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.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005979 regulation of glycogen biosynthetic process | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | 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. 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 Sources checked: PANTHER:PTN001634026 · ketohexokinase PANTHER node SUPPORTS SOURCE BUT NOT TARGET Family-level glycogen-regulation role reflects an indirect metabolic effect; KHK itself is the fructose-phosphorylating enzyme, not a regulator of glycogen synthesis. MGI:MGI:1096353 · mouse Khk SUPPORTS SOURCE BUT NOT TARGET |
| GO:0006000 fructose metabolic process | IBA GO_REF:0000033 | ACCEPT | 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. 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. Supporting Evidence: PMID:19237742 Fructose catabolism is initiated by its phosphorylation to fructose 1-phosphate, which is performed by ketohexokinase (KHK). |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | 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. 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. Supporting Evidence: Reactome:R-HSA-70333 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. |
| GO:0004454 ketohexokinase activity | IBA GO_REF:0000033 | ACCEPT | 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). Reason: This is the central, experimentally established molecular function of the gene product, corroborated by direct enzymology (PMID:12941785) and structural studies (PMID:19237742). Supporting Evidence: PMID:19237742 Fructose catabolism is initiated by its phosphorylation to fructose 1-phosphate, which is performed by ketohexokinase (KHK). |
| GO:0004454 ketohexokinase activity | IEA GO_REF:0000120 | ACCEPT | 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. Reason: The EC/RHEA/InterPro mapping to ketohexokinase activity is correct and matches the experimentally validated function; recombinant assays confirm both isoforms are active. Supporting Evidence: PMID:12941785 Here we show that both ketohexokinase isoforms are indeed active. |
| GO:0006000 fructose metabolic process | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic assignment (ARBA/InterPro, UniPathway UPA00202) of fructose metabolic process, matching the UniProt pathway annotation of carbohydrate metabolism / fructose metabolism. Reason: Correct process assignment consistent with the enzyme's characterized role in fructolysis. Supporting Evidence: PMID:19237742 Fructose catabolism is initiated by its phosphorylation to fructose 1-phosphate, which is performed by ketohexokinase (KHK). |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | MARK AS OVER ANNOTATED | 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. 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. |
| GO:0004454 ketohexokinase activity | IDA PMID:12941785 Properties of normal and mutant recombinant human ketohexoki... | ACCEPT | 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. 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. Supporting Evidence: PMID:12941785 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. |
| GO:0005524 ATP binding | IDA PMID:19237742 Structures of alternatively spliced isoforms of human ketohe... | ACCEPT | 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. 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. Supporting Evidence: PMID:19237742 one fructose molecule and one molecule of AMP-PNP |
| GO:0006000 fructose metabolic process | IDA PMID:12941785 Properties of normal and mutant recombinant human ketohexoki... | ACCEPT | 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. 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). Supporting Evidence: PMID:12941785 Hepatic ketohexokinase deficiency causes the benign disorder essential fructosuria. |
| GO:0042802 identical protein binding | IDA PMID:19237742 Structures of alternatively spliced isoforms of human ketohe... | ACCEPT | 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. 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). Supporting Evidence: PMID:19237742 Gel filtration and nondenaturing polyacrylamide gel electrophoresis (Bais et al., 1985 ▶; Raushel & Cleland, 1977 ▶) have demonstrated that KHK is a dimer in solution. |
| GO:0042803 protein homodimerization activity | IDA PMID:19237742 Structures of alternatively spliced isoforms of human ketohe... | ACCEPT | 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. 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. Supporting Evidence: PMID:19237742 The KHK-A ternary-complex structure reveals the presence of one active site per subunit |
| GO:0070061 fructose binding | IDA PMID:19237742 Structures of alternatively spliced isoforms of human ketohe... | ACCEPT | 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. 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). Supporting Evidence: PMID:19237742 All five hydroxyl groups of fructose form direct hydrogen bonds to protein residues |
| GO:0005829 cytosol | TAS Reactome:R-HSA-5656459 | ACCEPT | 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. Reason: Consistent with all other localization evidence (Reactome fructolysis reaction, IBA cytoplasm, IDA cytoplasm) and with the enzyme lacking targeting signals. Supporting Evidence: Reactome:R-HSA-5656459 the first step of fructose catabolism in the liver |
| GO:0070062 extracellular exosome | HDA PMID:19056867 Large-scale proteomics and phosphoproteomics of urinary exos... | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:19056867 profile the proteome of human urinary exosomes |
| GO:0005829 cytosol | TAS Reactome:R-HSA-70333 | ACCEPT | 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. Reason: Directly supported by the Reactome fructolysis reaction, which describes cytosolic KHK catalyzing fructose phosphorylation; consistent with all other localization evidence. Supporting Evidence: Reactome:R-HSA-70333 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. |
| GO:0005737 cytoplasm | IDA GO_REF:0000054 | ACCEPT | Summary: Direct assay (IDA, LIFEdb GFP-fusion localization) placing KHK in the cytoplasm, consistent with it being a soluble cytosolic kinase. 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. Supporting Evidence: Reactome:R-HSA-70333 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. |
| GO:0004454 ketohexokinase activity | TAS PMID:7833921 Molecular basis of essential fructosuria: molecular cloning ... | ACCEPT | 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. Reason: Consistent with the direct enzymatic and structural evidence for ketohexokinase activity; this paper established KHK as the fructokinase whose deficiency causes essential fructosuria. Supporting Evidence: PMID:7833921 This enzyme catalyses the first step of metabolism of dietary fructose, conversion of fructose to fructose-1-phosphate. |
| GO:0006001 fructose catabolic process | IDA PMID:12941785 Properties of normal and mutant recombinant human ketohexoki... | NEW | 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. 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. Supporting Evidence: PMID:19237742 Fructose catabolism is initiated by its phosphorylation to fructose 1-phosphate, which is performed by ketohexokinase (KHK). PMID:7833921 This enzyme catalyses the first step of metabolism of dietary fructose, conversion of fructose to fructose-1-phosphate. |
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)