac-00000010, pqac-00000014, pqac-00000004) | Watanabe et al., 2024, Blood Adv, doi:10.1182/bloodadvances.2023011131, https://doi.org/10.1182/bloodadvances.2023011131; Wu et al., 2024, Cell Death Discov, doi:10.1038/s41420-024-02140-x, https://doi.org/10.1038/s41420-024-02140-x; Li et al., 2024, https://doi.org/10.2147/jir.s440295; Fu et al., 2022, https://doi.org/10.3389/fonc.2022.904887 |
| Drug activation / biomarker use | UCK2 phosphorylates and thereby activates several cytotoxic ribonucleoside analogs. Strongest cited translational evidence is for RX-3117, where UCK2 knockdown protected tumor cells and drug nucleotide accumulation correlated with UCK2 expression (r = 0.803 and 0.915 in cell panels/xenografts); in pancreatic cancer, high UCK2 protein was observed in 21/25 tumors and associated with shorter mean overall survival (8.4 vs 34.3 months, p = 0.045). UCK2 is also implicated in activation of azacytidine and other analogs, and chromosome 1q gains may create a UCK2-linked therapeutic vulnerability. (pqac-00000001, pqac-00000003, pqac-00000013) | Sarkisjan et al., 2016, PLoS ONE, doi:10.1371/journal.pone.0162901, https://doi.org/10.1371/journal.pone.0162901; El Hassouni et al., 2019, AntiCancer Res, doi:10.21873/anticanres.13508, https://doi.org/10.21873/anticanres.13508; Xu et al., 2023, https://doi.org/10.1093/nar/gkad1002 |
| Antiviral context | In host-directed antiviral strategies, UCK2 is the salvage-pathway kinase whose activity can preserve pyrimidine pools when de novo synthesis is blocked, making it an attractive cotarget with DHODH. Small-molecule UCK2 inhibitors and allosteric leads have been developed to suppress uridine salvage, and 2023 work showed that UCK2 also potentiates mutagenic effects of β-d-N4-hydroxycytidine/molnupiravir in cells, with loss of UCK2 reducing mutagenicity. (pqac-00000007, pqac-00000005, pqac-00000013) | Okesli-Armlovich et al., 2019, Bioorg Med Chem Lett, doi:10.1016/j.bmcl.2019.08.010, https://doi.org/10.1016/j.bmcl.2019.08.010; Mashayekh et al., 2022, Biochemistry, doi:10.1021/acs.biochem.2c00451, https://doi.org/10.1021/acs.biochem.2c00451; Xu et al., 2023, Nucleic Acids Res, doi:10.1093/nar/gkad1002, https://doi.org/10.1093/nar/gkad1002 |


*Table: This table summarizes compact functional-annotation evidence for human UCK2/Q9BZX2 across enzymology, structure, regulation, expression, disease biology, and translational applications. It highlights both foundational mechanistic studies and recent 2023–2024 papers most relevant to current understanding.*