GALE is the human cytosolic, NAD+-dependent UDP-galactose 4-epimerase (galactowaldenase; EC 5.1.3.2), which catalyses the fourth, recycling step of the Leloir pathway of galactose catabolism: the reversible interconversion of UDP-alpha-D-glucose and UDP-alpha-D-galactose. This activity regenerates the UDP-glucose consumed by galactose-1-phosphate uridylyltransferase (GALT) and supplies UDP-galactose for the biosynthesis of glycoproteins, glycolipids and other galactosylated glycoconjugates. The human enzyme is bifunctional and also interconverts UDP-N-acetyl-alpha-D-glucosamine and UDP-N-acetyl-alpha-D- galactosamine (EC 5.1.3.7), providing UDP-GalNAc for glycosylation; this second activity, absent in the bacterial ortholog, reflects an enlarged active-site sugar-binding pocket. The active enzyme is a homodimer with one tightly bound NAD+ per subunit; catalysis proceeds through transient oxidation of the sugar at C4, rotation, and stereospecific reduction, with Tyr157 acting as the active-site proton acceptor. Loss-of-function variants cause epimerase-deficiency galactosemia (galactosemia type III), which ranges from a benign peripheral form limited to circulating blood cells to a rare severe generalized form, and biallelic variants that predominantly impair the UDP-GalNAc/GlcNAc activity and N-glycosylation cause a syndromic macrothrombocytopenia.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003978 UDP-glucose 4-epimerase activity | IBA GO_REF:0000033 | ACCEPT | Summary: UDP-glucose 4-epimerase (UDP-Glc <-> UDP-Gal, EC 5.1.3.2) is the defining, phylogenetically conserved molecular function of GALE across the panther family. This is the core molecular function and is independently supported by direct biochemical assay of the human enzyme. Reason: Core molecular function. The IBA is consistent with direct experimental characterization of purified human GALE catalysing UDP-glucose/UDP-galactose interconversion, and with the crystal structure of the enzyme in complex with NAD and UDP-glucose. Supporting Evidence: PMID:16302980 UDP-galactose 4-epimerase (GALE, EC 5.1.3.2) catalyses the interconversion of UDP-glucose and UDP-galactose. |
| GO:0033499 beta-D-galactose catabolic process via UDP-galactose, Leloir pathway | IBA GO_REF:0000033 | ACCEPT | Summary: GALE catalyses the recycling (fourth) step of the Leloir pathway, regenerating UDP-glucose and supplying UDP-galactose, so that dietary galactose is ultimately routed to glycolytic intermediates. This is the most specific and informative biological-process term for the gene and is the primary process annotation. Reason: Accurately captures GALE's role in the Leloir pathway of galactose catabolism. Supported by UniProt pathway annotation and by the enzyme's established biochemistry. Supporting Evidence: PMID:16302980 UDP-galactose 4-epimerase (GALE, EC 5.1.3.2) catalyses the interconversion of UDP-glucose and UDP-galactose. |
| GO:0005829 cytosol | IBA GO_REF:0000033 | ACCEPT | Summary: GALE is a soluble cytosolic enzyme; it has no signal peptide, transmembrane region, or organellar targeting sequence, and Reactome models the catalytic step as occurring in the cytosol. Reason: Consistent with the cytosolic localization documented by Reactome and with the absence of any targeting features in the UniProt sequence. Supporting Evidence: Reactome:R-HSA-70369 Cytosolic UDP-galactose 4-epimerase catalyzes the interconversion of UDP-D-galactose and UDP-D-glucose |
| GO:0003974 UDP-N-acetylglucosamine 4-epimerase activity | IEA GO_REF:0000120 | ACCEPT | Summary: The human enzyme is bifunctional and also catalyses the reversible epimerization of UDP-N-acetyl-D-glucosamine and UDP-N-acetyl-D-galactosamine (EC 5.1.3.7, RHEA:20517), supplying UDP-GalNAc for glycoconjugate synthesis. This second activity is experimentally established for human GALE (unlike the E. coli ortholog) and is central to the N-glycosylation defect seen in GALE-related thrombocytopenia. Reason: The EC 5.1.3.7 / RHEA:20517 mapping is a correct, experimentally corroborated secondary catalytic activity of the human enzyme, not an over-broad IEA. UniProt documents both catalytic activities (EC 5.1.3.2 and EC 5.1.3.7) on the basis of direct assay, and structural studies show the enlarged active site accommodates the N-acetyl group. No verbatim supporting quote is attached here because the cached publications and Reactome reaction document only the UDP-Glc/UDP-Gal (EC 5.1.3.2) activity; the N-acetylhexosamine activity rests on the UniProt catalytic-activity annotation (RHEA:20517). |
| GO:0003978 UDP-glucose 4-epimerase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic assertion of the core UDP-glucose 4-epimerase activity from the EC 5.1.3.2 / RHEA:22168 mapping and InterPro family membership. Redundant with, and corroborated by, the IBA and IDA annotations to the same term. Reason: Correct EC/RHEA/InterPro-based mapping of the core molecular function; consistent with direct experimental evidence. Supporting Evidence: PMID:16302980 UDP-galactose 4-epimerase (GALE, EC 5.1.3.2) catalyses the interconversion of UDP-glucose and UDP-galactose. |
| GO:0006012 galactose metabolic process | IEA GO_REF:0000120 | ACCEPT | Summary: Involvement in galactose metabolism is correct but is a broad parent of the more specific Leloir-pathway catabolic term (GO:0033499). Retained as a valid, if general, process annotation. Reason: Accurate at the general level (galactose metabolic process); the more specific GO:0033499 and GO:0019388 annotations provide the informative process terms. No reason to remove a correct broader parent. Supporting Evidence: PMID:16302980 the genetic disease, type III galactosemia |
| GO:0042802 identical protein binding | IPI PMID:16189514 Towards a proteome-scale map of the human protein-protein in... | MARK AS OVER ANNOTATED | Summary: Self-interaction (Q14376-Q14376) detected in a large-scale human protein interactome map. This reflects the enzyme's known homodimeric state but is a low-information molecular-function term. Reason: The self-interaction is real and consistent with the established homodimer, but "identical protein binding" is an uninformative term relative to the more specific GO:0042803 protein homodimerization activity already annotated for this gene, and it is repeated five times from high-throughput interactome screens. Kept but flagged as over-annotated; the homodimer is best represented by GO:0042803. Supporting Evidence: PMID:16189514 Towards a proteome-scale map of the human protein-protein interaction network. |
| GO:0042802 identical protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | MARK AS OVER ANNOTATED | Summary: Self-interaction reported in a proteome-scale human interactome map, consistent with the GALE homodimer. Low-information duplicate of the same identical-protein-binding assertion. Reason: Same rationale as the other identical-protein-binding annotations: real but uninformative relative to GO:0042803 protein homodimerization activity; one of five duplicate high-throughput self-interaction annotations. Supporting Evidence: PMID:25416956 A proteome-scale map of the human interactome network. |
| GO:0042802 identical protein binding | IPI PMID:25502805 A massively parallel pipeline to clone DNA variants and exam... | MARK AS OVER ANNOTATED | Summary: Self-interaction reported in a large-scale variant/interaction phenotyping pipeline, consistent with the GALE homodimer. Reason: Real self-interaction but uninformative relative to GO:0042803; duplicate high-throughput identical-protein-binding annotation. Supporting Evidence: PMID:25502805 A massively parallel pipeline to clone DNA variants and examine molecular phenotypes of human disease mutations. |
| GO:0042802 identical protein binding | IPI PMID:31515488 Extensive disruption of protein interactions by genetic vari... | MARK AS OVER ANNOTATED | Summary: Self-interaction reported in a systematic study of interaction disruption by human genetic variants, consistent with the GALE homodimer. Reason: Real self-interaction but uninformative relative to GO:0042803; duplicate high-throughput identical-protein-binding annotation. Supporting Evidence: PMID:31515488 Extensive disruption of protein interactions by genetic variants across the allele frequency spectrum in human populations. |
| GO:0042802 identical protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: Self-interaction reported in the HuRI reference map of the human binary protein interactome, consistent with the GALE homodimer. Reason: Real self-interaction but uninformative relative to GO:0042803; duplicate high-throughput identical-protein-binding annotation. Supporting Evidence: PMID:32296183 A reference map of the human binary protein interactome. |
| GO:0033499 beta-D-galactose catabolic process via UDP-galactose, Leloir pathway | IEA GO_REF:0000107 | ACCEPT | Summary: Electronic (Ensembl Compara ortholog-based) transfer of the Leloir-pathway galactose catabolic process, transferred from the mouse ortholog (Q8R059). Consistent with the IBA annotation to the same term and with GALE's established function. Reason: Correct process assignment; the ortholog-based inference agrees with the manual IBA annotation and with the enzyme's biochemistry. No over-propagation concern for this well-conserved Leloir enzyme. Supporting Evidence: PMID:16302980 UDP-galactose 4-epimerase (GALE, EC 5.1.3.2) catalyses the interconversion of UDP-glucose and UDP-galactose. |
| GO:0005829 cytosol | TAS Reactome:R-HSA-5610036 | ACCEPT | Summary: Reactome traceable-author-statement localizing GALE to the cytosol, in the context of defective GALE failing to epimerise UDP-Gal to UDP-Glc. Consistent with the other cytosol annotations. Reason: GALE is a soluble cytosolic enzyme; localization is well established. Duplicate of the other cytosol annotations, which is acceptable. Supporting Evidence: Reactome:R-HSA-70369 Cytosolic UDP-galactose 4-epimerase catalyzes the interconversion of UDP-D-galactose and UDP-D-glucose |
| GO:0005829 cytosol | TAS Reactome:R-HSA-70369 | ACCEPT | Summary: Reactome traceable-author-statement placing the GALE:NAD+ dimer epimerization reaction in the cytosol. Directly supports the cytosolic localization of the active enzyme. Reason: Well-supported cytosolic localization of the catalytically active homodimer. Supporting Evidence: Reactome:R-HSA-70369 Cytosolic UDP-galactose 4-epimerase catalyzes the interconversion of UDP-D-galactose and UDP-D-glucose |
| GO:0003978 UDP-glucose 4-epimerase activity | IDA PMID:16302980 Functional analysis of disease-causing mutations in human UD... | ACCEPT | Summary: Direct assay of recombinant purified human GALE established UDP-glucose/UDP-galactose epimerase activity and its kinetic parameters, including the effects of disease-causing mutations on turnover and specificity. This is the strongest direct evidence for the core molecular function. Reason: Experimental (IDA) confirmation of the core catalytic activity in the human enzyme. Supporting Evidence: PMID:16302980 UDP-galactose 4-epimerase (GALE, EC 5.1.3.2) catalyses the interconversion of UDP-glucose and UDP-galactose. |
| GO:0019388 galactose catabolic process | IDA PMID:16302980 Functional analysis of disease-causing mutations in human UD... | ACCEPT | Summary: GALE's epimerase activity is required for galactose catabolism, and impairment causes type III galactosemia; the direct functional characterization links the enzyme to galactose breakdown. GO:0033499 is the more specific Leloir-pathway child, but this general catabolic term is correct. Reason: Accurate process assignment supported by direct functional/disease evidence. Retained alongside the more specific GO:0033499. Supporting Evidence: PMID:16302980 Point mutations in this enzyme are associated with the genetic disease, type III galactosemia |
| GO:0042803 protein homodimerization activity | IPI PMID:16302980 Functional analysis of disease-causing mutations in human UD... | ACCEPT | Summary: The active enzyme is a homodimer; protein-protein crosslinking of wild-type and mutant GALE demonstrated homodimer formation, and the crystal structure is dimeric. This is the informative representation of GALE's self-association. Reason: Well-supported homodimerization, established both biochemically (crosslinking) and structurally. Preferred over the bare identical-protein-binding annotations for representing the same underlying self-association. Supporting Evidence: PMID:16302980 Protein-protein crosslinking suggests that none of the mutants are impaired in homodimer formation. |
| GO:0051287 NAD binding | IDA PMID:10801319 Crystallographic evidence for Tyr 157 functioning as the act... | NEW | Summary: GALE binds NAD+ as a tightly bound cofactor, one molecule per subunit, essential for the transient oxidation/reduction chemistry of epimerization. The crystal structure resolves the bound NAD, and UniProt annotates multiple NAD(+)-binding residues. Reason: NAD binding is a well-established, structurally and biochemically documented molecular function of GALE that is missing from the existing annotation set. Added as a new annotation for the enzyme's cofactor-binding role. Supporting Evidence: PMID:10801319 structures of human epimerase in the resting state (i.e., with bound NAD(+)) |
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Download this section (compressed HTML)Q: To what extent does the UDP-GalNAc/GlcNAc 4-epimerase activity of GALE, versus its UDP-Gal/Glc activity, account for the distinct clinical phenotypes (galactosemia versus syndromic macrothrombocytopenia) seen with different GALE variants?
Experiment: Systematically measure both UDP-Gal/Glc (EC 5.1.3.2) and UDP-GalNAc/GlcNAc (EC 5.1.3.7) epimerase activities for a panel of galactosemia- and thrombocytopenia-associated GALE variants, and correlate the differential loss of each activity with glycosylation status and clinical presentation.
Type: enzyme kinetics and glycomics
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