UDP-glucose 4-epimerase

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UDP-glucose 4-epimerase
AliasesUDPgalactose 4-epimerase4-epimeraseuridine diphosphate glucose 4-epimeraseUDPG-4-epimeraseUDP-galactose 4-epimeraseuridine diphosphoglucose epimeraseuridine diphospho-galactose-4-epimeraseUDP-D-galactose 4-epimeraseUDP-glucose epimeraseuridine diphosphoglucose 4-epimeraseuridine diphosphate galactose 4-epimerase
External IDsGeneCards: [1]
RefSeq (mRNA)



RefSeq (protein)



Location (UCSC)n/an/a
PubMed searchn/an/a
View/Edit Human
UDP-glucose 4-epimerase
H. sapiens UDP-glucose 4-epimerase homodimer bound to NADH and UDP-glucose. Domains: N-terminal and C-terminal.
EC no.
CAS no.9032-89-7
IntEnzIntEnz view
ExPASyNiceZyme view
MetaCycmetabolic pathway
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO
Human GALE bound to NAD+ and UDP-GlcNAc, with N- and C-terminal domains highlighted. Asn 207 contorts to accommodate UDP-GlcNAc within the active site.
NCBI gene2582
Other data
EC number5.1.3.2
LocusChr. 1 p36-p35
Search for
NAD-dependent epimerase/dehydratase
Available protein structures:
Pfam  structures / ECOD  
PDBsumstructure summary

The enzyme UDP-glucose 4-epimerase (EC, also known as UDP-galactose 4-epimerase or GALE, is a homodimeric epimerase found in bacterial, fungal, plant, and mammalian cells. This enzyme performs the final step in the Leloir pathway of galactose metabolism, catalyzing the reversible conversion of UDP-galactose to UDP-glucose.[1] GALE tightly binds nicotinamide adenine dinucleotide (NAD+), a co-factor required for catalytic activity.[2]

Additionally, human and some bacterial GALE isoforms reversibly catalyze the formation of UDP-N-acetylgalactosamine (UDP-GalNAc) from UDP-N-acetylglucosamine (UDP-GlcNAc) in the presence of NAD+, an initial step in glycoprotein or glycolipid synthesis.[3]

Historical significance[edit]

Dr. Luis Leloir deduced the role of GALE in galactose metabolism during his tenure at the Instituto de Investigaciones Bioquímicas del Fundación Campomar, initially terming the enzyme waldenase.[4] Dr. Leloir was awarded the 1970 Nobel Prize in Chemistry for his discovery of sugar nucleotides and their role in the biosynthesis of carbohydrates.[5]


GALE belongs to the short-chain dehydrogenase/reductase (SDR) superfamily of proteins.[6] This family is characterized by a conserved Tyr-X-X-X-Lys motif necessary for enzymatic activity; one or more Rossmann fold scaffolds; and the ability to bind NAD+.[6]

Tertiary structure[edit]

GALE structure has been resolved for a number of species, including E. coli[7] and humans.[8] GALE exists as a homodimer in various species.[8]

While subunit size varies from 68 amino acids (Enterococcus faecalis) to 564 amino acids (Rhodococcus jostii), a majority of GALE subunits cluster near 330 amino acids in length.[6] Each subunit contains two distinct domains. An N-terminal domain contains a 7-stranded parallel β-pleated sheet flanked by α-helices.[1] Paired Rossmann folds within this domain allow GALE to tightly bind one NAD+ cofactor per subunit.[2] A 6-stranded β-sheet and 5 α-helices comprise GALE's C-terminal domain.[1] C-terminal residues bind UDP, such that the subunit is responsible for correctly positioning UDP-glucose or UDP-galactose for catalysis.[1]

Active site[edit]

The cleft between GALE's N- and C-terminal domains constitutes the enzyme's active site. A conserved Tyr-X-X-X Lys motif is necessary for GALE catalytic activity; in humans, this motif is represented by Tyr 157-Gly-Lys-Ser-Lys 161,[6] while E. coli GALE contains Tyr 149-Gly-Lys-Ser-Lys 153.[8] The size and shape of GALE's active site varies across species, allowing for variable GALE substrate specificity.[3] Additionally, the conformation of the active site within a species-specific GALE is malleable; for instance, a bulky UDP-GlcNAc 2' N-acetyl group is accommodated within the human GALE active site by the rotation of the Asn 207 carboxamide side chain.[3]

Known E. coli GALE residue interactions with UDP-glucose and UDP-galactose.[9]
Residue Function
Ala 216, Phe 218 Anchor uracil ring to enzyme.
Asp 295 Interacts with ribose 2' hydroxyl group.
Asn 179, Arg 231, Arg 292 Interact with UDP phosphate groups.
Tyr 299, Asn 179 Interact with galactose 2' hydroxyl or glucose 6' hydroxyl group; properly position sugar within active site.
Tyr 177, Phe 178 Interact with galactose 3' hydroxyl or glucose 6' hydroxyl group; properly position sugar within active site.
Lys 153 Lowers pKa of Tyr 149, allows for abstraction or donation of a hydrogen atom to or from the sugar 4' hydroxyl group.
Tyr 149 Abstracts or donates a hydrogen atom to or from the sugar 4' hydroxyl group, catalyzing formation of 4-ketopyranose intermediate.


Conversion of UDP-galactose to UDP-glucose[edit]

GALE inverts the configuration of the 4' hydroxyl group of UDP-galactose through a series of 4 steps. Upon binding UDP-galactose, a conserved tyrosine residue in the active site abstracts a proton from the 4' hydroxyl group.[7][10]

Concomitantly, the 4' hydride is added to the si-face of NAD+, generating NADH and a 4-ketopyranose intermediate.[1] The 4-ketopyranose intermediate rotates 180° about the pyrophosphoryl linkage between the glycosyl oxygen and β-phosphorus atom, presenting the opposite face of the ketopyranose intermediate to NADH.[10] Hydride transfer from NADH to this opposite face inverts the stereochemistry of the 4' center. The conserved tyrosine residue then donates its proton, regenerating the 4' hydroxyl group.[1]

Conversion of UDP-GlcNAc to UDP-GalNAc[edit]

Human and some bacterial GALE isoforms reversibly catalyze the conversion of UDP-GlcNAc to UDP-GalNAc through an identical mechanism, inverting the stereochemical configuration at the sugar's 4' hydroxyl group.[3][11]

Biological function[edit]

Steps in the Leloir pathway of galactose metabolism.
Intermediates and enzymes in the Leloir pathway of galactose metabolism.[1]

Galactose metabolism[edit]

No direct catabolic pathways exist for galactose metabolism. Galactose is therefore preferentially converted into glucose-1-phosphate, which may be shunted into glycolysis or the inositol synthesis pathway.[12]

GALE functions as one of four enzymes in the Leloir pathway of galactose conversion of glucose-1-phosphate. First, galactose mutarotase converts β-D-galactose to α-D-galactose.[1] Galactokinase then phosphorylates α-D-galactose at the 1' hydroxyl group, yielding galactose-1-phosphate.[1] In the third step, galactose-1-phosphate uridyltransferase catalyzes the reversible transfer of a UMP moiety from UDP-glucose to galactose-1-phosphate, generating UDP-galactose and glucose-1-phosphate.[1] In the final Leloir step, UDP-glucose is regenerated from UDP-galactose by GALE; UDP-glucose cycles back to the third step of the pathway.[1] As such, GALE regenerates a substrate necessary for continued Leloir pathway cycling.

The glucose-1-phosphate generated in step 3 of the Leloir pathway may be isomerized to glucose-6-phosphate by phosphoglucomutase. Glucose-6-phosphate readily enters glycolysis, leading to the production of ATP and pyruvate.[13] Furthermore, glucose-6-phosphate may be converted to inositol-1-phosphate by inositol-3-phosphate synthase, generating a precursor needed for inositol biosynthesis.[14]

UDP-GalNAc synthesis[edit]

Human and selected bacterial GALE isoforms bind UDP-GlcNAc, reversibly catalyzing its conversion to UDP-GalNAc. A family of glycosyltransferases known as UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosamine transferases (ppGaNTases) transfers GalNAc from UDP-GalNAc to glycoprotein serine and threonine residues.[15] ppGaNTase-mediated glycosylation regulates protein sorting,[16][17][18][19][20] ligand signaling,[21][22][23] resistance to proteolytic attack,[24][25] and represents the first committed step in mucin biosynthesis.[15]

Role in disease[edit]

Human GALE deficiency or dysfunction results in Type III galactosemia, which may exist in a mild (peripheral) or more severe (generalized) form.[12]


  1. ^ a b c d e f g h i j k Holden HM, Rayment I, Thoden JB (November 2003). "Structure and function of enzymes of the Leloir pathway for galactose metabolism". J. Biol. Chem. 278 (45): 43885–8. doi:10.1074/jbc.R300025200. PMID 12923184.
  2. ^ a b Liu Y, Vanhooke JL, Frey PA (June 1996). "UDP-galactose 4-epimerase: NAD+ content and a charge-transfer band associated with the substrate-induced conformational transition". Biochemistry. 35 (23): 7615–20. doi:10.1021/bi960102v. PMID 8652544.
  3. ^ a b c d Thoden JB, Wohlers TM, Fridovich-Keil JL, Holden HM (May 2001). "Human UDP-galactose 4-epimerase. Accommodation of UDP-N-acetylglucosamine within the active site". J. Biol. Chem. 276 (18): 15131–6. doi:10.1074/jbc.M100220200. PMID 11279032.
  4. ^ LELOIR LF (September 1951). "The enzymatic transformation of uridine diphosphate glucose into a galactose derivative". Arch Biochem. 33 (2): 186–90. doi:10.1016/0003-9861(51)90096-3. hdl:11336/140700. PMID 14885999.
  5. ^ "The Nobel Prize in Chemistry 1970" (Press release). The Royal Swedish Academy of Science. 1970. Retrieved 2010-05-17.
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  7. ^ a b PDB: 1EK5​; Thoden JB, Wohlers TM, Fridovich-Keil JL, Holden HM (May 2000). "Crystallographic evidence for Tyr 157 functioning as the active site base in human UDP-galactose 4-epimerase". Biochemistry. 39 (19): 5691–701. doi:10.1021/bi000215l. PMID 10801319.
  8. ^ a b c PDB: 1XEL​; Thoden JB, Frey PA, Holden HM (April 1996). "Molecular structure of the NADH/UDP-glucose abortive complex of UDP-galactose 4-epimerase from Escherichia coli: implications for the catalytic mechanism". Biochemistry. 35 (16): 5137–44. doi:10.1021/bi9601114. PMID 8611497.
  9. ^ PDB: 1A9Z​; Thoden JB, Holden HM (August 1998). "Dramatic differences in the binding of UDP-galactose and UDP-glucose to UDP-galactose 4-epimerase from Escherichia coli". Biochemistry. 37 (33): 11469–77. doi:10.1021/bi9808969. PMID 9708982.
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