UCK1 (Uridine-cytidine kinase 1) is a nucleoside kinase that catalyzes the phosphorylation of uridine and cytidine to their respective monophosphates (UMP and CMP), functioning in the pyrimidine nucleotide salvage pathway. The enzyme can use ATP or GTP as phosphate donors and shows specificity for pyrimidine ribonucleosides, not phosphorylating deoxyribonucleosides or purine ribonucleosides. UCK1 is also capable of phosphorylating various nucleoside analogs used in cancer chemotherapy and antiviral therapy.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
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GO:0005737
cytoplasm
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: Cytoplasm is a broad, pathway-compatible location for UCK1, but the localization evidence is mixed. Reactome describes cytosolic UCK1 activity and the IBA row supports cytoplasm, while PMID:27239701 reports UCK1-GFP predominantly in the nucleus and UCK2-GFP in the cytosol.
Reason: Accept this broad cytoplasm row only as a non-exclusive location supported by GOA/Reactome-style pathway context. It should not be read as overriding the later GFP evidence for a nuclear UCK1 pool, and a direct nucleus annotation may be warranted after curator review.
Supporting Evidence:
Reactome:R-HSA-73599
Cytosolic uridine-cytidine kinase 1 (UCK1) catalyzes the reactions of cytidine or uridine with ATP to form CMP or UMP and ADP
file:human/UCK1/UCK1-deep-research-perplexity-lite.md
See deep research file for comprehensive analysis
PMID:27239701
Subcellular localization studies showed that UCK1-GFP and UCK2-GFP were localized in the cell nucleus and cytosol, respectively.
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GO:0004849
uridine kinase activity
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: Uridine kinase activity is a core function of UCK1, directly demonstrated by experimental evidence showing phosphorylation of uridine to UMP. This IBA annotation is consistent with direct experimental evidence (IDA) from PMID:11306702. The enzyme is highly specific for pyrimidine ribonucleosides.
Reason: This represents one of the two core molecular functions of UCK1. The enzyme catalyzes the phosphorylation of uridine to UMP using ATP or GTP as phosphate donor, as demonstrated experimentally in the original characterization study and confirmed through phylogenetic inference. The substrate specificity is well-established - UCK1 does not phosphorylate deoxyribonucleosides or purine ribonucleosides.
Supporting Evidence:
PMID:11306702
The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. The enzymes did not phosphorylate deoxyribonucleosides or purine ribonucleosides.
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GO:0009224
CMP biosynthetic process
|
IEA
GO_REF:0000108 |
MODIFY |
Summary: UCK1 does generate CMP from cytidine, but this GO term is a broad biosynthetic-process parent that does not distinguish the salvage pathway from de novo CMP biosynthesis. The more informative and already annotated term is GO:0044211 CTP salvage.
Reason: The evidence supports cytidine phosphorylation as the first step of CTP salvage, not a generic CMP biosynthetic process. Replace with the specific salvage-pathway term already supported by direct experimental evidence and UniProt pathway annotation.
Proposed replacements:
CTP salvage
Supporting Evidence:
file:human/UCK1/UCK1-uniprot.txt
PATHWAY: Pyrimidine metabolism; CTP biosynthesis via salvage pathway; CTP from cytidine: step 1/3.
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|
GO:0000166
nucleotide binding
|
IEA
GO_REF:0000043 |
MARK AS OVER ANNOTATED |
Summary: This is an overly general parent term that doesn't provide meaningful information about UCK1's specific function. While technically correct that UCK1 binds nucleotides (ATP/GTP as phosphate donors, and uridine/cytidine as substrates), this broad term lacks functional specificity. The more specific child term GO:0005524 (ATP binding) is already annotated.
Reason: This term is too vague and uninformative for curation purposes. It was automatically assigned based on UniProtKB keywords. UCK1 specifically binds ATP (and can also use GTP) as phosphate donors for its kinase activity. The more specific GO:0005524 (ATP binding) provides much more useful functional information and is already annotated. This overly broad parent term adds little value to the annotation set.
Supporting Evidence:
PMID:11306702
Uridine-cytidine kinases (UCK) have important roles for the phosphorylation of nucleoside analogs that are being investigated for possible use in chemotherapy of cancer.
|
|
GO:0004849
uridine kinase activity
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: This is a duplicate annotation of GO:0004849 with computational evidence (IEA), while the same term is also annotated with both IBA and IDA evidence. The IEA annotation is redundant but not incorrect.
Reason: Although this is a duplicate annotation with weaker evidence than the IBA and IDA annotations for the same term, it is still accurate and represents a core function of UCK1. Duplicate annotations with different evidence codes are acceptable in GO.
Supporting Evidence:
PMID:11306702
UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
|
|
GO:0005524
ATP binding
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: UCK1 binds ATP as a phosphate donor for its kinase activity. The enzyme can also use GTP, but ATP is the primary physiological phosphate donor. This IEA annotation is supported by structural data (PDB 2JEO, 2UVQ) showing ATP/ADP binding sites and by the biochemical characterization demonstrating ATP usage.
Reason: ATP binding is well-supported by multiple lines of evidence. The enzyme uses ATP (or GTP) as a phosphate donor in the reaction uridine/cytidine + ATP → UMP/CMP + ADP + H+. Crystallographic structures (PDB 2JEO, 2UVQ) directly demonstrate ATP/ADP binding. This is a functionally relevant molecular function term that is more informative than the parent term GO:0000166 (nucleotide binding). The annotation was correctly inferred from InterPro domains and UniProtKB keywords.
Supporting Evidence:
PMID:11306702
The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
|
|
GO:0016301
kinase activity
|
IEA
GO_REF:0000120 |
MARK AS OVER ANNOTATED |
Summary: This is a high-level parent term in the molecular function hierarchy. While UCK1 is indeed a kinase (specifically a nucleoside kinase), this term is overly general. The more specific and informative terms GO:0004849 (uridine kinase activity) and GO:0043771 (cytidine kinase activity) are already annotated with both IBA and IDA evidence.
Reason: This broad parent term was automatically inferred from InterPro domains and UniProtKB keywords. While technically accurate, it provides minimal functional information compared to the specific child terms (uridine kinase activity, cytidine kinase activity) that are already annotated with experimental evidence. In curation, such overly general parent terms are typically considered over-annotations when more specific child terms adequately describe the function.
Supporting Evidence:
PMID:11306702
The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
|
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GO:0016740
transferase activity
|
IEA
GO_REF:0000043 |
MARK AS OVER ANNOTATED |
Summary: This is an extremely high-level parent term in the molecular function ontology. UCK1 is indeed a transferase (kinases transfer phosphate groups from ATP to substrates), but this root-level term provides virtually no functional specificity. Multiple more informative descendant terms are already annotated (kinase activity, uridine kinase activity, cytidine kinase activity).
Reason: This is a classic example of over-annotation with an overly broad parent term. It was automatically assigned based on UniProtKB keywords. While technically correct, GO:0016740 is so general that it encompasses a vast array of different enzyme activities. The more specific child terms (GO:0016301 kinase activity, GO:0004849 uridine kinase activity, GO:0043771 cytidine kinase activity) provide far more useful functional information. Annotating to such high-level parent terms when specific child terms exist is generally discouraged in manual curation.
Supporting Evidence:
PMID:11306702
The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
|
|
GO:0043771
cytidine kinase activity
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: This is a duplicate annotation of GO:0043771 with computational evidence (IEA), while the same term is also annotated with experimental evidence (IDA from PMID:11306702). Cytidine kinase activity is one of the two core molecular functions of UCK1. The IEA annotation was correctly inferred from RHEA reactions and EC number 2.7.1.48.
Reason: Although this is a duplicate annotation with weaker evidence than the IDA annotation for the same term, it is still accurate and represents a core function of UCK1. The enzyme catalyzes the reaction cytidine + ATP = CMP + ADP + H+. Duplicate annotations with different evidence codes are acceptable in GO and can arise from independent annotation pipelines (manual curation vs automated inference). Both contribute to the overall evidence for this function.
Supporting Evidence:
PMID:11306702
The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
|
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GO:0044206
UMP salvage
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: This is a duplicate annotation of GO:0044206 with computational evidence (IEA), while the same term is also annotated with experimental evidence (IDA from PMID:11306702). UMP salvage is a core biological process for UCK1, representing the salvage pathway alternative to de novo UMP biosynthesis. The IEA was correctly inferred from UniPathway UPA00574.
Reason: Although this is a duplicate annotation with weaker evidence than the IDA annotation, it accurately represents UCK1's role in the pyrimidine salvage pathway. UCK1 catalyzes the sole step in UMP biosynthesis via the salvage pathway (uridine + ATP → UMP + ADP + H+). This is distinct from de novo pyrimidine synthesis. Duplicate annotations with different evidence codes are acceptable and can arise from independent curation and automated inference pipelines.
Supporting Evidence:
PMID:11306702
The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
|
|
GO:0044211
CTP salvage
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: This is a duplicate annotation of GO:0044211 with computational evidence (IEA), while the same term is also annotated with experimental evidence (IDA from PMID:11306702). CTP salvage is a core biological process for UCK1, representing the salvage pathway alternative to de novo CTP biosynthesis. The IEA was correctly inferred from UniPathway UPA00579.
Reason: Although this is a duplicate annotation with weaker evidence than the IDA annotation, it accurately represents UCK1's role in the pyrimidine salvage pathway. UCK1 catalyzes the first step of three in CTP biosynthesis via the salvage pathway (cytidine + ATP → CMP + ADP + H+, followed by CMP → CDP → CTP by other enzymes). This is distinct from de novo pyrimidine synthesis. Duplicate annotations with different evidence codes are acceptable and can arise from independent curation and automated inference pipelines.
Supporting Evidence:
PMID:11306702
The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
|
|
GO:0004849
uridine kinase activity
|
IDA
PMID:11306702 Phosphorylation of uridine and cytidine nucleoside analogs b... |
ACCEPT |
Summary: Uridine kinase activity is a core molecular function of UCK1, directly demonstrated by experimental evidence (IDA). This annotation is based on the original biochemical characterization study by Van Rompay et al. (2001) showing that recombinant UCK1 phosphorylates uridine to UMP. The enzyme exhibits strict substrate specificity, not phosphorylating deoxyribonucleosides or purines.
Reason: This is the gold standard annotation with direct experimental evidence (IDA) from the original characterization paper PMID:11306702. The study cloned and expressed recombinant UCK1 in E. coli and directly demonstrated its ability to catalyze uridine phosphorylation using biochemical assays. This represents one of the two core enzymatic activities of UCK1 (the other being cytidine kinase activity). The annotation is well-supported, specific, and represents a primary molecular function.
Supporting Evidence:
PMID:11306702
The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. The enzymes did not phosphorylate deoxyribonucleosides or purine ribonucleosides.
|
|
GO:0043771
cytidine kinase activity
|
IDA
PMID:11306702 Phosphorylation of uridine and cytidine nucleoside analogs b... |
ACCEPT |
Summary: Cytidine kinase activity is a core molecular function of UCK1, directly demonstrated by experimental evidence (IDA). This annotation is based on the original biochemical characterization study by Van Rompay et al. (2001) showing that recombinant UCK1 phosphorylates cytidine to CMP. The enzyme exhibits strict substrate specificity, not phosphorylating deoxyribonucleosides or purines.
Reason: This is the gold standard annotation with direct experimental evidence (IDA) from the original characterization paper PMID:11306702. The study cloned and expressed recombinant UCK1 in E. coli and directly demonstrated its ability to catalyze cytidine phosphorylation using biochemical assays. This represents one of the two core enzymatic activities of UCK1 (the other being uridine kinase activity). The annotation is well-supported, specific, and represents a primary molecular function. UCK1's dual substrate specificity for both uridine and cytidine distinguishes it from other nucleoside kinases.
Supporting Evidence:
PMID:11306702
The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. The enzymes did not phosphorylate deoxyribonucleosides or purine ribonucleosides.
|
|
GO:0044206
UMP salvage
|
IDA
PMID:11306702 Phosphorylation of uridine and cytidine nucleoside analogs b... |
ACCEPT |
Summary: UMP salvage is a core biological process for UCK1, directly supported by experimental evidence (IDA). UCK1 catalyzes the conversion of uridine to UMP (uridine + ATP → UMP + ADP + H+), which is the sole enzymatic step in UMP biosynthesis via the salvage pathway. This salvage pathway provides an energy-efficient alternative to de novo pyrimidine synthesis.
Reason: This is an accurate and well-supported annotation with direct experimental evidence (IDA) from PMID:11306702. UCK1 performs the complete UMP salvage pathway in a single step, converting free uridine (from nucleotide turnover or dietary sources) back into UMP for nucleotide pool maintenance. This is a core biological process function. The UniProt entry explicitly states this is "step 1/1" in UMP biosynthesis via salvage pathway. The salvage pathway is particularly important in tissues with high nucleotide turnover and for activating nucleoside analog drugs.
Supporting Evidence:
PMID:11306702
The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
|
|
GO:0044211
CTP salvage
|
IDA
PMID:11306702 Phosphorylation of uridine and cytidine nucleoside analogs b... |
ACCEPT |
Summary: CTP salvage is a core biological process for UCK1, directly supported by experimental evidence (IDA). UCK1 catalyzes the first step in CTP biosynthesis via the salvage pathway by converting cytidine to CMP (cytidine + ATP → CMP + ADP + H+). CMP is subsequently phosphorylated to CDP and then CTP by other enzymes (CMP kinase and nucleoside diphosphate kinase). This salvage pathway provides an energy-efficient alternative to de novo pyrimidine synthesis.
Reason: This is an accurate and well-supported annotation with direct experimental evidence (IDA) from PMID:11306702. UCK1 performs the rate-limiting first step (step 1 of 3) in the CTP salvage pathway, converting free cytidine (from nucleotide turnover or dietary sources) to CMP. The complete pathway is cytidine → CMP → CDP → CTP. This is a core biological process function. The UniProt entry explicitly states this is "step 1/3" in CTP biosynthesis via salvage pathway. The salvage pathway is particularly important in rapidly dividing cells and for activating cytidine analog drugs used in cancer chemotherapy.
Supporting Evidence:
PMID:11306702
The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
|
|
GO:0005829
cytosol
|
TAS
Reactome:R-HSA-73599 |
ACCEPT |
Summary: Reactome explicitly describes UCK1 as cytosolic in the cytidine/uridine phosphorylation reaction, but this conflicts with later UCK1-GFP evidence reporting predominant nuclear localization. The cytosol row is therefore accepted as Reactome pathway-supported but not as the sole or best localization for UCK1.
Reason: Keep the cytosol annotation because it is directly supported by the Reactome TAS source for the metabolic reaction. However, this annotation should be interpreted alongside PMID:27239701, which reports nuclear UCK1-GFP localization and UCK1-dependent relocalization of UCK2. The review flags nucleus as a suggested follow-up annotation rather than treating cytosol as exclusive.
Supporting Evidence:
Reactome:R-HSA-73599
Cytosolic uridine-cytidine kinase 1 (UCK1) catalyzes the reactions of cytidine or uridine with ATP to form CMP or UMP and ADP
PMID:27239701
Subcellular localization studies showed that UCK1-GFP and UCK2-GFP were localized in the cell nucleus and cytosol, respectively.
|
Q: Should UCK1 receive a direct nucleus cellular-component annotation based on the GFP localization evidence from PMID:27239701?
Suggested experts: GO cellular component curators, nucleotide metabolism specialists
Q: Which UCK1 isoforms or cell states determine nuclear versus cytosolic pyrimidine-salvage activity?
Suggested experts: cell biologists, metabolic enzyme specialists
Experiment: Use tagged knock-in or validated antibody staining to compare endogenous UCK1 localization with UCK1-GFP localization across proliferating and differentiated cells.
Hypothesis: UCK1 has a substantial nuclear pool that is not captured by current cytosol/cytoplasm pathway annotations.
Type: Endogenous localization
Experiment: Express individual UCK1 isoforms in UCK1/UCK2-deficient cells and measure uridine, cytidine, and nucleoside-analog phosphorylation.
Hypothesis: UCK1 isoforms differ in localization and in their contribution to pyrimidine salvage and analog activation.
Type: Isoform rescue biochemistry
The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
Target verified. The requested protein is human uridine–cytidine kinase 1 (UCK1), an enzyme classified as EC 2.7.1.48 that phosphorylates the ribonucleosides uridine (Urd) and cytidine (Cyd). Foundational cloning/biochemistry work identified two human uridine–cytidine kinases (UCK1 and UCK2) as ~30 kDa proteins and mapped UCK1 to chromosome 9q34.2–9q34.3; UCK1 mRNA was reported as two isoforms and was broadly expressed across tissues (Van Rompay et al., May 2001, Molecular Pharmacology, https://doi.org/10.1124/mol.59.5.1181). (rompay2001phosphorylationofuridine pages 1-2)
Conceptual definition (pyrimidine salvage). UCK1 catalyzes the first (often rate-limiting) ATP-dependent step in the pyrimidine ribonucleoside salvage pathway, converting imported or recycled Urd/Cyd into nucleotide monophosphates that can be further phosphorylated to triphosphates for RNA/DNA and phospholipid biosynthesis. (kuilenburg2016thepivotalrole pages 1-2, rompay2001phosphorylationofuridine pages 1-2)
UCK1 catalyzes:
This ATP-dependent monophosphorylation is the defining biochemical activity of human UCK1 described in recombinant enzyme experiments. (rompay2001phosphorylationofuridine pages 1-2)
Recombinant human UCK1 phosphorylates uridine and cytidine, and does not phosphorylate deoxyribonucleosides or purine ribonucleosides, supporting a role specific to pyrimidine ribonucleoside salvage rather than deoxynucleoside salvage (which is handled by other kinases such as DCK in separate pathways). (rompay2001phosphorylationofuridine pages 1-2)
UCK1 can phosphorylate multiple pyrimidine ribonucleoside analogs, supporting its pharmacological relevance as a “bioactivating kinase” for certain nucleoside analog drugs/prodrugs. In a systematic substrate survey (28 analogs tested), examples phosphorylated by human UCK1 included 6-azauridine, 5-fluorouridine, 4-thiouridine, 5-bromouridine, N4-acetylcytidine, N4-benzoylcytidine, 5-fluorocytidine, 2-thiocytidine, 5-methylcytidine, and N4-anisoylcytidine (Van Rompay et al., May 2001). (rompay2001phosphorylationofuridine pages 1-2)
Quantitative substrate preference evidence (Table 2). Van Rompay et al. provide a table of relative phosphorylation efficiencies for many analogs by UCK1 vs UCK2, normalized to uridine phosphorylation (Table 2). This table is the most direct comparative dataset in the retrieved evidence for analog substrate breadth and differential activation potential between the isoenzymes. (rompay2001phosphorylationofuridine media 4d67e9df)
A key UCK1-specific mechanistic observation is subcellular compartmentalization:
Moreover, co-expression of UCK1 with UCK2 could relocalize UCK2 to the nucleus and impair UCK2 function; physical association between UCK1 and UCK2 was supported by pull-down experiments (His-tag pull-down). These findings imply that UCK1 may influence pyrimidine salvage not only by catalysis but also by modulating UCK2 localization and activity through interaction. (van Kuilenburg & Meinsma, Sep 2016, BBA – Molecular Basis of Disease, https://doi.org/10.1016/j.bbadis.2016.05.012). (kuilenburg2016thepivotalrole pages 1-2)
UCK1 functions in pyrimidine nucleotide supply by enabling salvage of extracellular or recycled uridine/cytidine into nucleotide pools (UMP/CMP → UDP/CDP → UTP/CTP via downstream kinases). This positions UCK1 as part of the broader metabolic program supporting proliferation and RNA synthesis. (kuilenburg2016thepivotalrole pages 1-2, rompay2001phosphorylationofuridine pages 1-2)
Reviews and primary studies emphasize that UCK2 often has higher catalytic efficiency for uridine/cytidine and may dominate pyrimidine salvage flux in some cancer contexts, while UCK1 is more ubiquitously expressed and can have distinct localization/interaction roles. (kuilenburg2016thepivotalrole pages 1-2, matchett2022characterizationofuridinecytidine pages 1-2)
A clinically actionable application of UCK1 biology is its role in activating the hypomethylating agent azacitidine (AZA), a cytidine analog used in myelodysplastic syndromes (MDS).
Patient cohort statistics (MDS, AZA-treated). In a cohort of 57 MDS patients treated with AZA, lower UCK1 expression was observed in non-responders compared with responders (median 0.2 vs 0.49, P = 0.07). Patients with below-median UCK1 expression had shorter overall survival from AZA onset (P = 0.049). (Valencia et al., Nov 2014, Leukemia, https://doi.org/10.1038/leu.2013.330). (valencia2014expressionofnucleosidemetabolizing pages 1-2)
Functional validation (in vitro). In K562 cells, siRNA silencing of UCK1 reduced AZA-induced apoptosis from 31% ± 0.65% (control) to 11% ± 0.34% (effective siUCK1), consistent with UCK1 being a determinant of intracellular AZA activation and cytotoxic effect (P = 0.0015). (valencia2014expressionofnucleosidemetabolizing pages 4-5)
Mechanistic notes. In the same study, differences in UCK1 expression were not explained by detectable UCK1 promoter hypermethylation, and common coding polymorphisms were not associated with UCK1 expression or AZA response, suggesting additional regulatory layers (e.g., transcriptional/post-transcriptional or protein stability mechanisms) may be responsible in patients. (valencia2014expressionofnucleosidemetabolizing pages 1-2, valencia2014expressionofnucleosidemetabolizing pages 4-5)
The recombinant cloning and substrate profiling of UCK1 and UCK2 was explicitly motivated by improving the development/characterization of pyrimidine ribonucleoside analogs for chemotherapy, because the first phosphorylation step can be rate-limiting and therefore affect drug sensitivity and tissue-specific toxicity. (rompay2001phosphorylationofuridine pages 1-2, rompay2001phosphorylationofuridine media 4d67e9df)
A 2023 authoritative review in Nature Reviews Cancer frames nucleotide metabolism (including salvage) as a pan-cancer metabolic dependency and explicitly includes UCK1/UCK2 among enzymes in the pyrimidine nucleoside salvage pathway. This review emphasizes that cancers frequently upregulate nucleotide supply to support proliferation and therapy resistance, motivating therapeutic strategies that combine inhibitors of nucleotide production with other treatments. (Mullen & Singh, Mar 2023, https://doi.org/10.1038/s41568-023-00557-7). (shu2025uridinecytidinekinases pages 1-4)
Within the documents retrieved here, most 2023–2024 primary mechanistic work is focused on UCK2 rather than UCK1, reflecting a broader trend that UCK2 is frequently overexpressed in cancers and has higher catalytic efficiency. Consequently, the most direct UCK1-specific, experimentally grounded annotations remain anchored in the earlier primary and mid-period review literature (2001–2016) plus clinically oriented UCK1/AZA studies (2014). (shu2025uridinecytidinekinases pages 1-4, valencia2014expressionofnucleosidemetabolizing pages 1-2, kuilenburg2016thepivotalrole pages 1-2)
| Aspect | Key points | Evidence (DOI/URL + year) | Notes |
|---|---|---|---|
| Verified identity | Human UCK1 encodes uridine-cytidine kinase 1, a 277 aa protein of about 31 kDa; UCK1 is broadly/ubiquitously expressed, with two mRNA isoforms reported, and the gene maps to chromosome 9q34.2-q34.3. | Van Rompay et al., Mol Pharmacol (2001), DOI: 10.1124/mol.59.5.1181; van Kuilenburg & Meinsma, BBA Mol Basis Dis (2016), DOI: 10.1016/j.bbadis.2016.05.012 (rompay2001phosphorylationofuridine pages 1-2, kuilenburg2016thepivotalrole pages 1-2) | Matches the requested target: human UCK1 / UniProt Q9HA47; distinct from UCK2 and UCKL1. |
| Core enzymatic function | UCK1 catalyzes the ATP-dependent phosphorylation of uridine and cytidine to UMP and CMP, the first/rate-limiting step of the pyrimidine salvage pathway. It does not phosphorylate deoxyribonucleosides or purine ribonucleosides. | Van Rompay et al. (2001), DOI: 10.1124/mol.59.5.1181; van Kuilenburg & Meinsma (2016), DOI: 10.1016/j.bbadis.2016.05.012 (rompay2001phosphorylationofuridine pages 1-2, kuilenburg2016thepivotalrole pages 1-2) | Foundational biochemical definition of human UCK1 activity. |
| Substrate specificity and analog activation | Recombinant human UCK1 phosphorylates multiple pyrimidine ribonucleoside analogs in addition to uridine/cytidine. Examples reported include 6-azauridine, 5-fluorouridine, 4-thiouridine, 5-bromouridine, N4-acetylcytidine, 5-fluorocytidine, 2-thiocytidine, 5-methylcytidine, and N4-anisoylcytidine; Table 2 quantifies relative phosphorylation efficiencies for analogs. | Van Rompay et al. (2001), DOI: 10.1124/mol.59.5.1181 and Table 2 view (rompay2001phosphorylationofuridine pages 4-5, rompay2001phosphorylationofuridine media 4d67e9df) | Table 2 is the key source for analog examples and relative activity comparisons between UCK1 and UCK2. |
| Subcellular localization | In GFP-fusion experiments, UCK1-GFP localized to the nucleus, whereas UCK2-GFP was cytosolic. Co-expression of UCK1 with UCK2 could relocalize UCK2 to the nucleus and impair its function; a physical UCK1-UCK2 association was supported by pull-down. | van Kuilenburg & Meinsma (2016), DOI: 10.1016/j.bbadis.2016.05.012 (kuilenburg2016thepivotalrole pages 1-2) | Important for annotation because it places human UCK1 function primarily in the nuclear compartment in these studies. |
| Comparative enzymology | UCK1 and UCK2 share about 72% sequence similarity, but UCK2 has higher catalytic efficiency for uridine/cytidine; in neuroblastoma, UCK2 was the predominant active isoform in the reviewed studies. | van Kuilenburg & Meinsma (2016), DOI: 10.1016/j.bbadis.2016.05.012; Matchett et al. (2022), DOI: 10.1042/bcj20210770 (kuilenburg2016thepivotalrole pages 1-2, matchett2022characterizationofuridinecytidine pages 1-2) | Useful context: UCK1 is biochemically active but often less efficient than UCK2. |
| Clinical relevance: azacitidine activation | UCK1 contributes to azacitidine (AZA) activation. In a cohort of 57 MDS patients, non-responders had lower UCK1 expression than responders (median 0.2 vs 0.49, P = 0.07). Patients with below-median UCK1 had shorter overall survival (P = 0.049). | Valencia et al., Leukemia (2014), DOI: 10.1038/leu.2013.330 (valencia2014expressionofnucleosidemetabolizing pages 1-2) | UCK1 promoter methylation and common coding polymorphisms did not explain the expression-response difference in this study. |
| Functional resistance evidence | In K562 cells, siRNA knockdown of UCK1 blunted AZA response: AZA-induced apoptosis fell from 31% ± 0.65% in controls to 11% ± 0.34% with effective siUCK1 (P = 0.0015). | Valencia et al. (2014), DOI: 10.1038/leu.2013.330 (valencia2014expressionofnucleosidemetabolizing pages 4-5, valencia2014expressionofnucleosidemetabolizing pages 5-7) | Direct experimental support that UCK1 abundance can influence drug sensitivity. |
| Recent review framing | Recent cancer metabolism review literature places UCK1/UCK2 in the pyrimidine nucleoside salvage pathway, where salvaged uridine/cytidine are phosphorylated as part of nucleotide supply for proliferating cells. | Mullen & Singh, Nat Rev Cancer (2023), DOI: 10.1038/s41568-023-00557-7 (shu2025uridinecytidinekinases pages 1-4) | The 2023 review is pathway-framing rather than UCK1-specific mechanistic primary evidence. |
Table: This table summarizes the core functional annotation of human UCK1 (UniProt Q9HA47), including identity, enzymatic activity, localization, drug-relevant biology, and recent pathway framing. It is useful as a compact evidence map anchored to the provided source snippets.
References
(rompay2001phosphorylationofuridine pages 1-2): An R. Van Rompay, Ameli Norda, Karin Lindén, Magnus Johansson, and Anna Karlsson. Phosphorylation of uridine and cytidine nucleoside analogs by two human uridine-cytidine kinases. Molecular pharmacology, 59 5:1181-6, May 2001. URL: https://doi.org/10.1124/mol.59.5.1181, doi:10.1124/mol.59.5.1181. This article has 220 citations and is from a domain leading peer-reviewed journal.
(kuilenburg2016thepivotalrole pages 1-2): André B.P. van Kuilenburg and Rutger Meinsma. The pivotal role of uridine-cytidine kinases in pyrimidine metabolism and activation of cytotoxic nucleoside analogues in neuroblastoma. Biochimica et biophysica acta, 1862 9:1504-12, Sep 2016. URL: https://doi.org/10.1016/j.bbadis.2016.05.012, doi:10.1016/j.bbadis.2016.05.012. This article has 44 citations.
(rompay2001phosphorylationofuridine media 4d67e9df): An R. Van Rompay, Ameli Norda, Karin Lindén, Magnus Johansson, and Anna Karlsson. Phosphorylation of uridine and cytidine nucleoside analogs by two human uridine-cytidine kinases. Molecular pharmacology, 59 5:1181-6, May 2001. URL: https://doi.org/10.1124/mol.59.5.1181, doi:10.1124/mol.59.5.1181. This article has 220 citations and is from a domain leading peer-reviewed journal.
(matchett2022characterizationofuridinecytidine pages 1-2): Emily C. Matchett, Elise C. Ambrose, and Jacki Kornbluth. Characterization of uridine-cytidine kinase like-1 nucleoside kinase activity and its role in tumor growth. Biochemical Journal, 479:1149-1164, Jun 2022. URL: https://doi.org/10.1042/bcj20210770, doi:10.1042/bcj20210770. This article has 16 citations and is from a domain leading peer-reviewed journal.
(valencia2014expressionofnucleosidemetabolizing pages 1-2): Ana Valencia, E. Masala, A. Rossi, Alessandro Martino, A. Sanna, Francesca Buchi, F. Canzian, D. Cilloni, Gaidano, M. Voso, O. Kosmider, M. Fontenay, Antonella Gozzini, Alberto Bosi, and Santini. Expression of nucleoside-metabolizing enzymes in myelodysplastic syndromes and modulation of response to azacitidine. Leukemia, 28:621-628, Nov 2014. URL: https://doi.org/10.1038/leu.2013.330, doi:10.1038/leu.2013.330. This article has 109 citations and is from a highest quality peer-reviewed journal.
(valencia2014expressionofnucleosidemetabolizing pages 4-5): Ana Valencia, E. Masala, A. Rossi, Alessandro Martino, A. Sanna, Francesca Buchi, F. Canzian, D. Cilloni, Gaidano, M. Voso, O. Kosmider, M. Fontenay, Antonella Gozzini, Alberto Bosi, and Santini. Expression of nucleoside-metabolizing enzymes in myelodysplastic syndromes and modulation of response to azacitidine. Leukemia, 28:621-628, Nov 2014. URL: https://doi.org/10.1038/leu.2013.330, doi:10.1038/leu.2013.330. This article has 109 citations and is from a highest quality peer-reviewed journal.
(shu2025uridinecytidinekinases pages 1-4): Huazhang Shu, Sushma Sharm, Seher Alam, Lilian Frank, Marianna Tampere, André B.P. van Kuilenburg, Nicholas C.K. Valerie, Mikael Altun, Andrei Chabes, Sean G. Rudd, and Si Min Zhang. Uridine cytidine kinases dictate the therapeutic response of molnupiravir via its bioactivation. bioRxiv, May 2025. URL: https://doi.org/10.1101/2025.05.13.653844, doi:10.1101/2025.05.13.653844. This article has 0 citations.
(rompay2001phosphorylationofuridine pages 4-5): An R. Van Rompay, Ameli Norda, Karin Lindén, Magnus Johansson, and Anna Karlsson. Phosphorylation of uridine and cytidine nucleoside analogs by two human uridine-cytidine kinases. Molecular pharmacology, 59 5:1181-6, May 2001. URL: https://doi.org/10.1124/mol.59.5.1181, doi:10.1124/mol.59.5.1181. This article has 220 citations and is from a domain leading peer-reviewed journal.
(valencia2014expressionofnucleosidemetabolizing pages 5-7): Ana Valencia, E. Masala, A. Rossi, Alessandro Martino, A. Sanna, Francesca Buchi, F. Canzian, D. Cilloni, Gaidano, M. Voso, O. Kosmider, M. Fontenay, Antonella Gozzini, Alberto Bosi, and Santini. Expression of nucleoside-metabolizing enzymes in myelodysplastic syndromes and modulation of response to azacitidine. Leukemia, 28:621-628, Nov 2014. URL: https://doi.org/10.1038/leu.2013.330, doi:10.1038/leu.2013.330. This article has 109 citations and is from a highest quality peer-reviewed journal.
UCK1 (Uridine-Cytidine Kinase 1) encodes a uridine-cytidine kinase that catalyzes the phosphorylation of uridine and cytidine nucleosides to their monophosphate forms—uridine monophosphate (UMP) and cytidine monophosphate (CMP), respectively[1][6]. This enzyme is classified as a kinase and transferase based on its molecular function[9].
A critical characteristic of UCK1 is its substrate specificity. The enzyme does not phosphorylate deoxyribonucleosides or purine ribonucleosides, demonstrating selective recognition of pyrimidine ribonucleosides[1][11]. Additionally, UCK1 can utilize both ATP and GTP as phosphate donors, providing metabolic flexibility in nucleotide synthesis[1][6].
Beyond natural substrates, UCK1 demonstrates remarkable versatility in phosphorylating various nucleoside analogs, including 6-azauridine, 5-fluorouridine, 4-thiouridine, 5-bromouridine, N(4)-acetylcytidine, N(4)-benzoylcytidine, 5-fluorocytidine, 2-thiocytidine, 5-methylcytidine, and N(4)-anisoylcytidine[1]. This capacity is particularly significant for pharmaceutical applications, as many anticancer and antiviral drugs are nucleoside analogs that require UCK1-mediated activation.
UCK1 functions as a key enzyme in the nucleotide salvage pathway, which recycles nucleosides back into nucleotide pools for DNA and RNA synthesis[3]. The nucleotide salvage pathway represents an energy-efficient alternative to de novo nucleotide synthesis, allowing cells to reuse existing nucleoside building blocks.
Human uridine-cytidine kinases, including UCK1 and its paralog UCK2, display distinct functions in cellular metabolism[3]. While both enzymes catalyze similar phosphorylation reactions, they exhibit different tissue distributions and regulatory properties, suggesting specialized roles in different cellular contexts.
UCK1 is located on chromosome 9 (positions 131,523,786-131,531,280 on the reverse strand)[10]. The gene exhibits significant structural complexity, encoding a ~30 kDa protein and generating 15 different transcripts through alternative splicing, which produce distinct protein isoforms[1][10]. This alternative splicing mechanism allows for functional diversity and tissue-specific expression patterns.
The gene has 202 orthologues across different species and 3 paralogues within the human genome, with UCK2 being identified as an important paralog[1][10]. This evolutionary conservation underscores the fundamental importance of pyrimidine nucleoside phosphorylation in cellular metabolism.
UCK1 has been associated with several pathological conditions[1]:
Additionally, UCK1 expression levels correlate with sensitivity to certain anticancer agents. For example, sensitivity of human cancer cells to the anticancer ribonucleoside TAS-106 is correlated with UCK1 and UCK2 expression levels, as these enzymes are required for drug activation[4].
The ability of UCK1 to phosphorylate nucleoside analogs makes it a critical determinant of drug efficacy and resistance in cancer chemotherapy and antiviral therapy. Tumors with reduced UCK1 expression may develop resistance to nucleoside-based therapeutics, while high expression correlates with increased drug sensitivity. This relationship has important implications for personalized medicine approaches in cancer treatment and for understanding variable responses to antiviral medications.
id: Q9HA47
gene_symbol: UCK1
product_type: PROTEIN
status: COMPLETE
taxon:
id: NCBITaxon:9606
label: Homo sapiens
description: UCK1 (Uridine-cytidine kinase 1) is a nucleoside kinase that catalyzes the
phosphorylation of uridine and cytidine to their respective monophosphates (UMP and CMP),
functioning in the pyrimidine nucleotide salvage pathway. The enzyme can use ATP or GTP as
phosphate donors and shows specificity for pyrimidine ribonucleosides, not phosphorylating
deoxyribonucleosides or purine ribonucleosides. UCK1 is also capable of phosphorylating various
nucleoside analogs used in cancer chemotherapy and antiviral therapy.
existing_annotations:
- term:
id: GO:0005737
label: cytoplasm
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: >-
Cytoplasm is a broad, pathway-compatible location for UCK1, but the localization evidence is mixed.
Reactome describes cytosolic UCK1 activity and the IBA row supports cytoplasm, while PMID:27239701
reports UCK1-GFP predominantly in the nucleus and UCK2-GFP in the cytosol.
action: ACCEPT
reason: >-
Accept this broad cytoplasm row only as a non-exclusive location supported by GOA/Reactome-style
pathway context. It should not be read as overriding the later GFP evidence for a nuclear UCK1 pool,
and a direct nucleus annotation may be warranted after curator review.
supported_by:
- reference_id: Reactome:R-HSA-73599
supporting_text: Cytosolic uridine-cytidine kinase 1 (UCK1) catalyzes the reactions of
cytidine or uridine with ATP to form CMP or UMP and ADP
- reference_id: file:human/UCK1/UCK1-deep-research-perplexity-lite.md
supporting_text: See deep research file for comprehensive analysis
- reference_id: PMID:27239701
supporting_text: >-
Subcellular localization studies showed that UCK1-GFP and UCK2-GFP were localized in the cell
nucleus and cytosol, respectively.
- term:
id: GO:0004849
label: uridine kinase activity
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: Uridine kinase activity is a core function of UCK1, directly demonstrated by
experimental evidence showing phosphorylation of uridine to UMP. This IBA annotation is
consistent with direct experimental evidence (IDA) from PMID:11306702. The enzyme is highly
specific for pyrimidine ribonucleosides.
action: ACCEPT
reason: This represents one of the two core molecular functions of UCK1. The enzyme catalyzes
the phosphorylation of uridine to UMP using ATP or GTP as phosphate donor, as demonstrated
experimentally in the original characterization study and confirmed through phylogenetic
inference. The substrate specificity is well-established - UCK1 does not phosphorylate
deoxyribonucleosides or purine ribonucleosides.
supported_by:
- reference_id: PMID:11306702
supporting_text: The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in
Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. The enzymes did
not phosphorylate deoxyribonucleosides or purine ribonucleosides.
- term:
id: GO:0009224
label: CMP biosynthetic process
evidence_type: IEA
original_reference_id: GO_REF:0000108
review:
summary: >-
UCK1 does generate CMP from cytidine, but this GO term is a broad biosynthetic-process parent that
does not distinguish the salvage pathway from de novo CMP biosynthesis. The more informative and
already annotated term is GO:0044211 CTP salvage.
action: MODIFY
reason: >-
The evidence supports cytidine phosphorylation as the first step of CTP salvage, not a generic CMP
biosynthetic process. Replace with the specific salvage-pathway term already supported by direct
experimental evidence and UniProt pathway annotation.
supported_by:
- reference_id: file:human/UCK1/UCK1-uniprot.txt
supporting_text: >-
PATHWAY: Pyrimidine metabolism; CTP biosynthesis via salvage pathway; CTP from cytidine: step
1/3.
proposed_replacement_terms:
- id: GO:0044211
label: CTP salvage
- term:
id: GO:0000166
label: nucleotide binding
evidence_type: IEA
original_reference_id: GO_REF:0000043
review:
summary: This is an overly general parent term that doesn't provide meaningful information about
UCK1's specific function. While technically correct that UCK1 binds nucleotides (ATP/GTP as
phosphate donors, and uridine/cytidine as substrates), this broad term lacks functional
specificity. The more specific child term GO:0005524 (ATP binding) is already annotated.
action: MARK_AS_OVER_ANNOTATED
reason: This term is too vague and uninformative for curation purposes. It was automatically
assigned based on UniProtKB keywords. UCK1 specifically binds ATP (and can also use GTP) as
phosphate donors for its kinase activity. The more specific GO:0005524 (ATP binding) provides
much more useful functional information and is already annotated. This overly broad parent
term adds little value to the annotation set.
supported_by:
- reference_id: PMID:11306702
supporting_text: Uridine-cytidine kinases (UCK) have important roles for the phosphorylation
of nucleoside analogs that are being investigated for possible use in chemotherapy of
cancer.
- term:
id: GO:0004849
label: uridine kinase activity
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: This is a duplicate annotation of GO:0004849 with computational evidence (IEA), while
the same term is also annotated with both IBA and IDA evidence. The IEA annotation is
redundant but not incorrect.
action: ACCEPT
reason: Although this is a duplicate annotation with weaker evidence than the IBA and IDA
annotations for the same term, it is still accurate and represents a core function of UCK1.
Duplicate annotations with different evidence codes are acceptable in GO.
supported_by:
- reference_id: PMID:11306702
supporting_text: UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the
phosphorylation of Urd and Cyd.
- term:
id: GO:0005524
label: ATP binding
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: UCK1 binds ATP as a phosphate donor for its kinase activity. The enzyme can also use
GTP, but ATP is the primary physiological phosphate donor. This IEA annotation is supported by
structural data (PDB 2JEO, 2UVQ) showing ATP/ADP binding sites and by the biochemical
characterization demonstrating ATP usage.
action: ACCEPT
reason: ATP binding is well-supported by multiple lines of evidence. The enzyme uses ATP (or
GTP) as a phosphate donor in the reaction uridine/cytidine + ATP → UMP/CMP + ADP + H+.
Crystallographic structures (PDB 2JEO, 2UVQ) directly demonstrate ATP/ADP binding. This is a
functionally relevant molecular function term that is more informative than the parent term
GO:0000166 (nucleotide binding). The annotation was correctly inferred from InterPro domains
and UniProtKB keywords.
supported_by:
- reference_id: PMID:11306702
supporting_text: The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in
Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
- term:
id: GO:0016301
label: kinase activity
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: This is a high-level parent term in the molecular function hierarchy. While UCK1 is
indeed a kinase (specifically a nucleoside kinase), this term is overly general. The more
specific and informative terms GO:0004849 (uridine kinase activity) and GO:0043771 (cytidine
kinase activity) are already annotated with both IBA and IDA evidence.
action: MARK_AS_OVER_ANNOTATED
reason: This broad parent term was automatically inferred from InterPro domains and UniProtKB
keywords. While technically accurate, it provides minimal functional information compared to
the specific child terms (uridine kinase activity, cytidine kinase activity) that are already
annotated with experimental evidence. In curation, such overly general parent terms are
typically considered over-annotations when more specific child terms adequately describe the
function.
supported_by:
- reference_id: PMID:11306702
supporting_text: The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in
Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
- term:
id: GO:0016740
label: transferase activity
evidence_type: IEA
original_reference_id: GO_REF:0000043
review:
summary: This is an extremely high-level parent term in the molecular function ontology. UCK1 is
indeed a transferase (kinases transfer phosphate groups from ATP to substrates), but this
root-level term provides virtually no functional specificity. Multiple more informative
descendant terms are already annotated (kinase activity, uridine kinase activity, cytidine
kinase activity).
action: MARK_AS_OVER_ANNOTATED
reason: This is a classic example of over-annotation with an overly broad parent term. It was
automatically assigned based on UniProtKB keywords. While technically correct, GO:0016740 is
so general that it encompasses a vast array of different enzyme activities. The more specific
child terms (GO:0016301 kinase activity, GO:0004849 uridine kinase activity, GO:0043771
cytidine kinase activity) provide far more useful functional information. Annotating to such
high-level parent terms when specific child terms exist is generally discouraged in manual
curation.
supported_by:
- reference_id: PMID:11306702
supporting_text: The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in
Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
- term:
id: GO:0043771
label: cytidine kinase activity
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: This is a duplicate annotation of GO:0043771 with computational evidence (IEA), while
the same term is also annotated with experimental evidence (IDA from PMID:11306702). Cytidine
kinase activity is one of the two core molecular functions of UCK1. The IEA annotation was
correctly inferred from RHEA reactions and EC number 2.7.1.48.
action: ACCEPT
reason: Although this is a duplicate annotation with weaker evidence than the IDA annotation for
the same term, it is still accurate and represents a core function of UCK1. The enzyme
catalyzes the reaction cytidine + ATP = CMP + ADP + H+. Duplicate annotations with different
evidence codes are acceptable in GO and can arise from independent annotation pipelines
(manual curation vs automated inference). Both contribute to the overall evidence for this
function.
supported_by:
- reference_id: PMID:11306702
supporting_text: The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in
Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
- term:
id: GO:0044206
label: UMP salvage
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: This is a duplicate annotation of GO:0044206 with computational evidence (IEA), while
the same term is also annotated with experimental evidence (IDA from PMID:11306702). UMP
salvage is a core biological process for UCK1, representing the salvage pathway alternative to
de novo UMP biosynthesis. The IEA was correctly inferred from UniPathway UPA00574.
action: ACCEPT
reason: Although this is a duplicate annotation with weaker evidence than the IDA annotation, it
accurately represents UCK1's role in the pyrimidine salvage pathway. UCK1 catalyzes the sole
step in UMP biosynthesis via the salvage pathway (uridine + ATP → UMP + ADP + H+). This is
distinct from de novo pyrimidine synthesis. Duplicate annotations with different evidence
codes are acceptable and can arise from independent curation and automated inference
pipelines.
supported_by:
- reference_id: PMID:11306702
supporting_text: The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in
Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
- term:
id: GO:0044211
label: CTP salvage
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: This is a duplicate annotation of GO:0044211 with computational evidence (IEA), while
the same term is also annotated with experimental evidence (IDA from PMID:11306702). CTP
salvage is a core biological process for UCK1, representing the salvage pathway alternative to
de novo CTP biosynthesis. The IEA was correctly inferred from UniPathway UPA00579.
action: ACCEPT
reason: Although this is a duplicate annotation with weaker evidence than the IDA annotation, it
accurately represents UCK1's role in the pyrimidine salvage pathway. UCK1 catalyzes the first
step of three in CTP biosynthesis via the salvage pathway (cytidine + ATP → CMP + ADP + H+,
followed by CMP → CDP → CTP by other enzymes). This is distinct from de novo pyrimidine
synthesis. Duplicate annotations with different evidence codes are acceptable and can arise
from independent curation and automated inference pipelines.
supported_by:
- reference_id: PMID:11306702
supporting_text: The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in
Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
- term:
id: GO:0004849
label: uridine kinase activity
evidence_type: IDA
original_reference_id: PMID:11306702
review:
summary: Uridine kinase activity is a core molecular function of UCK1, directly demonstrated by
experimental evidence (IDA). This annotation is based on the original biochemical
characterization study by Van Rompay et al. (2001) showing that recombinant UCK1
phosphorylates uridine to UMP. The enzyme exhibits strict substrate specificity, not
phosphorylating deoxyribonucleosides or purines.
action: ACCEPT
reason: This is the gold standard annotation with direct experimental evidence (IDA) from the
original characterization paper PMID:11306702. The study cloned and expressed recombinant UCK1
in E. coli and directly demonstrated its ability to catalyze uridine phosphorylation using
biochemical assays. This represents one of the two core enzymatic activities of UCK1 (the
other being cytidine kinase activity). The annotation is well-supported, specific, and
represents a primary molecular function.
supported_by:
- reference_id: PMID:11306702
supporting_text: The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in
Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. The enzymes did
not phosphorylate deoxyribonucleosides or purine ribonucleosides.
- term:
id: GO:0043771
label: cytidine kinase activity
evidence_type: IDA
original_reference_id: PMID:11306702
review:
summary: Cytidine kinase activity is a core molecular function of UCK1, directly demonstrated by
experimental evidence (IDA). This annotation is based on the original biochemical
characterization study by Van Rompay et al. (2001) showing that recombinant UCK1
phosphorylates cytidine to CMP. The enzyme exhibits strict substrate specificity, not
phosphorylating deoxyribonucleosides or purines.
action: ACCEPT
reason: This is the gold standard annotation with direct experimental evidence (IDA) from the
original characterization paper PMID:11306702. The study cloned and expressed recombinant UCK1
in E. coli and directly demonstrated its ability to catalyze cytidine phosphorylation using
biochemical assays. This represents one of the two core enzymatic activities of UCK1 (the
other being uridine kinase activity). The annotation is well-supported, specific, and
represents a primary molecular function. UCK1's dual substrate specificity for both uridine
and cytidine distinguishes it from other nucleoside kinases.
supported_by:
- reference_id: PMID:11306702
supporting_text: The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in
Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. The enzymes did
not phosphorylate deoxyribonucleosides or purine ribonucleosides.
- term:
id: GO:0044206
label: UMP salvage
evidence_type: IDA
original_reference_id: PMID:11306702
review:
summary: UMP salvage is a core biological process for UCK1, directly supported by experimental
evidence (IDA). UCK1 catalyzes the conversion of uridine to UMP (uridine + ATP → UMP + ADP +
H+), which is the sole enzymatic step in UMP biosynthesis via the salvage pathway. This
salvage pathway provides an energy-efficient alternative to de novo pyrimidine synthesis.
action: ACCEPT
reason: This is an accurate and well-supported annotation with direct experimental evidence
(IDA) from PMID:11306702. UCK1 performs the complete UMP salvage pathway in a single step,
converting free uridine (from nucleotide turnover or dietary sources) back into UMP for
nucleotide pool maintenance. This is a core biological process function. The UniProt entry
explicitly states this is "step 1/1" in UMP biosynthesis via salvage pathway. The salvage
pathway is particularly important in tissues with high nucleotide turnover and for activating
nucleoside analog drugs.
supported_by:
- reference_id: PMID:11306702
supporting_text: The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in
Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
- term:
id: GO:0044211
label: CTP salvage
evidence_type: IDA
original_reference_id: PMID:11306702
review:
summary: CTP salvage is a core biological process for UCK1, directly supported by experimental
evidence (IDA). UCK1 catalyzes the first step in CTP biosynthesis via the salvage pathway by
converting cytidine to CMP (cytidine + ATP → CMP + ADP + H+). CMP is subsequently
phosphorylated to CDP and then CTP by other enzymes (CMP kinase and nucleoside diphosphate
kinase). This salvage pathway provides an energy-efficient alternative to de novo pyrimidine
synthesis.
action: ACCEPT
reason: This is an accurate and well-supported annotation with direct experimental evidence
(IDA) from PMID:11306702. UCK1 performs the rate-limiting first step (step 1 of 3) in the CTP
salvage pathway, converting free cytidine (from nucleotide turnover or dietary sources) to
CMP. The complete pathway is cytidine → CMP → CDP → CTP. This is a core biological process
function. The UniProt entry explicitly states this is "step 1/3" in CTP biosynthesis via
salvage pathway. The salvage pathway is particularly important in rapidly dividing cells and
for activating cytidine analog drugs used in cancer chemotherapy.
supported_by:
- reference_id: PMID:11306702
supporting_text: The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in
Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
- term:
id: GO:0005829
label: cytosol
evidence_type: TAS
original_reference_id: Reactome:R-HSA-73599
review:
summary: >-
Reactome explicitly describes UCK1 as cytosolic in the cytidine/uridine phosphorylation reaction,
but this conflicts with later UCK1-GFP evidence reporting predominant nuclear localization. The
cytosol row is therefore accepted as Reactome pathway-supported but not as the sole or best localization
for UCK1.
action: ACCEPT
reason: >-
Keep the cytosol annotation because it is directly supported by the Reactome TAS source for the
metabolic reaction. However, this annotation should be interpreted alongside PMID:27239701, which
reports nuclear UCK1-GFP localization and UCK1-dependent relocalization of UCK2. The review flags
nucleus as a suggested follow-up annotation rather than treating cytosol as exclusive.
supported_by:
- reference_id: Reactome:R-HSA-73599
supporting_text: Cytosolic uridine-cytidine kinase 1 (UCK1) catalyzes the reactions of
cytidine or uridine with ATP to form CMP or UMP and ADP
- reference_id: PMID:27239701
supporting_text: >-
Subcellular localization studies showed that UCK1-GFP and UCK2-GFP were localized in the cell
nucleus and cytosol, respectively.
references:
- id: GO_REF:0000033
title: Annotation inferences using phylogenetic trees
findings:
- statement: Provides phylogenetic support for cytoplasmic localization and uridine kinase
activity
- id: GO_REF:0000043
title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping
findings:
- statement: Automated annotations based on UniProtKB keywords for nucleotide binding and
transferase activity
- id: GO_REF:0000108
title: Automatic assignment of GO terms using logical inference, based on on inter-ontology links.
findings:
- statement: Inferred CMP biosynthetic process from molecular function annotations
- id: GO_REF:0000120
title: Combined Automated Annotation using Multiple IEA Methods.
findings:
- statement: Automated annotations combining multiple computational methods for molecular
functions and biological processes
- id: PMID:11306702
title: Phosphorylation of uridine and cytidine nucleoside analogs by two human uridine-cytidine
kinases.
findings:
- statement: UCK1 phosphorylates uridine and cytidine to UMP and CMP respectively
supporting_text: The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in
Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd.
- statement: Does not phosphorylate deoxyribonucleosides or purine ribonucleosides
supporting_text: The enzymes did not phosphorylate deoxyribonucleosides or purine
ribonucleosides.
- statement: Can use ATP or GTP as phosphate donor
supporting_text: Uridine-cytidine kinases (UCK) have important roles for the phosphorylation of
nucleoside analogs that are being investigated for possible use in chemotherapy of cancer.
- statement: Phosphorylates various nucleoside analogs including 6-azauridine, 5-fluorouridine,
4-thiouridine, 5-bromouridine, N(4)-acetylcytidine, N(4)-benzoylcytidine, 5-fluorocytidine,
2-thiocytidine, 5-methylcytidine, and N(4)-anisoylcytidine
supporting_text: The enzymes phosphorylated several of the analogs, such as 6-azauridine,
5-fluorouridine, 4-thiouridine, 5-bromouridine, N(4)-acetylcytidine, N(4)-benzoylcytidine,
5-fluorocytidine, 2-thiocytidine, 5-methylcytidine, and N(4)-anisoylcytidine.
- statement: UCK1 mRNA detected in multiple tissues with ubiquitous expression
supporting_text: The 2.7-kb band was ubiquitously expressed in the investigated tissues.
- id: Reactome:R-HSA-73599
title: cytidine or uridine + ATP => CMP or UMP + ADP [UCK1]
findings:
- statement: UCK1 localizes to the cytosol
supporting_text: Cytosolic uridine-cytidine kinase 1 (UCK1) catalyzes the reactions of cytidine
or uridine with ATP to form CMP or UMP and ADP
- statement: Catalyzes phosphorylation of cytidine or uridine with ATP to form CMP or UMP and ADP
supporting_text: Cytosolic uridine-cytidine kinase 1 (UCK1) catalyzes the reactions of cytidine
or uridine with ATP to form CMP or UMP and ADP
- statement: Functions as a tetramer based on crystallographic data
supporting_text: Unpublished crystallographic data show the enzyme to be a tetramer (PDB -
2JEO).
- id: file:human/UCK1/UCK1-deep-research-perplexity-lite.md
title: Deep research on UCK1 function
findings: []
- id: PMID:27239701
title: The pivotal role of uridine-cytidine kinases in pyrimidine metabolism and activation of
cytotoxic nucleoside analogues in neuroblastoma.
findings:
- statement: UCK1-GFP localized to the nucleus, while UCK2-GFP localized to the cytosol, and UCK1
co-expression redirected UCK2 to the nucleus.
supporting_text: >-
Subcellular localization studies showed that UCK1-GFP and UCK2-GFP were localized in the cell nucleus
and cytosol, respectively.
- id: file:human/UCK1/UCK1-deep-research-falcon.md
title: Falcon deep research on UCK1 function
findings: []
- id: file:human/UCK1/UCK1-uniprot.txt
title: UniProt record for human UCK1
findings: []
core_functions:
- description: >-
Phosphorylates uridine to UMP using ATP or GTP as phosphate donor, supporting UMP salvage and pyrimidine
ribonucleoside salvage metabolism.
molecular_function:
id: GO:0004849
label: uridine kinase activity
directly_involved_in:
- id: GO:0044206
label: UMP salvage
locations:
- id: GO:0005829
label: cytosol
substrates:
- id: CHEBI:16704
label: uridine
supported_by:
- reference_id: PMID:11306702
supporting_text: >-
The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown
to catalyze the phosphorylation of Urd and Cyd. The enzymes did not phosphorylate deoxyribonucleosides
or purine ribonucleosides.
- reference_id: Reactome:R-HSA-73599
supporting_text: >-
Cytosolic uridine-cytidine kinase 1 (UCK1) catalyzes the reactions of cytidine or uridine with ATP
to form CMP or UMP and ADP
- reference_id: file:human/UCK1/UCK1-deep-research-falcon.md
supporting_text: >-
Falcon report summarizes UCK1 as an ATP-dependent uridine/cytidine kinase in pyrimidine salvage,
with specificity for pyrimidine ribonucleosides and drug-analog activation.
- description: >-
Phosphorylates cytidine to CMP using ATP or GTP as phosphate donor, providing the first step of CTP
salvage from cytidine.
molecular_function:
id: GO:0043771
label: cytidine kinase activity
directly_involved_in:
- id: GO:0044211
label: CTP salvage
locations:
- id: GO:0005829
label: cytosol
substrates:
- id: CHEBI:17562
label: cytidine
supported_by:
- reference_id: PMID:11306702
supporting_text: >-
The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown
to catalyze the phosphorylation of Urd and Cyd. The enzymes did not phosphorylate deoxyribonucleosides
or purine ribonucleosides.
- reference_id: file:human/UCK1/UCK1-uniprot.txt
supporting_text: >-
Reaction=cytidine + ATP = CMP + ADP + H(+); EC=2.7.1.48; Evidence={ECO:0000269|PubMed:11306702};
- reference_id: file:human/UCK1/UCK1-deep-research-falcon.md
supporting_text: >-
Falcon report summarizes UCK1 as an ATP-dependent uridine/cytidine kinase in pyrimidine salvage,
with specificity for pyrimidine ribonucleosides and drug-analog activation.
proposed_new_terms: []
suggested_questions:
- question: Should UCK1 receive a direct nucleus cellular-component annotation based on the GFP
localization evidence from PMID:27239701?
experts:
- GO cellular component curators
- nucleotide metabolism specialists
- question: Which UCK1 isoforms or cell states determine nuclear versus cytosolic pyrimidine-salvage
activity?
experts:
- cell biologists
- metabolic enzyme specialists
suggested_experiments:
- experiment_type: Endogenous localization
description: Use tagged knock-in or validated antibody staining to compare endogenous UCK1
localization with UCK1-GFP localization across proliferating and differentiated cells.
hypothesis: UCK1 has a substantial nuclear pool that is not captured by current cytosol/cytoplasm
pathway annotations.
- experiment_type: Isoform rescue biochemistry
description: Express individual UCK1 isoforms in UCK1/UCK2-deficient cells and measure uridine,
cytidine, and nucleoside-analog phosphorylation.
hypothesis: UCK1 isoforms differ in localization and in their contribution to pyrimidine salvage
and analog activation.