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UDP-glucose 4-epimerase
Identifiers
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 IDs GeneCards: [1]; OMA: - orthologs
Orthologs
SpeciesHumanMouse
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UniProt

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RefSeq (mRNA)

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RefSeq (protein)

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Location (UCSC)n/an/a
PubMed searchn/an/a
Wikidata
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.
Identifiers
EC no. 5.1.3.2
CAS no. 9032-89-7
Databases
IntEnz IntEnz view
BRENDA BRENDA entry
ExPASy NiceZyme view
KEGG KEGG entry
MetaCyc metabolic pathway
PRIAM profile
PDB structures RCSB PDB PDBe PDBsum
Gene Ontology AmiGO / QuickGO
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PMC articles
PubMed articles
NCBI proteins
UDP-galactose-4-epimerase
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.
Identifiers
SymbolGALE
NCBI gene 2582
HGNC 4116
OMIM 606953
RefSeq NM_000403
UniProt Q14376
Other data
EC number 5.1.3.2
Locus Chr. 1 p36-p35
Search for
Structures Swiss-model
Domains InterPro
NAD-dependent epimerase/dehydratase
Identifiers
Symbol?
Pfam PF01370
InterPro IPR001509
Membranome 330
Available protein structures:
Pfam   structures / ECOD  
PDB RCSB PDB; PDBe; PDBj
PDBsum structure summary

The enzyme UDP-glucose 4-epimerase ( EC 5.1.3.2), 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

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 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

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. ^ 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.
  6. ^ a b c d Kavanagh KL, Jörnvall H, Persson B, Oppermann U (December 2008). "Medium- and short-chain dehydrogenase/reductase gene and protein families : the SDR superfamily: functional and structural diversity within a family of metabolic and regulatory enzymes". Cell. Mol. Life Sci. 65 (24): 3895–906. doi: 10.1007/s00018-008-8588-y. PMC  2792337. PMID  19011750.
  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.
  10. ^ a b Liu Y, Thoden JB, Kim J, Berger E, Gulick AM, Ruzicka FJ, Holden HM, Frey PA (September 1997). "Mechanistic roles of tyrosine 149 and serine 124 in UDP-galactose 4-epimerase from Escherichia coli". Biochemistry. 36 (35): 10675–84. doi: 10.1021/bi970430a. PMID  9271498.
  11. ^ Kingsley DM, Kozarsky KF, Hobbie L, Krieger M (March 1986). "Reversible defects in O-linked glycosylation and LDL receptor expression in a UDP-Gal/UDP-GalNAc 4-epimerase deficient mutant". Cell. 44 (5): 749–59. doi: 10.1016/0092-8674(86)90841-X. PMID  3948246. S2CID  28293937.
  12. ^ a b Lai K, Elsas LJ, Wierenga KJ (November 2009). "Galactose toxicity in animals". IUBMB Life. 61 (11): 1063–74. doi: 10.1002/iub.262. PMC  2788023. PMID  19859980.
  13. ^ Stryer, Lubert; Berg, Jeremy Mark; Tymoczko, John L. (2008). Biochemistry (Looseleaf). San Francisco: W. H. Freeman. pp.  443–58. ISBN  9780716718437.
  14. ^ Michell RH (February 2008). "Inositol derivatives: evolution and functions". Nat. Rev. Mol. Cell Biol. 9 (2): 151–61. doi: 10.1038/nrm2334. PMID  18216771. S2CID  3245927.
  15. ^ a b Ten Hagen KG, Fritz TA, Tabak LA (January 2003). "All in the family: the UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases". Glycobiology. 13 (1): 1R–16R. doi: 10.1093/glycob/cwg007. PMID  12634319.
  16. ^ Alfalah M, Jacob R, Preuss U, Zimmer KP, Naim H, Naim HY (June 1999). "O-linked glycans mediate apical sorting of human intestinal sucrase-isomaltase through association with lipid rafts". Curr. Biol. 9 (11): 593–6. doi: 10.1016/S0960-9822(99)80263-2. PMID  10359703. S2CID  16866875.
  17. ^ Altschuler Y, Kinlough CL, Poland PA, Bruns JB, Apodaca G, Weisz OA, Hughey RP (March 2000). "Clathrin-mediated endocytosis of MUC1 is modulated by its glycosylation state". Mol. Biol. Cell. 11 (3): 819–31. doi: 10.1091/mbc.11.3.819. PMC  14813. PMID  10712502.
  18. ^ Breuza L, Garcia M, Delgrossi MH, Le Bivic A (February 2002). "Role of the membrane-proximal O-glycosylation site in sorting of the human receptor for neurotrophins to the apical membrane of MDCK cells". Exp. Cell Res. 273 (2): 178–86. doi: 10.1006/excr.2001.5442. PMID  11822873.
  19. ^ Naim HY, Joberty G, Alfalah M, Jacob R (June 1999). "Temporal association of the N- and O-linked glycosylation events and their implication in the polarized sorting of intestinal brush border sucrase-isomaltase, aminopeptidase N, and dipeptidyl peptidase IV". J. Biol. Chem. 274 (25): 17961–7. doi: 10.1074/jbc.274.25.17961. PMID  10364244.
  20. ^ Zheng X, Sadler JE (March 2002). "Mucin-like domain of enteropeptidase directs apical targeting in Madin-Darby canine kidney cells". J. Biol. Chem. 277 (9): 6858–63. doi: 10.1074/jbc.M109857200. PMID  11878264.
  21. ^ Hooper LV, Gordon JI (February 2001). "Glycans as legislators of host-microbial interactions: spanning the spectrum from symbiosis to pathogenicity". Glycobiology. 11 (2): 1R–10R. doi: 10.1093/glycob/11.2.1R. PMID  11287395.
  22. ^ Yeh JC, Hiraoka N, Petryniak B, Nakayama J, Ellies LG, Rabuka D, Hindsgaul O, Marth JD, Lowe JB, Fukuda M (June 2001). "Novel sulfated lymphocyte homing receptors and their control by a Core1 extension beta 1,3-N-acetylglucosaminyltransferase". Cell. 105 (7): 957–69. doi: 10.1016/S0092-8674(01)00394-4. PMID  11439191. S2CID  18674112.
  23. ^ Somers WS, Tang J, Shaw GD, Camphausen RT (October 2000). "Insights into the molecular basis of leukocyte tethering and rolling revealed by structures of P- and E-selectin bound to SLe(X) and PSGL-1". Cell. 103 (3): 467–79. doi: 10.1016/S0092-8674(00)00138-0. PMID  11081633. S2CID  12719907.
  24. ^ Sauer J, Sigurskjold BW, Christensen U, Frandsen TP, Mirgorodskaya E, Harrison M, Roepstorff P, Svensson B (December 2000). "Glucoamylase: structure/function relationships, and protein engineering". Biochim. Biophys. Acta. 1543 (2): 275–293. doi: 10.1016/s0167-4838(00)00232-6. PMID  11150611.
  25. ^ Garner B, Merry AH, Royle L, Harvey DJ, Rudd PM, Thillet J (June 2001). "Structural elucidation of the N- and O-glycans of human apolipoprotein(a): role of o-glycans in conferring protease resistance". J. Biol. Chem. 276 (25): 22200–8. doi: 10.1074/jbc.M102150200. PMID  11294842.

Further reading

External links

  1. ^ Beebe, Jane A.; Frey, Perry A. (1998-10-01). "Galactose Mutarotase: Purification, Characterization, and Investigations of Two Important Histidine Residues". Biochemistry. 37 (42): 14989–14997. doi: 10.1021/bi9816047. ISSN  0006-2960.
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