UDP-glucose 4-epimerase

Source: Wikipedia, the free encyclopedia.
"GALE" redirects here. For the DARPA program, see
Global Autonomous Language Exploitation
.
UDP-glucose 4-epimerase
Identifiers
Ensembl

n/a

n/a

UniProt

n
a

n/a

RefSeq (mRNA)

n/a

n/a

RefSeq (protein)

n/a

n/a

Location (UCSC)n/an/a
PubMed searchn/an/a
Wikidata
View/Edit Human
UDP-glucose 4-epimerase
ExPASy
NiceZyme view
KEGGKEGG entry
MetaCycmetabolic pathway
PRIAMprofile
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO
Search
PMCarticles
PubMedarticles
NCBIproteins
UDP-galactose-4-epimerase
Chr. 1 p36-p35
Search for
StructuresSwiss-model
DomainsInterPro
NAD-dependent epimerase/dehydratase
Identifiers
Symbol?
PfamPF01370
InterProIPR001509
Membranome330
Available protein structures:
Pfam  structures / ECOD  
PDBRCSB PDB; PDBe; PDBj
PDBsumstructure summary

The

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

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]

Structure

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

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

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.

Mechanism

Conversion of UDP-galactose to UDP-glucose

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

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

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

Galactose metabolism

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

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

inositol-1-phosphate by inositol-3-phosphate synthase, generating a precursor needed for inositol biosynthesis.[14]

UDP-GalNAc synthesis

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

Main article: Galactose epimerase deficiency

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

References

  1. ^
    PMID 12923184
    .
  2. ^
    PMID 8652544
    .
  3. ^
    PMID 11279032
    .
  4. PMID 14885999
    .
  5. ^ "The Nobel Prize in Chemistry 1970" (Press release). The Royal Swedish Academy of Science. 1970. Retrieved 2010-05-17.
  6. ^
    PMID 19011750
    .
  7. ^
    PMID 10801319
    .
  8. ^
    PMID 8611497
    .
  9. PMID 9708982
    .
  10. ^
    PMID 9271498
    .
  11. S2CID 28293937
    .
  12. ^
    PMID 19859980
    .
  13. ISBN 9780716718437
    .
  14. S2CID 3245927
    .
  15. ^
    PMID 12634319
    .
  16. S2CID 16866875
    .
  17. PMID 10712502
    .
  18. PMID 11822873
    .
  19. PMID 10364244
    .
  20. PMID 11878264
    .
  21. PMID 11287395
    .
  22. S2CID 18674112
    .
  23. S2CID 12719907
    .
  24. PMID 11150611
    .
  25. PMID 11294842
    .

Further reading

External links
Retrieved from "https://en.wikipedia.org/w/index.php?title=UDP-glucose_4-epimerase&oldid=1172364639"