id: O00142
gene_symbol: TK2
product_type: PROTEIN
status: INITIALIZED
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  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.
alternative_products:
- name: '1'
  id: O00142-1
- name: '2'
  id: O00142-2
  sequence_note: VSP_003028
- name: '3'
  id: O00142-3
  sequence_note: VSP_043503
- name: '4'
  id: O00142-4
  sequence_note: VSP_044459
- name: '5'
  id: O00142-5
  sequence_note: VSP_054606
- name: '6'
  id: O00142-6
  sequence_note: VSP_058694
existing_annotations:
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    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.
    action: MARK_AS_OVER_ANNOTATED
    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
      source_entities:
      - source_id: PANTHER:PTN000053758
        source_label: deoxyribonucleoside kinase family
        source_status: SUPPORTS_SOURCE_BUT_NOT_TARGET
        comment: >-
          Family includes cytosolic members (e.g. dCK); the cytoplasm term leaks to
          TK2, which is mitochondrial-matrix localized.
    supported_by:
    - reference_id: file:human/TK2/TK2-uniprot.txt
      supporting_text: >-
        SUBCELLULAR LOCATION: Mitochondrion
- term:
    id: GO:0004136
    label: deoxyadenosine kinase activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    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.
    action: MARK_AS_OVER_ANNOTATED
    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
      source_entities:
      - source_id: PANTHER:PTN000828170
        source_label: deoxyribonucleoside kinase family node
        source_status: SUPPORTS_SOURCE_BUT_NOT_TARGET
        comment: >-
          Deoxyadenosine kinase activity belongs to the purine arm (DGUOK) and the
          broad-specificity insect dnk; human TK2 does not phosphorylate deoxyadenosine.
      - source_id: FB:FBgn0022338
        source_label: Drosophila dnk (multisubstrate deoxyribonucleoside kinase)
        source_status: SUPPORTS_SOURCE_BUT_NOT_TARGET
        comment: >-
          Insect dnk is broad-specificity; its purine activity should not transfer to the
          pyrimidine-specific human TK2.
    supported_by:
    - reference_id: PMID:11687801
      supporting_text: >-
        Two of the four human deoxyribonucleoside kinases, deoxyguanosine kinase (dGK) and
        thymidine kinase-2
    - reference_id: PMID:11687801
      supporting_text: >-
        TK2 phosphorylates deoxythymidine, deoxycytidine and deoxyuridine.
- term:
    id: GO:0004137
    label: deoxycytidine kinase activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      IBA phylogenetic annotation to deoxycytidine kinase activity, one of the three
      characterized pyrimidine deoxyribonucleoside kinase activities of TK2.
    action: ACCEPT
    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.
    supported_by:
    - reference_id: PMID:9079672
      supporting_text: >-
        phosphorylates thymidine, deoxycytidine, and deoxyuridine.
- term:
    id: GO:0004797
    label: thymidine kinase activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      IBA phylogenetic annotation to thymidine kinase activity, the defining and
      eponymous molecular function of TK2 (EC 2.7.1.21).
    action: ACCEPT
    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.
    supported_by:
    - reference_id: PMID:9079672
      supporting_text: >-
        Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that
        phosphorylates thymidine, deoxycytidine, and deoxyuridine.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: >-
      IBA phylogenetic annotation placing TK2 activity in the mitochondrion, consistent
      with its cleaved mitochondrial transit peptide and experimental localization.
    action: ACCEPT
    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.
    supported_by:
    - reference_id: file:human/TK2/TK2-uniprot.txt
      supporting_text: >-
        Phosphorylates thymidine, deoxycytidine, and deoxyuridine in
- term:
    id: GO:0004137
    label: deoxycytidine kinase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    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.
    action: ACCEPT
    reason: >-
      Correct electronic mapping to a characterized TK2 activity; agrees with the
      experimental (IDA) annotations.
    supported_by:
    - reference_id: PMID:9079672
      supporting_text: >-
        phosphorylates thymidine, deoxycytidine, and deoxyuridine.
- term:
    id: GO:0004797
    label: thymidine kinase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Automated (ARBA/RHEA/EC-mapped) annotation to thymidine kinase activity
      (EC 2.7.1.21, RHEA:19129), duplicating the experimental evidence.
    action: ACCEPT
    reason: >-
      Correct electronic mapping to the defining TK2 activity; agrees with the IDA and
      TAS annotations.
    supported_by:
    - reference_id: PMID:9079672
      supporting_text: >-
        Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that
        phosphorylates thymidine, deoxycytidine, and deoxyuridine.
- term:
    id: GO:0005524
    label: ATP binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    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.
    action: ACCEPT
    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.
    supported_by:
    - reference_id: file:human/TK2/TK2-uniprot.txt
      supporting_text: >-
        BINDING         57..65
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: located_in
  review:
    summary: >-
      Automated subcellular-location annotation (UniProtKB-SubCell SL-0173) to
      mitochondrion, consistent with all experimental evidence.
    action: ACCEPT
    reason: >-
      TK2 has a cleaved mitochondrial transit peptide and is experimentally localized to
      mitochondria (IDA, PubMed:9989599); this IEA is correct.
    supported_by:
    - reference_id: file:human/TK2/TK2-uniprot.txt
      supporting_text: >-
        SUBCELLULAR LOCATION: Mitochondrion
- term:
    id: GO:0006170
    label: dAMP biosynthetic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000108
  qualifier: involved_in
  review:
    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.
    action: MARK_AS_OVER_ANNOTATED
    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.
    supported_by:
    - reference_id: PMID:11687801
      supporting_text: >-
        TK2 phosphorylates deoxythymidine, deoxycytidine and deoxyuridine.
- term:
    id: GO:0009157
    label: deoxyribonucleoside monophosphate biosynthetic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: involved_in
  review:
    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).
    action: ACCEPT
    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.
    supported_by:
    - reference_id: PMID:9079672
      supporting_text: >-
        Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that
        phosphorylates thymidine, deoxycytidine, and deoxyuridine.
- term:
    id: GO:0009165
    label: nucleotide biosynthetic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000108
  qualifier: involved_in
  review:
    summary: >-
      Logical-inference annotation to the broad term nucleotide biosynthetic process,
      derived from GO:0019206 nucleoside kinase activity.
    action: MARK_AS_OVER_ANNOTATED
    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.
    supported_by:
    - reference_id: PMID:9079672
      supporting_text: >-
        phosphorylates thymidine, deoxycytidine, and deoxyuridine.
- term:
    id: GO:0019136
    label: deoxynucleoside kinase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      InterPro/RHEA-based annotation to the parent term deoxynucleoside kinase activity
      (RHEA:28206 corresponds to the deoxyuridine kinase reaction).
    action: ACCEPT
    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.
    supported_by:
    - reference_id: file:human/TK2/TK2-uniprot.txt
      supporting_text: >-
        Reaction=2'-deoxyuridine + ATP = dUMP + ADP + H(+);
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:33961781
  qualifier: enables
  review:
    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.
    action: MARK_AS_OVER_ANNOTATED
    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.
    supported_by:
    - reference_id: PMID:33961781
      supporting_text: >-
        In tandem, these cell-specific interaction networks begin to reveal how
        interactomes vary with cellular state.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:40205054
  qualifier: enables
  review:
    summary: >-
      Second high-throughput interactome/imaging (multimodal cell map) IPI recording the
      same TK2-GSTK1 (Q9Y2Q3) physical interaction. Bare "protein binding" is uninformative.
    action: MARK_AS_OVER_ANNOTATED
    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.
    supported_by:
    - reference_id: PMID:40205054
      supporting_text: >-
        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.
- term:
    id: GO:0046092
    label: deoxycytidine metabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ortholog-transfer annotation (from rat Tk2) to deoxycytidine metabolic process,
      reflecting TK2's deoxycytidine kinase activity.
    action: ACCEPT
    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.
    supported_by:
    - reference_id: PMID:9079672
      supporting_text: >-
        phosphorylates thymidine, deoxycytidine, and deoxyuridine.
- term:
    id: GO:0046104
    label: thymidine metabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: >-
      Ortholog-transfer annotation (from rat Tk2) to thymidine metabolic process,
      reflecting TK2's thymidine kinase activity.
    action: ACCEPT
    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.
    supported_by:
    - reference_id: PMID:9079672
      supporting_text: >-
        Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that
        phosphorylates thymidine, deoxycytidine, and deoxyuridine.
- term:
    id: GO:0043097
    label: pyrimidine nucleoside salvage
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-73614
  qualifier: involved_in
  review:
    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.
    action: ACCEPT
    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.
    supported_by:
    - reference_id: PMID:11687801
      supporting_text: >-
        mtDNA synthesis depends solely on the mitochondrial
        salvage pathway enzymes, the deoxyribonucleosides kinases.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: HTP
  original_reference_id: PMID:34800366
  qualifier: located_in
  review:
    summary: >-
      High-throughput mitochondrial-proteome annotation (MitoCoP) placing TK2 in the
      mitochondrion, consistent with experimental IDA localization.
    action: ACCEPT
    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.
    supported_by:
    - reference_id: PMID:34800366
      supporting_text: >-
        defined a
        mitochondrial high-confidence proteome of >1,100 proteins (MitoCoP).
- term:
    id: GO:0004137
    label: deoxycytidine kinase activity
  evidence_type: IDA
  original_reference_id: PMID:11687801
  qualifier: enables
  review:
    summary: >-
      Direct experimental (IDA) demonstration that TK2 phosphorylates deoxycytidine;
      MTDPS2 disease variants reduce this activity in patient muscle mitochondria.
    action: ACCEPT
    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.
    supported_by:
    - reference_id: PMID:11687801
      supporting_text: >-
        TK2 phosphorylates deoxythymidine, deoxycytidine and deoxyuridine.
- term:
    id: GO:0004137
    label: deoxycytidine kinase activity
  evidence_type: IDA
  original_reference_id: PMID:9989599
  qualifier: enables
  review:
    summary: >-
      Direct experimental (IDA) characterization of recombinant human TK2 confirming
      deoxycytidine as a substrate (KM ~36 uM), with broad pyrimidine substrate specificity.
    action: ACCEPT
    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.
    supported_by:
    - reference_id: PMID:9989599
      supporting_text: >-
        The results demonstrate a broad
        substrate specificity and complex kinetics
- term:
    id: GO:0004797
    label: thymidine kinase activity
  evidence_type: IDA
  original_reference_id: PMID:11687801
  qualifier: enables
  review:
    summary: >-
      Direct experimental (IDA) demonstration of TK2 thymidine kinase activity, reduced in
      MTDPS2 patient muscle mitochondria (to 14-45% of control).
    action: ACCEPT
    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.
    supported_by:
    - reference_id: PMID:11687801
      supporting_text: >-
        the activity of TK2 in muscle mitochondria is
        reduced to 14-45% of the mean value in healthy control individuals.
- term:
    id: GO:0004797
    label: thymidine kinase activity
  evidence_type: IDA
  original_reference_id: PMID:9989599
  qualifier: enables
  review:
    summary: >-
      Direct experimental (IDA) characterization of recombinant human TK2 confirming
      thymidine kinase activity (KM ~16 uM for thymidine) as the reference activity.
    action: ACCEPT
    reason: >-
      Wang et al. characterized human TK2 with thymidine and multiple nucleoside analogues,
      establishing thymidine kinase activity as the defining TK2 function. Core function.
    supported_by:
    - reference_id: PMID:9989599
      supporting_text: >-
        Based on amino acid sequence information from purified mitochondrial thymidine
        kinase (TK2), a cDNA of 1930 bp was cloned
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IDA
  original_reference_id: PMID:9989599
  qualifier: is_active_in
  review:
    summary: >-
      Direct experimental (IDA) evidence that TK2 is a mitochondrial enzyme, from cloning
      and characterization of the protein purified from human mitochondria.
    action: ACCEPT
    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.
    supported_by:
    - reference_id: PMID:9989599
      supporting_text: >-
        Based on amino acid sequence information from purified mitochondrial thymidine
        kinase (TK2), a cDNA of 1930 bp was cloned
- term:
    id: GO:0019206
    label: nucleoside kinase activity
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-109759
  qualifier: enables
  review:
    summary: >-
      Reactome TAS annotation to the broad parent term nucleoside kinase activity for the
      TK2 reaction (deoxycytidine/thymidine/deoxyuridine + ATP => dCMP/TMP/dUMP + ADP).
    action: MARK_AS_OVER_ANNOTATED
    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.
    supported_by:
    - reference_id: PMID:9079672
      supporting_text: >-
        Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that
        phosphorylates thymidine, deoxycytidine, and deoxyuridine.
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-109759
  qualifier: located_in
  review:
    summary: >-
      Reactome TAS annotation localizing TK2 activity to the mitochondrial matrix, the most
      specific and correct compartment for this enzyme.
    action: ACCEPT
    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.
    supported_by:
    - reference_id: file:human/TK2/TK2-uniprot.txt
      supporting_text: >-
        Phosphorylates thymidine, deoxycytidine, and deoxyuridine in
- term:
    id: GO:0004797
    label: thymidine kinase activity
  evidence_type: TAS
  original_reference_id: PMID:9079672
  qualifier: enables
  review:
    summary: >-
      TAS annotation to thymidine kinase activity from the original cloning paper, which
      established the substrate specificity of human TK2.
    action: ACCEPT
    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.
    supported_by:
    - reference_id: PMID:9079672
      supporting_text: >-
        Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that
        phosphorylates thymidine, deoxycytidine, and deoxyuridine.
- term:
    id: GO:0006139
    label: nucleobase-containing compound metabolic process
  evidence_type: TAS
  original_reference_id: PMID:9079672
  qualifier: involved_in
  review:
    summary: >-
      TAS annotation to the very broad term nucleobase-containing compound metabolic
      process from the cloning paper.
    action: MARK_AS_OVER_ANNOTATED
    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.
    supported_by:
    - reference_id: PMID:9079672
      supporting_text: >-
        Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that
        phosphorylates thymidine, deoxycytidine, and deoxyuridine.
core_functions:
- description: >-
    Thymidine kinase activity: ATP-dependent phosphorylation of thymidine to dTMP in the
    mitochondrial matrix, the defining (eponymous) function of TK2 and the pyrimidine arm
    of mitochondrial deoxyribonucleoside salvage that supplies dTMP for mitochondrial DNA
    replication.
  molecular_function:
    id: GO:0004797
    label: thymidine kinase activity
  directly_involved_in:
  - id: GO:0046104
    label: thymidine metabolic process
  - id: GO:0043097
    label: pyrimidine nucleoside salvage
  locations:
  - id: GO:0005759
    label: mitochondrial matrix
  supported_by:
  - reference_id: PMID:9079672
    supporting_text: >-
      Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that
      phosphorylates thymidine, deoxycytidine, and deoxyuridine.
  - reference_id: PMID:11687801
    supporting_text: >-
      the activity of TK2 in muscle mitochondria is
      reduced to 14-45% of the mean value in healthy control individuals.
- description: >-
    Deoxycytidine kinase activity: ATP-dependent phosphorylation of deoxycytidine to dCMP
    (and also deoxyuridine to dUMP) in the mitochondrial matrix. Together with the
    thymidine kinase activity this makes TK2 a multisubstrate pyrimidine deoxyribonucleoside
    kinase supplying dCMP/dUMP precursors for the mitochondrial dNTP pool.
  molecular_function:
    id: GO:0004137
    label: deoxycytidine kinase activity
  directly_involved_in:
  - id: GO:0046092
    label: deoxycytidine metabolic process
  - id: GO:0009157
    label: deoxyribonucleoside monophosphate biosynthetic process
  locations:
  - id: GO:0005759
    label: mitochondrial matrix
  supported_by:
  - reference_id: PMID:9989599
    supporting_text: >-
      The results demonstrate a broad
      substrate specificity and complex kinetics
  - reference_id: PMID:11687801
    supporting_text: >-
      TK2 phosphorylates deoxythymidine, deoxycytidine and deoxyuridine.
proposed_new_terms: []
suggested_questions:
- question: >-
    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?
- question: >-
    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?
suggested_experiments:
- description: >-
    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.
- description: >-
    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.
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000107
  title: Automatic transfer of experimentally verified manual GO annotation data to
    orthologs using Ensembl Compara
  findings: []
- id: GO_REF:0000108
  title: Automatic assignment of GO terms using logical inference, based on on inter-ontology
    links
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:11687801
  title: Mutant mitochondrial thymidine kinase in mitochondrial DNA depletion myopathy.
  findings:
  - statement: >-
      TK2 phosphorylates the pyrimidine deoxyribonucleosides thymidine, deoxycytidine and
      deoxyuridine; in non-replicating cells mtDNA synthesis depends solely on the
      mitochondrial salvage kinases TK2 and DGUOK. TK2 mutations reduce muscle mitochondrial
      TK2 activity to 14-45% of control and cause mtDNA depletion myopathy.
    supporting_text: >-
      TK2 phosphorylates deoxythymidine, deoxycytidine and deoxyuridine.
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PubMed-verified primary paper establishing TK2 substrate specificity, its role in the
      mitochondrial salvage pathway, and the MTDPS2 disease link. Abstract-only in cache
      but the substrate/activity statements are quoted verbatim.
- id: PMID:33961781
  title: Dual proteome-scale networks reveal cell-specific remodeling of the human
    interactome.
  findings:
  - statement: >-
      Proteome-scale AP-MS interactome (BioPlex) reporting a physical interaction of TK2
      with GSTK1 (Q9Y2Q3); the basis for a bare protein-binding IPI with no functional
      specificity for TK2.
    supporting_text: >-
      In tandem, these cell-specific interaction networks begin to reveal how
      interactomes vary with cellular state.
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: >-
      High-throughput interactome screen; correctly cited but supports only a bare
      protein-binding annotation (TK2-GSTK1) that is uninformative about TK2 function.
- id: PMID:34800366
  title: Quantitative high-confidence human mitochondrial proteome and its dynamics
    in cellular context.
  findings:
  - statement: >-
      Defined the high-confidence human mitochondrial proteome (MitoCoP, >1,100 proteins),
      which includes TK2, corroborating its mitochondrial localization by high-throughput
      proteomics.
    supporting_text: >-
      defined a
      mitochondrial high-confidence proteome of >1,100 proteins (MitoCoP).
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      High-throughput (HTP) mitochondrial-proteome study; TK2 is a MitoCoP member (in
      supplementary data, not named in the extracted text), supporting the mitochondrion
      location annotation.
- id: PMID:40205054
  title: Multimodal cell maps as a foundation for structural and functional genomics.
  findings:
  - statement: >-
      Multimodal (AP-MS + immunofluorescence) cell map of U2OS cells; source of a second
      bare protein-binding IPI for the TK2-GSTK1 interaction.
    supporting_text: >-
      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.
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: >-
      High-throughput multimodal interactome/imaging screen; correctly cited but supports
      only an uninformative protein-binding annotation for TK2.
- id: PMID:9079672
  title: Cloning of the cDNA and chromosome localization of the gene for human thymidine
    kinase 2.
  findings:
  - statement: >-
      Original cloning of human TK2, establishing it as a deoxyribonucleoside kinase that
      phosphorylates thymidine, deoxycytidine and deoxyuridine, ~30% similar to dCK and
      DGUOK, predominantly expressed in liver, pancreas, muscle and brain.
    supporting_text: >-
      Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that
      phosphorylates thymidine, deoxycytidine, and deoxyuridine.
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PubMed-verified original cloning/characterization paper; directly establishes TK2
      substrate specificity and family membership.
- id: PMID:9989599
  title: 'Human thymidine kinase 2: molecular cloning and characterisation of the
    enzyme activity with antiviral and cytostatic nucleoside substrates.'
  findings:
  - statement: >-
      Molecular cloning and biochemical characterization of human TK2 (purified
      mitochondrial thymidine kinase), demonstrating broad pyrimidine substrate specificity
      and complex kinetics, including phosphorylation of antiviral/cytostatic nucleoside
      analogues.
    supporting_text: >-
      The results demonstrate a broad
      substrate specificity and complex kinetics
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PubMed-verified primary paper; source of the mitochondrial localization (IDA) and
      catalytic-activity annotations for TK2.
- id: Reactome:R-HSA-109759
  title: deoxycytidine, thymidine, or deoxyuridine + ATP => dCMP, TMP, or dUMP + ADP
    [TK2]
  findings: []
- id: Reactome:R-HSA-73614
  title: Pyrimidine salvage
  findings: []
- id: file:human/TK2/TK2-uniprot.txt
  title: UniProtKB O00142 (KITM_HUMAN) TK2 record
  findings:
  - statement: >-
      TK2 phosphorylates thymidine, deoxycytidine and deoxyuridine in the mitochondrial
      matrix using ATP; in non-replicating cells mtDNA synthesis depends solely on TK2 and
      DGUOK. Has a cleaved N-terminal mitochondrial transit peptide (1-33) and belongs to
      the DCK/DGK family. Deficiency causes MTDPS2 and PEOB3.
    supporting_text: >-
      Phosphorylates thymidine, deoxycytidine, and deoxyuridine in