id: P50053
gene_symbol: KHK
product_type: PROTEIN
status: INITIALIZED
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  Ketohexokinase (fructokinase; EC 2.7.1.3) catalyzes the committed first step of dietary
  fructose catabolism (fructolysis), phosphorylating fructose to fructose-1-phosphate using
  ATP (beta-D-fructose plus ATP yields beta-D-fructose 1-phosphate plus ADP). It is a soluble
  cytosolic enzyme of the carbohydrate-kinase PfkB (ribokinase-like) family that acts as a
  homodimer, requires potassium, and is inhibited by ADP. Two catalytically active isoforms are
  produced by alternative splicing of two adjacent exons: KHK-C (the high-affinity, predominantly
  hepatic/renal/intestinal isoform responsible for the bulk of fructose phosphorylation in vivo)
  and KHK-A (widely but weakly expressed, lower fructose affinity, more thermostable, with no
  clearly defined physiological role). Fructose entering metabolism via KHK bypasses the
  phosphofructokinase-controlled regulatory step of glycolysis, so KHK-driven fructolysis is
  largely unregulated. Recessive loss of hepatic KHK activity causes essential fructosuria, a
  benign inborn error of metabolism characterized by elevated blood and urinary fructose after
  fructose, sucrose, or sorbitol ingestion.
alternative_products:
- name: C {ECO:0000269|PubMed:9799106} (Central,)
  id: P50053-1
- name: A {ECO:0000269|PubMed:9799106} (Peripheral)
  id: P50053-2
  sequence_note: VSP_004669
existing_annotations:
- term:
    id: GO:0005979
    label: regulation of glycogen biosynthetic process
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: PAN-GO phylogenetic (IBA) annotation placing KHK in regulation of glycogen
      biosynthesis. KHK produces fructose-1-phosphate, feeding hepatic carbohydrate flux that
      can be channeled toward glycogen, but KHK is a committed catabolic kinase, not a regulator
      of glycogen synthesis. There is no direct evidence that KHK regulates glycogen biosynthesis;
      this is a broad, indirect propagation across the ketohexokinase family. The current
      UniProt DR record shows the corresponding PAN-GO term as GO:0070873 (regulation of glycogen
      metabolic process), i.e. the family-level annotation is itself imprecise/indirect for KHK.
    action: MARK_AS_OVER_ANNOTATED
    reason: KHK's molecular function is fructose phosphorylation; any effect on glycogen is a
      downstream metabolic consequence of feeding the fructolysis product into hepatic carbohydrate
      metabolism, not a regulatory activity of KHK. Retaining as a core function would overstate
      KHK's role. Better captured as a non-core, indirect metabolic contribution.
    propagation_review:
      root_cause: PROPAGATION_BAD
      failure_modes:
      - ROLE_CONFLATION
      source_entities:
      - source_id: PANTHER:PTN001634026
        source_label: ketohexokinase PANTHER node
        source_status: SUPPORTS_SOURCE_BUT_NOT_TARGET
        comment: Family-level glycogen-regulation role reflects an indirect metabolic effect;
          KHK itself is the fructose-phosphorylating enzyme, not a regulator of glycogen synthesis.
      - source_id: MGI:MGI:1096353
        source_label: mouse Khk
        source_status: SUPPORTS_SOURCE_BUT_NOT_TARGET
- term:
    id: GO:0006000
    label: fructose metabolic process
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: KHK is the entry enzyme of fructose metabolism, phosphorylating fructose to
      fructose-1-phosphate; this IBA phylogenetic annotation correctly captures the core biological
      process for the ketohexokinase family.
    action: ACCEPT
    reason: Well supported by biochemistry (EC 2.7.1.3), by the enzyme's characterized catalytic
      activity, and by the essential-fructosuria disease phenotype. This is a core process for KHK.
      A more precise child term (GO:0006001 fructose catabolic process) is proposed in core_functions.
    supported_by:
    - reference_id: PMID:19237742
      supporting_text: "Fructose catabolism is initiated by its phosphorylation to fructose 1-phosphate, which is performed by ketohexokinase (KHK)."
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: Phylogenetic (IBA) annotation that KHK is active in the cytoplasm. KHK is a soluble
      cytosolic kinase with no membrane-targeting or organellar features, consistent with cytosolic
      activity.
    action: ACCEPT
    reason: Consistent with the Reactome cytosolic reaction and with the protein having no
      signal/transmembrane features. Cytosol (GO:0005829) is the more precise child; cytoplasm is
      acceptable and correct.
    supported_by:
    - reference_id: Reactome:R-HSA-70333
      supporting_text: "Cytosolic ketohexokinase (KHK, also known as fructokinase) catalyzes the reaction of D-fructose (Fru) and ATP to form D-fructose 1-phosphate (Fru 1-P) and ADP."
- term:
    id: GO:0004454
    label: ketohexokinase activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: Core molecular function. Phylogenetic (IBA) assignment of ketohexokinase activity,
      the defining catalytic activity of KHK (EC 2.7.1.3; beta-D-fructose plus ATP yields
      beta-D-fructose 1-phosphate plus ADP).
    action: ACCEPT
    reason: This is the central, experimentally established molecular function of the gene product,
      corroborated by direct enzymology (PMID:12941785) and structural studies (PMID:19237742).
    supported_by:
    - reference_id: PMID:19237742
      supporting_text: "Fructose catabolism is initiated by its phosphorylation to fructose 1-phosphate, which is performed by ketohexokinase (KHK)."
- term:
    id: GO:0004454
    label: ketohexokinase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: Electronic assignment of ketohexokinase activity via ARBA/InterPro (IPR034093) and the
      RHEA:18145 / EC:2.7.1.3 mapping. Redundant with, and confirmed by, the experimental annotations.
    action: ACCEPT
    reason: The EC/RHEA/InterPro mapping to ketohexokinase activity is correct and matches the
      experimentally validated function; recombinant assays confirm both isoforms are active.
    supported_by:
    - reference_id: PMID:12941785
      supporting_text: "Here we show that both ketohexokinase isoforms are indeed active."
- term:
    id: GO:0006000
    label: fructose metabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: involved_in
  review:
    summary: Electronic assignment (ARBA/InterPro, UniPathway UPA00202) of fructose metabolic process,
      matching the UniProt pathway annotation of carbohydrate metabolism / fructose metabolism.
    action: ACCEPT
    reason: Correct process assignment consistent with the enzyme's characterized role in fructolysis.
    supported_by:
    - reference_id: PMID:19237742
      supporting_text: "Fructose catabolism is initiated by its phosphorylation to fructose 1-phosphate, which is performed by ketohexokinase (KHK)."
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:25416956
  qualifier: enables
  review:
    summary: IPI annotation to bare "protein binding" from a proteome-scale human interactome map,
      recording an interaction with LHX9 (UniProtKB:Q9NQ69). The term "protein binding" conveys no
      specific molecular function, and the interaction (a high-throughput screen hit) has no
      established functional significance for KHK's catalytic role.
    action: MARK_AS_OVER_ANNOTATED
    reason: Per curation guidelines, bare "protein binding" is uninformative and should not be
      treated as a core function. The single high-throughput interactome hit (LHX9) is retained as
      an over-annotated, low-information annotation rather than removed, since it is an experimental
      IPI whose underlying dataset I cannot fully adjudicate.
- term:
    id: GO:0004454
    label: ketohexokinase activity
  evidence_type: IDA
  original_reference_id: PMID:12941785
  qualifier: enables
  review:
    summary: Direct assay (IDA) of ketohexokinase activity using recombinant human KHK. Both KHK-C
      and KHK-A isoforms were shown to be enzymatically active, with KHK-C the high-affinity fructose
      enzyme; the disease mutant G40R is null. This directly establishes the core catalytic function.
    action: ACCEPT
    reason: Gold-standard direct enzymatic evidence for ketohexokinase activity, including
      determination of kinetic parameters (KM for fructose and Mg-ATP) and characterization of
      disease-causing mutations.
    supported_by:
    - reference_id: PMID:12941785
      supporting_text: "Here we show that both ketohexokinase isoforms are indeed active. Ketohexokinase-A has much poorer substrate affinity than ketohexokinase-C for fructose but is considerably more thermostable."
- term:
    id: GO:0005524
    label: ATP binding
  evidence_type: IDA
  original_reference_id: PMID:19237742
  qualifier: enables
  review:
    summary: Direct structural evidence (IDA) for ATP binding, from the crystal structure of KHK-A
      in a ternary complex with fructose and the ATP analogue AMP-PNP. ATP is the phosphate donor
      for the ketohexokinase reaction, so ATP binding is an integral part of the catalytic function.
    action: ACCEPT
    reason: The co-crystal structure with AMP-PNP (a non-hydrolysable ATP analogue) directly
      demonstrates the nucleotide-binding site; consistent with UniProt ATP-binding residue features.
    supported_by:
    - reference_id: PMID:19237742
      supporting_text: "one fructose molecule and one molecule of AMP-PNP"
- term:
    id: GO:0006000
    label: fructose metabolic process
  evidence_type: IDA
  original_reference_id: PMID:12941785
  qualifier: involved_in
  review:
    summary: Direct evidence (IDA) placing KHK in fructose metabolism, from characterization of
      recombinant enzyme activity on fructose and the link of KHK deficiency to essential fructosuria.
    action: ACCEPT
    reason: Core biological process, directly supported by the enzyme's assayed activity on fructose
      and its disease role. A more precise child term is proposed in core_functions (GO:0006001).
    supported_by:
    - reference_id: PMID:12941785
      supporting_text: "Hepatic ketohexokinase deficiency causes the benign disorder essential fructosuria."
- term:
    id: GO:0042802
    label: identical protein binding
  evidence_type: IDA
  original_reference_id: PMID:19237742
  qualifier: enables
  review:
    summary: KHK is a homodimer, established by gel filtration, native PAGE, and the crystal
      structures; "identical protein binding" captures the self-association. Real and structurally
      characterized, though it is a structural/quaternary property rather than the core catalytic
      function.
    action: ACCEPT
    reason: Directly supported by biophysical and crystallographic evidence that KHK exists as a
      dimer in solution with a defined dimer interface. Homodimerization is a genuine property but
      is not itself the core molecular function (see GO:0042803 for the more specific term).
    supported_by:
    - reference_id: PMID:19237742
      supporting_text: "Gel filtration and nondenaturing polyacrylamide gel electrophoresis (Bais et al., 1985 ▶; Raushel & Cleland, 1977 ▶) have demonstrated that KHK is a dimer in solution."
- term:
    id: GO:0042803
    label: protein homodimerization activity
  evidence_type: IDA
  original_reference_id: PMID:19237742
  qualifier: enables
  review:
    summary: KHK forms a catalytically active homodimer with a dimer interface formed by extended
      four-stranded beta-sheets; one active site per subunit. This is the more informative term for
      the self-association captured also as identical protein binding.
    action: ACCEPT
    reason: Directly supported by the crystal structures of both isoforms and solution studies
      demonstrating dimerization. Well characterized quaternary structure; retained but secondary
      to the catalytic core function.
    supported_by:
    - reference_id: PMID:19237742
      supporting_text: "The KHK-A ternary-complex structure reveals the presence of one active site per subunit"
- term:
    id: GO:0070061
    label: fructose binding
  evidence_type: IDA
  original_reference_id: PMID:19237742
  qualifier: enables
  review:
    summary: Direct structural evidence (IDA) for fructose binding, from the KHK-A ternary complex in
      which the fructose substrate is almost completely buried in the active site with all five
      hydroxyls hydrogen-bonded to protein residues. Fructose binding is the substrate-recognition
      component of the ketohexokinase catalytic function.
    action: ACCEPT
    reason: The co-crystal structure directly resolves the bound fructose molecule and its
      interactions, consistent with UniProt fructose-binding residue features (Asp15, Gly41, Asn42,
      Asn45, Asp258).
    supported_by:
    - reference_id: PMID:19237742
      supporting_text: "All five hydroxyl groups of fructose form direct hydrogen bonds to protein residues"
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-5656459
  qualifier: located_in
  review:
    summary: Reactome TAS annotation placing KHK in the cytosol, from the "Defective KHK does not
      phosphorylate beta-D-fructose" reaction (the essential-fructosuria variant reaction). KHK is a
      soluble cytosolic enzyme.
    action: ACCEPT
    reason: Consistent with all other localization evidence (Reactome fructolysis reaction, IBA
      cytoplasm, IDA cytoplasm) and with the enzyme lacking targeting signals.
    supported_by:
    - reference_id: Reactome:R-HSA-5656459
      supporting_text: "the first step of fructose catabolism in the liver"
- term:
    id: GO:0070062
    label: extracellular exosome
  evidence_type: HDA
  original_reference_id: PMID:19056867
  qualifier: located_in
  review:
    summary: High-throughput mass-spectrometry detection (HDA) of KHK in human urinary exosomes.
      This is a proteomic identification, consistent with KHK's high expression in renal
      epithelium, but does not indicate that KHK functions in exosomes; it is not a site of
      catalytic activity.
    action: KEEP_AS_NON_CORE
    reason: Bulk proteomic detection of a highly expressed cytosolic enzyme in exosomes is common
      and not indicative of a functional extracellular/vesicular role. Retained as non-core
      localization evidence rather than accepted as a core cellular component.
    supported_by:
    - reference_id: PMID:19056867
      supporting_text: "profile the proteome of human urinary exosomes"
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-70333
  qualifier: located_in
  review:
    summary: Reactome TAS annotation placing KHK in the cytosol, from the "KHK dimer phosphorylates
      Fru to Fru 1-P" reaction. The enzyme is explicitly described as cytosolic.
    action: ACCEPT
    reason: Directly supported by the Reactome fructolysis reaction, which describes cytosolic KHK
      catalyzing fructose phosphorylation; consistent with all other localization evidence.
    supported_by:
    - reference_id: Reactome:R-HSA-70333
      supporting_text: "Cytosolic ketohexokinase (KHK, also known as fructokinase) catalyzes the reaction of D-fructose (Fru) and ATP to form D-fructose 1-phosphate (Fru 1-P) and ADP."
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IDA
  original_reference_id: GO_REF:0000054
  qualifier: located_in
  review:
    summary: Direct assay (IDA, LIFEdb GFP-fusion localization) placing KHK in the cytoplasm,
      consistent with it being a soluble cytosolic kinase.
    action: ACCEPT
    reason: Consistent with all other localization evidence and with the absence of targeting
      signals. Cytosol (GO:0005829) is the more precise term but cytoplasm is correct.
    supported_by:
    - reference_id: Reactome:R-HSA-70333
      supporting_text: "Cytosolic ketohexokinase (KHK, also known as fructokinase) catalyzes the reaction of D-fructose (Fru) and ATP to form D-fructose 1-phosphate (Fru 1-P) and ADP."
- term:
    id: GO:0004454
    label: ketohexokinase activity
  evidence_type: TAS
  original_reference_id: PMID:7833921
  qualifier: enables
  review:
    summary: TAS annotation of ketohexokinase activity from the original cloning/mutational-analysis
      paper that defined the molecular basis of essential fructosuria, identifying KHK as hepatic
      fructokinase (EC 2.7.1.3) that catalyzes the first step of dietary fructose metabolism.
    action: ACCEPT
    reason: Consistent with the direct enzymatic and structural evidence for ketohexokinase activity;
      this paper established KHK as the fructokinase whose deficiency causes essential fructosuria.
    supported_by:
    - reference_id: PMID:7833921
      supporting_text: "This enzyme catalyses the first step of metabolism of dietary fructose, conversion of fructose to fructose-1-phosphate."
- term:
    id: GO:0006001
    label: fructose catabolic process
  evidence_type: IDA
  original_reference_id: PMID:12941785
  qualifier: involved_in
  review:
    summary: Proposed more precise process annotation. KHK catalyzes the committed, first step
      of fructose catabolism (fructolysis), phosphorylating fructose to fructose-1-phosphate;
      fructose catabolic process (GO:0006001) is the precise child of the existing fructose
      metabolic process (GO:0006000) annotations and better reflects KHK's committed catabolic role.
    action: NEW
    reason: The existing GO:0006000 (fructose metabolic process) annotations are correct but
      general; KHK specifically initiates fructose catabolism, so the catabolic child term is a
      more informative representation of its biological process. Proposed as a refinement, not a
      replacement, of the accepted fructose-metabolic-process annotations.
    supported_by:
    - reference_id: PMID:19237742
      supporting_text: "Fructose catabolism is initiated by its phosphorylation to fructose 1-phosphate, which is performed by ketohexokinase (KHK)."
    - reference_id: PMID:7833921
      supporting_text: "This enzyme catalyses the first step of metabolism of dietary fructose, conversion of fructose to fructose-1-phosphate."
core_functions:
- description: Ketohexokinase activity - phosphorylation of fructose to fructose-1-phosphate using
    ATP, the committed first step of fructose catabolism (fructolysis), performed by cytosolic
    KHK homodimers.
  molecular_function:
    id: GO:0004454
    label: ketohexokinase activity
  directly_involved_in:
  - id: GO:0006001
    label: fructose catabolic process
  locations:
  - id: GO:0005829
    label: cytosol
  supported_by:
  - reference_id: PMID:12941785
    supporting_text: "Here we show that both ketohexokinase isoforms are indeed active."
  - reference_id: PMID:19237742
    supporting_text: "Fructose catabolism is initiated by its phosphorylation to fructose 1-phosphate, which is performed by ketohexokinase (KHK)."
references:
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000054
  title: Gene Ontology annotation based on curation of intracellular localizations
    of expressed fusion proteins in living cells
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:12941785
  title: Properties of normal and mutant recombinant human ketohexokinases and implications
    for the pathogenesis of essential fructosuria.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Primary enzymology of recombinant human KHK; establishes ketohexokinase
      activity (EC 2.7.1.3), that both KHK-A and KHK-C isoforms are active, kinetic parameters,
      and the disease mechanism of essential fructosuria (G40R null, A43T thermolabile). Directly
      supports the core catalytic function and fructose metabolic process annotations. Cached
      entry is abstract-only (full_text_available false).
- id: PMID:19056867
  title: Large-scale proteomics and phosphoproteomics of urinary exosomes.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: High-throughput urinary exosome proteomics; source of the extracellular exosome
      HDA localization. Detection of a highly expressed cytosolic enzyme in exosomes, not a
      functional localization. Abstract-only cache does not name KHK individually (part of the
      1132-protein dataset).
- id: PMID:19237742
  title: Structures of alternatively spliced isoforms of human ketohexokinase.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Full-text-available crystal structures of KHK-A and KHK-C and the KHK-A ternary
      complex with fructose and AMP-PNP. Directly supports ATP binding, fructose binding,
      homodimerization/identical protein binding, and the committed fructolysis role. Verified
      against cached full text.
- id: PMID:25416956
  title: A proteome-scale map of the human interactome network.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Proteome-scale Y2H interactome; source of the bare "protein binding" IPI
      (KHK-LHX9, Q9NQ69). The cached full text does not mention KHK or LHX9 specifically (the
      interaction lives in the supplementary dataset). Low information value for KHK's molecular
      function; retained as over-annotated.
- id: PMID:7833921
  title: 'Molecular basis of essential fructosuria: molecular cloning and mutational
    analysis of human ketohexokinase (fructokinase).'
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Original cloning and mutational analysis defining KHK as hepatic fructokinase
      (EC 2.7.1.3) and the molecular basis of essential fructosuria (compound heterozygous G40R /
      A43T). Supports ketohexokinase activity and disease role. Abstract-only cache.
- id: Reactome:R-HSA-5656459
  title: Defective KHK does not phosphorylate beta-D-fructose
  findings: []
- id: Reactome:R-HSA-70333
  title: KHK dimer phosphorylates Fru to Fru 1-P
  findings: []