UDP-glucose 4-epimerase

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]

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 number5.1.3.2
CAS number9032-89-7
Databases
IntEnzIntEnz view
BRENDABRENDA entry
ExPASyNiceZyme view
KEGGKEGG entry
MetaCycmetabolic pathway
PRIAMprofile
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO
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 gene2582
HGNC4116
OMIM606953
RefSeqNM_000403
UniProtQ14376
Other data
EC number5.1.3.2
LocusChr. 1 p36-p35
NAD-dependent epimerase/dehydratase
Identifiers
Symbol?
PfamPF01370
InterProIPR001509
Membranome330

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 218Anchor uracil ring to enzyme.
Asp 295Interacts with ribose 2' hydroxyl group.
Asn 179, Arg 231, Arg 292Interact with UDP phosphate groups.
Tyr 299, Asn 179Interact with galactose 2' hydroxyl or glucose 6' hydroxyl group; properly position sugar within active site.
Tyr 177, Phe 178Interact with galactose 3' hydroxyl or glucose 6' hydroxyl group; properly position sugar within active site.
Lys 153Lowers pKa of Tyr 149, allows for abstraction or donation of a hydrogen atom to or from the sugar 4' hydroxyl group.
Tyr 149Abstracts 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
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]

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References
  1. 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. 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. 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. PMID 14885999.
  5. "The Nobel Prize in Chemistry 1970" (Press release). The Royal Swedish Academy of Science. 1970. Retrieved 2010-05-17.
  6. 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. 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. 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. 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.
  12. 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.
  15. 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.
  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.
  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.
  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
  • Leloir LF (1953). Enzymic isomerization and related processes. Adv. Enzymol. Relat. Subj. Biochem. Advances in Enzymology - and Related Areas of Molecular Biology. 14. pp. 193–218. doi:10.1002/9780470122594.ch6. ISBN 9780470122594. PMID 13057717.
  • Maxwell ES, de Robichon-Szulmajster H (1960). "Purification of uridine diphosphate galactose-4-epimerase from yeast and the identification of protein-bound diphosphopyridine nucleotide". J. Biol. Chem. 235: 308–312.
  • Wilson DB, Hogness DS (August 1964). "The enzymes of the galactose operon in Escherichia coli. I Purification and characterization of uridine diphosphogalactose 4-epimerase". J. Biol. Chem. 239: 2469–81. PMID 14235524.
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