TK2 encodes mitochondrial thymidine kinase 2, a multisubstrate deoxyribonucleoside kinase of the DCK/DGK (deoxyribonucleoside kinase) family. Following cleavage of an N-terminal mitochondrial transit peptide, the mature enzyme resides in the mitochondrial matrix, where it phosphorylates the pyrimidine deoxyribonucleosides thymidine, deoxycytidine and deoxyuridine to their respective deoxyribonucleoside monophosphates (dTMP, dCMP, dUMP) using ATP as phosphate donor. TK2 constitutes the pyrimidine arm of the mitochondrial deoxyribonucleoside salvage pathway; together with deoxyguanosine kinase (DGUOK), which handles the purine arm, it supplies the deoxyribonucleotide precursors needed for mitochondrial DNA replication. Because the mitochondrial dNTP pool is separated from the cytosolic pool and mitochondria lack de novo dNTP synthesis, in non-replicating/quiescent post-mitotic tissues (where cytosolic dNTP production is low) mtDNA synthesis depends on this salvage pathway. Loss-of-function variants cause a myopathic mitochondrial DNA depletion syndrome (MTDPS2) and TK2-related progressive external ophthalmoplegia with multiple mtDNA deletions (PEOB3). TK2 also activates several antiviral and chemotherapeutic pyrimidine nucleoside analogues.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: IBA phylogenetic annotation to cytoplasm. TK2 is a mitochondrial-matrix enzyme with a cleaved N-terminal mitochondrial transit peptide, not a general cytoplasmic protein. Reason: The mature protein localizes to the mitochondrial matrix, and both UniProt (IDA, PubMed:9989599) and Reactome place its activity there. The generic "cytoplasm" IBA is over-broad, likely propagated from paralogous cytosolic deoxyribonucleoside kinases (e.g. cytosolic dCK) across the shared PANTHER family; the mitochondrion / mitochondrial matrix terms are the informative locations for TK2. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: COMPARTMENT OR COMPLEX MISMATCH Sources checked: PANTHER:PTN000053758 · deoxyribonucleoside kinase family SUPPORTS SOURCE BUT NOT TARGET Family includes cytosolic members (e.g. dCK); the cytoplasm term leaks to TK2, which is mitochondrial-matrix localized. Supporting Evidence: file:human/TK2/TK2-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0004136 deoxyadenosine kinase activity | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: IBA phylogenetic annotation to deoxyadenosine kinase activity. TK2 is the pyrimidine arm of mitochondrial salvage; deoxyadenosine (a purine deoxyribonucleoside) is the substrate of the paralog DGUOK (deoxyguanosine kinase), not of TK2. Reason: Characterized substrate specificity of human TK2 is restricted to the pyrimidine deoxyribonucleosides thymidine, deoxycytidine and deoxyuridine; deoxyguanosine and deoxyadenosine are handled by the sibling mitochondrial kinase DGUOK. This IBA is a paralog over-annotation propagated across the shared deoxyribonucleoside-kinase PANTHER family (note the WITH/FROM includes Drosophila dnk, a broad-specificity insect enzyme). It is not wrong for the family, but it over-annotates TK2, whose informative activities are thymidine/deoxycytidine/deoxyuridine kinase. Propagation Review Root cause: PROPAGATION BAD Failure modes: WRONG ORTHOLOG OR PARALOG FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTN000828170 · deoxyribonucleoside kinase family node SUPPORTS SOURCE BUT NOT TARGET Deoxyadenosine kinase activity belongs to the purine arm (DGUOK) and the broad-specificity insect dnk; human TK2 does not phosphorylate deoxyadenosine. FB:FBgn0022338 · Drosophila dnk (multisubstrate deoxyribonucleoside kinase) SUPPORTS SOURCE BUT NOT TARGET Insect dnk is broad-specificity; its purine activity should not transfer to the pyrimidine-specific human TK2. Supporting Evidence: PMID:11687801 Two of the four human deoxyribonucleoside kinases, deoxyguanosine kinase (dGK) and thymidine kinase-2 PMID:11687801 TK2 phosphorylates deoxythymidine, deoxycytidine and deoxyuridine. |
| GO:0004137 deoxycytidine kinase activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA phylogenetic annotation to deoxycytidine kinase activity, one of the three characterized pyrimidine deoxyribonucleoside kinase activities of TK2. Reason: Deoxycytidine is a well-documented physiological substrate of TK2 (EC 2.7.1.74), confirmed biochemically (KM ~36 uM). This IBA agrees with the IDA and IEA annotations and represents a core molecular function. Supporting Evidence: PMID:9079672 phosphorylates thymidine, deoxycytidine, and deoxyuridine. |
| GO:0004797 thymidine kinase activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA phylogenetic annotation to thymidine kinase activity, the defining and eponymous molecular function of TK2 (EC 2.7.1.21). Reason: Thymidine kinase activity is the core, experimentally established function of TK2 (KM ~16 uM for thymidine). This IBA is fully consistent with the IDA/TAS/IEA annotations for the same term. Supporting Evidence: PMID:9079672 Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that phosphorylates thymidine, deoxycytidine, and deoxyuridine. |
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | ACCEPT | Summary: IBA phylogenetic annotation placing TK2 activity in the mitochondrion, consistent with its cleaved mitochondrial transit peptide and experimental localization. Reason: TK2 is a bona fide mitochondrial-matrix enzyme (UniProt IDA, PubMed:9989599; Reactome matrix TAS). The is_active_in qualifier is appropriate since its catalytic function is exerted in the mitochondrion. Supporting Evidence: file:human/TK2/TK2-uniprot.txt Phosphorylates thymidine, deoxycytidine, and deoxyuridine in |
| GO:0004137 deoxycytidine kinase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Automated (ARBA/RHEA/EC-mapped) annotation to deoxycytidine kinase activity (EC 2.7.1.74, RHEA:46040), duplicating the IDA/IBA evidence for this substrate. Reason: Correct electronic mapping to a characterized TK2 activity; agrees with the experimental (IDA) annotations. Supporting Evidence: PMID:9079672 phosphorylates thymidine, deoxycytidine, and deoxyuridine. |
| GO:0004797 thymidine kinase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Automated (ARBA/RHEA/EC-mapped) annotation to thymidine kinase activity (EC 2.7.1.21, RHEA:19129), duplicating the experimental evidence. Reason: Correct electronic mapping to the defining TK2 activity; agrees with the IDA and TAS annotations. Supporting Evidence: PMID:9079672 Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that phosphorylates thymidine, deoxycytidine, and deoxyuridine. |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO annotation (IPR002624, DCK/DGK) to ATP binding. TK2 uses ATP as the phosphate donor for deoxyribonucleoside phosphorylation and has a defined ATP-binding P-loop. Reason: ATP is the phosphate donor for all three TK2 kinase reactions, and UniProt annotates an ATP-binding P-loop motif (residues 57-65). The ATP-binding molecular function is correct and supported. Supporting Evidence: file:human/TK2/TK2-uniprot.txt BINDING 57..65 |
| GO:0005739 mitochondrion | IEA GO_REF:0000120 | ACCEPT | Summary: Automated subcellular-location annotation (UniProtKB-SubCell SL-0173) to mitochondrion, consistent with all experimental evidence. Reason: TK2 has a cleaved mitochondrial transit peptide and is experimentally localized to mitochondria (IDA, PubMed:9989599); this IEA is correct. Supporting Evidence: file:human/TK2/TK2-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0006170 dAMP biosynthetic process | IEA GO_REF:0000108 | MARK AS OVER ANNOTATED | Summary: Logical-inference annotation to dAMP biosynthetic process, derived from the GO:0004136 deoxyadenosine kinase activity annotation. dAMP synthesis is the purine (DGUOK) arm of mitochondrial salvage, not the TK2 pyrimidine arm. Reason: This process term was inferred (ECO:0000364) from the GO:0004136 deoxyadenosine kinase annotation, which is itself a paralog over-annotation of TK2 (deoxyadenosine is a DGUOK substrate). TK2 produces dTMP, dCMP and dUMP, not dAMP, so this downstream process annotation over-annotates TK2 by the same reasoning. Supporting Evidence: PMID:11687801 TK2 phosphorylates deoxythymidine, deoxycytidine and deoxyuridine. |
| GO:0009157 deoxyribonucleoside monophosphate biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: Automated annotation to deoxyribonucleoside monophosphate biosynthetic process, inferred from the deoxycytidine/thymidine/deoxynucleoside kinase MF annotations. This is exactly the class of product TK2 makes (dTMP, dCMP, dUMP). Reason: The direct products of TK2 are deoxyribonucleoside 5'-monophosphates; this process term correctly summarizes the biological outcome of TK2 catalysis and is the appropriate general product-class term for the pyrimidine salvage arm. Supporting Evidence: PMID:9079672 Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that phosphorylates thymidine, deoxycytidine, and deoxyuridine. |
| GO:0009165 nucleotide biosynthetic process | IEA GO_REF:0000108 | MARK AS OVER ANNOTATED | Summary: Logical-inference annotation to the broad term nucleotide biosynthetic process, derived from GO:0019206 nucleoside kinase activity. Reason: Correct in direction but far too general. The specific, informative process for TK2 is the salvage biosynthesis of pyrimidine deoxyribonucleoside monophosphates (dTMP/dCMP/dUMP) for the mitochondrial dNTP pool; the parent term "nucleotide biosynthetic process" adds no specific information beyond that. Supporting Evidence: PMID:9079672 phosphorylates thymidine, deoxycytidine, and deoxyuridine. |
| GO:0019136 deoxynucleoside kinase activity | IEA GO_REF:0000120 | ACCEPT | Summary: InterPro/RHEA-based annotation to the parent term deoxynucleoside kinase activity (RHEA:28206 corresponds to the deoxyuridine kinase reaction). Reason: TK2 is a deoxyribonucleoside (deoxynucleoside) kinase, and RHEA:28206 is the 2'-deoxyuridine + ATP => dUMP reaction that TK2 catalyzes. The parent term is appropriate and additionally captures the deoxyuridine-kinase arm that lacks its own specific GO term; it complements (does not conflict with) the specific thymidine- and deoxycytidine-kinase annotations. Supporting Evidence: file:human/TK2/TK2-uniprot.txt Reaction=2'-deoxyuridine + ATP = dUMP + ADP + H(+); |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | MARK AS OVER ANNOTATED | Summary: High-throughput AP-MS interaction (BioPlex) recording a physical interaction between TK2 and GSTK1 (glutathione S-transferase kappa 1, Q9Y2Q3). The bare "protein binding" term is uninformative about TK2 molecular function. Reason: Per curation policy the experimental IPI is retained rather than removed, but bare GO:0005515 protein binding does not describe a specific TK2 function. The GSTK1 interaction comes from a proteome-scale AP-MS interactome screen and has no established functional or complex-level meaning for TK2 (TK2 is a monomeric enzyme per UniProt). No more informative molecular-function term is warranted from this evidence. Supporting Evidence: PMID:33961781 In tandem, these cell-specific interaction networks begin to reveal how interactomes vary with cellular state. |
| GO:0005515 protein binding | IPI PMID:40205054 Multimodal cell maps as a foundation for structural and func... | MARK AS OVER ANNOTATED | Summary: Second high-throughput interactome/imaging (multimodal cell map) IPI recording the same TK2-GSTK1 (Q9Y2Q3) physical interaction. Bare "protein binding" is uninformative. Reason: As with the BioPlex IPI, this proteome-scale screen supports a physical interaction but not a specific, informative molecular function for TK2. The experimental annotation is retained (not removed) but marked over-annotated because GO:0005515 protein binding conveys no functional specificity. Supporting Evidence: PMID:40205054 Here we construct a global map of human subcellular architecture through joint measurement of biophysical interactions and immunofluorescence images for over 5,100 proteins in U2OS osteosarcoma cells. |
| GO:0046092 deoxycytidine metabolic process | IEA GO_REF:0000107 | ACCEPT | Summary: Ortholog-transfer annotation (from rat Tk2) to deoxycytidine metabolic process, reflecting TK2's deoxycytidine kinase activity. Reason: Deoxycytidine is a physiological TK2 substrate; TK2 participates in deoxycytidine metabolism by phosphorylating it to dCMP. This process term correctly reflects a core TK2 function. Supporting Evidence: PMID:9079672 phosphorylates thymidine, deoxycytidine, and deoxyuridine. |
| GO:0046104 thymidine metabolic process | IEA GO_REF:0000107 | ACCEPT | Summary: Ortholog-transfer annotation (from rat Tk2) to thymidine metabolic process, reflecting TK2's thymidine kinase activity. Reason: Thymidine is the eponymous physiological TK2 substrate; TK2 participates in thymidine metabolism by phosphorylating it to dTMP. This process term correctly reflects a core TK2 function. Supporting Evidence: PMID:9079672 Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that phosphorylates thymidine, deoxycytidine, and deoxyuridine. |
| GO:0043097 pyrimidine nucleoside salvage | TAS Reactome:R-HSA-73614 | ACCEPT | Summary: Reactome TAS annotation to pyrimidine nucleoside salvage, capturing the core pathway role of TK2 as the pyrimidine deoxyribonucleoside kinase of the mitochondrial salvage pathway. Reason: TK2 phosphorylates the pyrimidine deoxyribonucleosides thymidine, deoxycytidine and deoxyuridine in the salvage route that supplies mitochondrial dNTPs; this is precisely a pyrimidine (deoxyribo)nucleoside salvage process. This is a core, well-supported biological process for TK2. Supporting Evidence: PMID:11687801 mtDNA synthesis depends solely on the mitochondrial salvage pathway enzymes, the deoxyribonucleosides kinases. |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: High-throughput mitochondrial-proteome annotation (MitoCoP) placing TK2 in the mitochondrion, consistent with experimental IDA localization. Reason: TK2 is included in the high-confidence human mitochondrial proteome (MitoCoP) defined in this study, corroborating its mitochondrial localization established by targeted experiments. Consistent with the UniProt IDA and Reactome matrix annotations. Supporting Evidence: PMID:34800366 defined a mitochondrial high-confidence proteome of >1,100 proteins (MitoCoP). |
| GO:0004137 deoxycytidine kinase activity | IDA PMID:11687801 Mutant mitochondrial thymidine kinase in mitochondrial DNA d... | ACCEPT | Summary: Direct experimental (IDA) demonstration that TK2 phosphorylates deoxycytidine; MTDPS2 disease variants reduce this activity in patient muscle mitochondria. Reason: Saada et al. characterized TK2 substrate use and showed disease-associated loss of TK2 activity in muscle mitochondria, directly supporting deoxycytidine kinase activity as a core TK2 function. Supporting Evidence: PMID:11687801 TK2 phosphorylates deoxythymidine, deoxycytidine and deoxyuridine. |
| GO:0004137 deoxycytidine kinase activity | IDA PMID:9989599 Human thymidine kinase 2: molecular cloning and characterisa... | ACCEPT | Summary: Direct experimental (IDA) characterization of recombinant human TK2 confirming deoxycytidine as a substrate (KM ~36 uM), with broad pyrimidine substrate specificity. Reason: Wang et al. cloned and biochemically characterized human TK2, demonstrating deoxycytidine kinase activity (and activity on deoxycytidine analogues such as gemcitabine). A core, experimentally established molecular function. Supporting Evidence: PMID:9989599 The results demonstrate a broad substrate specificity and complex kinetics |
| GO:0004797 thymidine kinase activity | IDA PMID:11687801 Mutant mitochondrial thymidine kinase in mitochondrial DNA d... | ACCEPT | Summary: Direct experimental (IDA) demonstration of TK2 thymidine kinase activity, reduced in MTDPS2 patient muscle mitochondria (to 14-45% of control). Reason: Saada et al. measured TK2 (thymidine kinase) activity in muscle mitochondria and found it markedly reduced in patients with TK2 mutations, directly supporting thymidine kinase as the core TK2 function. Supporting Evidence: PMID:11687801 the activity of TK2 in muscle mitochondria is reduced to 14-45% of the mean value in healthy control individuals. |
| GO:0004797 thymidine kinase activity | IDA PMID:9989599 Human thymidine kinase 2: molecular cloning and characterisa... | ACCEPT | Summary: Direct experimental (IDA) characterization of recombinant human TK2 confirming thymidine kinase activity (KM ~16 uM for thymidine) as the reference activity. Reason: Wang et al. characterized human TK2 with thymidine and multiple nucleoside analogues, establishing thymidine kinase activity as the defining TK2 function. Core function. Supporting Evidence: PMID:9989599 Based on amino acid sequence information from purified mitochondrial thymidine kinase (TK2), a cDNA of 1930 bp was cloned |
| GO:0005739 mitochondrion | IDA PMID:9989599 Human thymidine kinase 2: molecular cloning and characterisa... | ACCEPT | Summary: Direct experimental (IDA) evidence that TK2 is a mitochondrial enzyme, from cloning and characterization of the protein purified from human mitochondria. Reason: TK2 was cloned from and characterized as the purified mitochondrial thymidine kinase, and UniProt annotates the mitochondrial subcellular location from this work. The is_active_in qualifier is appropriate as catalysis occurs in the mitochondrion. Supporting Evidence: PMID:9989599 Based on amino acid sequence information from purified mitochondrial thymidine kinase (TK2), a cDNA of 1930 bp was cloned |
| GO:0019206 nucleoside kinase activity | TAS Reactome:R-HSA-109759 | MARK AS OVER ANNOTATED | Summary: Reactome TAS annotation to the broad parent term nucleoside kinase activity for the TK2 reaction (deoxycytidine/thymidine/deoxyuridine + ATP => dCMP/TMP/dUMP + ADP). Reason: Not wrong, but too general: TK2 is specifically a deoxyribonucleoside (pyrimidine deoxynucleoside) kinase. The informative molecular-function terms are the specific thymidine kinase, deoxycytidine kinase and deoxynucleoside kinase activities already annotated; the generic "nucleoside kinase activity" adds no specificity. Supporting Evidence: PMID:9079672 Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that phosphorylates thymidine, deoxycytidine, and deoxyuridine. |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-109759 | ACCEPT | Summary: Reactome TAS annotation localizing TK2 activity to the mitochondrial matrix, the most specific and correct compartment for this enzyme. Reason: TK2 phosphorylates its substrates in the mitochondrial matrix (UniProt FUNCTION). This is the appropriate, specific cellular-component term and is preferred over the broader "mitochondrion" and the over-broad "cytoplasm" annotations. Supporting Evidence: file:human/TK2/TK2-uniprot.txt Phosphorylates thymidine, deoxycytidine, and deoxyuridine in |
| GO:0004797 thymidine kinase activity | TAS PMID:9079672 Cloning of the cDNA and chromosome localization of the gene ... | ACCEPT | Summary: TAS annotation to thymidine kinase activity from the original cloning paper, which established the substrate specificity of human TK2. Reason: Johansson & Karlsson cloned human TK2 and confirmed it as a deoxyribonucleoside kinase phosphorylating thymidine (and deoxycytidine and deoxyuridine). This TAS supports the core, defining thymidine kinase activity. Supporting Evidence: PMID:9079672 Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that phosphorylates thymidine, deoxycytidine, and deoxyuridine. |
| GO:0006139 nucleobase-containing compound metabolic process | TAS PMID:9079672 Cloning of the cDNA and chromosome localization of the gene ... | MARK AS OVER ANNOTATED | Summary: TAS annotation to the very broad term nucleobase-containing compound metabolic process from the cloning paper. Reason: This is a high-level parent term that conveys little specific information. TK2's role is specifically pyrimidine deoxyribonucleoside salvage / dNMP biosynthesis (thymidine and deoxycytidine metabolic processes), which the more specific process annotations already capture. Supporting Evidence: PMID:9079672 Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that phosphorylates thymidine, deoxycytidine, and deoxyuridine. |
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Download this section (compressed HTML)Q: What determines the strict tissue-dependence of the TK2-deficiency phenotype (skeletal muscle and CNS versus sparing of mitotic tissues), given that TK2 is broadly expressed?
Q: Does TK2 physically or functionally associate with the mitochondrial dNTP-handling machinery (DGUOK, SUCLA2/SUCLG1, mitochondrial nucleotide transporters), or is the reported GSTK1 interaction functionally meaningful?
Experiment: Quantify the individual and combined contributions of TK2 and DGUOK to the mitochondrial dNTP pool in post-mitotic (differentiated myotube/neuron) versus proliferating cells, using isotope-labelled deoxyribonucleoside tracing.
Experiment: Measure kinetic parameters (kcat/KM) of purified recombinant TK2 for thymidine, deoxycytidine and deoxyuridine side-by-side, and test disease variants, to define the relative fluxes of each pyrimidine arm and the biochemical basis of MTDPS2 severity.
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