词条 | UDP-glucose 4-epimerase | ||||||||||||||||
释义 |
| Name = UDP-glucose 4-epimerase | EC_number = 5.1.3.2 | CAS_number = 9032-89-7 | IUBMB_EC_number = 5/1/3/2 | GO_code = 0003978 | image = Human GALE bound to NADH and UDP-glucose.png | width = | caption = H. sapiens UDP-glucose 4-epimerase homodimer bound to NADH and UDP-glucose. Domains: N-terminal and C-terminal. }}{{infobox protein |Name=UDP-galactose-4-epimerase |caption=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. |image=Human GALE bound to NAD+ and UDP-GlcNAc.png |width= |HGNCid=4116 |Symbol=GALE |AltSymbols= |EntrezGene=2582 |OMIM=606953 |RefSeq=NM_000403 |UniProt=Q14376 |PDB= |ECnumber=5.1.3.2 |Chromosome=1 |Arm=p |Band=36 |LocusSupplementaryData=-p35 }}{{Pfam_box | Symbol = | Name = NAD-dependent epimerase/dehydratase | image = | width = | caption = | Pfam= PF01370 | InterPro= IPR001509 | SMART= | Prosite = | SCOP = | TCDB = | OPM family= | OPM protein= | PDB= |Membranome superfamily =330 }} The enzyme UDP-glucose 4-epimerase ({{EC number|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 significanceDr. 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] StructureGALE 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 structureGALE 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 [https://www.ncbi.nlm.nih.gov/protein/ZP_05580029.1 (Enterococcus faecalis)] to 564 amino acids [https://www.ncbi.nlm.nih.gov/protein/YP_701629.1 (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 siteThe 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]
MechanismConversion of UDP-galactose to UDP-glucoseGALE 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-GalNAcHuman 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 functionGalactose metabolismNo 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 synthesisHuman 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|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] References1. ^1 2 3 4 5 6 7 8 9 10 {{cite journal |vauthors=Holden HM, Rayment I, Thoden JB | title = Structure and function of enzymes of the Leloir pathway for galactose metabolism | journal = J. Biol. Chem. | volume = 278 | issue = 45 | pages = 43885–8 |date=November 2003 | pmid = 12923184 | doi = 10.1074/jbc.R300025200 | url = | issn = }} 2. ^1 {{cite journal |vauthors=Liu Y, Vanhooke JL, Frey PA | title = UDP-galactose 4-epimerase: NAD+ content and a charge-transfer band associated with the substrate-induced conformational transition | journal = Biochemistry | volume = 35 | issue = 23 | pages = 7615–20 |date=June 1996 | pmid = 8652544 | doi = 10.1021/bi960102v | url = | issn = }} 3. ^1 2 3 {{cite journal |vauthors=Thoden JB, Wohlers TM, Fridovich-Keil JL, Holden HM | title = Human UDP-galactose 4-epimerase. Accommodation of UDP-N-acetylglucosamine within the active site | journal = J. Biol. Chem. | volume = 276 | issue = 18 | pages = 15131–6 |date=May 2001 | pmid = 11279032 | doi = 10.1074/jbc.M100220200 | url = | issn = }} 4. ^{{cite journal | author = LELOIR LF | title = The enzymatic transformation of uridine diphosphate glucose into a galactose derivative | journal = Arch Biochem | volume = 33 | issue = 2 | pages = 186–90 |date=September 1951 | pmid = 14885999 | doi = 10.1016/0003-9861(51)90096-3| url = | issn = }} 5. ^{{cite press release | url = http://nobelprize.org/nobel_prizes/chemistry/laureates/1970/press.html | title=The Nobel Prize in Chemistry 1970 | publisher = The Royal Swedish Academy of Science | year = 1970 | accessdate = 2010-05-17}} 6. ^1 2 3 {{cite journal |vauthors=Kavanagh KL, Jörnvall H, Persson B, Oppermann U | title = 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 | journal = Cell. Mol. Life Sci. | volume = 65 | issue = 24 | pages = 3895–906 |date=December 2008 | pmid = 19011750 | pmc = 2792337 | doi = 10.1007/s00018-008-8588-y | url = | issn = }} 7. ^1 {{PDB|1EK5}}; {{cite journal |vauthors=Thoden JB, Wohlers TM, Fridovich-Keil JL, Holden HM | title = Crystallographic evidence for Tyr 157 functioning as the active site base in human UDP-galactose 4-epimerase | journal = Biochemistry | volume = 39 | issue = 19 | pages = 5691–701 |date=May 2000 | pmid = 10801319 | doi = 10.1021/bi000215l| url = | issn = }} 8. ^1 2 {{PDB|1XEL}}; {{cite journal |vauthors=Thoden JB, Frey PA, Holden HM | title = Molecular structure of the NADH/UDP-glucose abortive complex of UDP-galactose 4-epimerase from Escherichia coli: implications for the catalytic mechanism | journal = Biochemistry | volume = 35 | issue = 16 | pages = 5137–44 |date=April 1996 | pmid = 8611497 | doi = 10.1021/bi9601114 | url = | issn = }} 9. ^{{PDB|1A9Z}}; {{cite journal |vauthors=Thoden JB, Holden HM | title = Dramatic differences in the binding of UDP-galactose and UDP-glucose to UDP-galactose 4-epimerase from Escherichia coli | journal = Biochemistry | volume = 37 | issue = 33 | pages = 11469–77 |date=August 1998 | pmid = 9708982 | doi = 10.1021/bi9808969 | url = | issn = }} 10. ^1 {{cite journal |vauthors=Liu Y, Thoden JB, Kim J, Berger E, Gulick AM, Ruzicka FJ, Holden HM, Frey PA | title = Mechanistic roles of tyrosine 149 and serine 124 in UDP-galactose 4-epimerase from Escherichia coli | journal = Biochemistry | volume = 36 | issue = 35 | pages = 10675–84 |date=September 1997 | pmid = 9271498 | doi = 10.1021/bi970430a | url = | issn = }} 11. ^{{cite journal |vauthors=Kingsley DM, Kozarsky KF, Hobbie L, Krieger M | title = Reversible defects in O-linked glycosylation and LDL receptor expression in a UDP-Gal/UDP-GalNAc 4-epimerase deficient mutant | journal = Cell | volume = 44 | issue = 5 | pages = 749–59 |date=March 1986 | pmid = 3948246 | doi = 10.1016/0092-8674(86)90841-X| url = | issn = }} 12. ^1 {{cite journal |vauthors=Lai K, Elsas LJ, Wierenga KJ | title = Galactose toxicity in animals | journal = IUBMB Life | volume = 61 | issue = 11 | pages = 1063–74 |date=November 2009 | pmid = 19859980 | pmc = 2788023 | doi = 10.1002/iub.262 | url = | issn = }} 13. ^{{cite book |author1=Stryer, Lubert |author2=Berg, Jeremy Mark |author3=Tymoczko, John L. | title = Biochemistry (Looseleaf) | edition = | language = | publisher = W. 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Biophys. Acta | volume = 1543 | issue = 2 | pages = 275–293 |date=December 2000 | pmid = 11150611 | doi = 10.1016/s0167-4838(00)00232-6| url = | issn = }} 25. ^{{cite journal |vauthors=Garner B, Merry AH, Royle L, Harvey DJ, Rudd PM, Thillet J | title = Structural elucidation of the N- and O-glycans of human apolipoprotein(a): role of o-glycans in conferring protease resistance | journal = J. Biol. Chem. | volume = 276 | issue = 25 | pages = 22200–8 |date=June 2001 | pmid = 11294842 | doi = 10.1074/jbc.M102150200 | url = | issn = }} Further reading{{refbegin}}
External links
3 : EC 5.1.3|Enzymes of known structure|NADH-dependent enzymes |
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