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MetaCyc Pathway: D-galactose degradation III
Inferred from experiment

Enzyme View:

Pathway diagram: D-galactose degradation III

This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.

Synonyms: UDP-D-galatose biosynthesis (from D-galactose)

Superclasses: BiosynthesisCarbohydrates BiosynthesisSugars BiosynthesisSugar Nucleotides BiosynthesisUDP-sugar Biosynthesis
Degradation/Utilization/AssimilationCarbohydrates DegradationSugars DegradationGalactose Degradation

Some taxa known to possess this pathway include : Arabidopsis thaliana col, Cucumis melo, Cucumis sativus, Glycine max, Oryza sativa, Vicia faba

Expected Taxonomic Range: Embryophyta

Free galactose and its derivatives are toxic to plants even at low concentrations [Ordin57]. This has been shown to be due to the inhibition of glucose incorporation into cellulose, which in turn results in an inhibition of elongation of growth.

The molecular basis of galactose toxicity is unclear. Feeding inhibitory concentrations of external galactose to intact roots is linked to an increase in galactose-1-phosphate but a decrease of UDP-galactose. It was shown that galactose-1-phosphate can act as a competitive inhibitor of UTP:glucose-1-phosphate uridyl transferase thereby inhibiting the formation of UDP-glucose, a precursor of cellulose biosynthesis (note that it was subsequently argued that the concentration of galactose-1-phosphate used in the experiment is unlikely to be physiological [Kaplan97].

Other studies using galactose-resistant cell lines proposed that UDP-galactose is the main toxic metabolite (resistant lines were characterized by increased levels of UDP-galactose-4-epimerase) [Maretzki78]. Irrespective of the exact mode of toxicity, plants must exercise a tight control over the levels of free galactose and its phosphorylated derivatives. Although galactose degradation is often understood to occur via the transfer of the UDP moiety of UDP-glucose to galactose-1-phosphate to form UDP-galactose (Leloir pathway, see D-galactose degradation I (Leloir pathway)), the degradation of galactose has also been shown to occur in plants via the conversion of galactose-1-phosphate to UDP-galactose by an uridyltransferase, using UTP as uridine diphosphate source. The uridylation of galactose-1-phosphate is followed by the epimerization of UDP-galactose into UDP-glucose which is thought to play a central role as 'branch point' metabolite (formation of glucose-1-phosphate for energy needs, synthesis of transportable sucrose, or cell-wall synthesis in developing seedlings).

Variants: D-galactose degradation I (Leloir pathway), D-galactose degradation II, D-galactose degradation IV, D-galactose degradation V (Leloir pathway), L-galactose degradation, lactose and galactose degradation I

Unification Links: AraCyc:PWY-3821

Revised 15-Aug-2013 by Caspi R, SRI International


Kaplan97: Kaplan CP, Tugal HB, Baker A (1997). "Isolation of a cDNA encoding an Arabidopsis galactokinase by functional expression in yeast." Plant Mol Biol 34(3);497-506. PMID: 9225860

Maretzki78: Maretzki A. "Characteristics of a galactose adapted sugar cane cell line grown in suspension culture." Plant Physiol. (1978) 61:544-548.

Ordin57: Ordin L., Bonner J. "Effect of galactose on growth and metabolism of Avena coleoptile sections." Plant Physiol. (1957) 32:212-215.

Seifert02: Seifert GJ, Barber C, Wells B, Dolan L, Roberts K (2002). "Galactose biosynthesis in Arabidopsis: genetic evidence for substrate channeling from UDP-D-galactose into cell wall polymers." Curr Biol 12(21);1840-5. PMID: 12419184

Seifert04: Seifert GJ (2004). "Nucleotide sugar interconversions and cell wall biosynthesis: how to bring the inside to the outside." Curr Opin Plant Biol 7(3);277-84. PMID: 15134748

Studer99: Studer Feusi M.E., Burton J.D., Williamson J.D., Pharr D.M. "Galactosyl-sucrose metabolism and UDP-galactose pyrophosphorylase from Cucumis melo L. fruit." Physiologia Plantarum (1999) 106:9-16.

Other References Related to Enzymes, Genes, Subpathways, and Substrates of this Pathway

Arabshahi88: Arabshahi A, Flentke GR, Frey PA (1988). "Uridine diphosphate galactose 4-epimerase. pH dependence of the reduction of NAD+ by a substrate analog." J Biol Chem 263(6);2638-43. PMID: 3277958

Barber06: Barber C, Rosti J, Rawat A, Findlay K, Roberts K, Seifert GJ (2006). "Distinct properties of the five UDP-D-glucose/UDP-D-galactose 4-epimerase isoforms of arabidopsis thaliana." J Biol Chem. PMID: 16644739

Berger01: Berger E, Arabshahi A, Wei Y, Schilling JF, Frey PA (2001). "Acid-base catalysis by UDP-galactose 4-epimerase: correlations of kinetically measured acid dissociation constants with thermodynamic values for tyrosine 149." Biochemistry 40(22);6699-705. PMID: 11380265

Blackburn76: Blackburn P, Ferdinand W (1976). "The concerted inactivation of Escherichia coli uridine diphosphate galactose 4-epimerase by sugar nucleotide together with a free sugar." Biochem J 155(2);225-9. PMID: 779771

BRENDA14: BRENDA team (2014). Imported from BRENDA version existing on Aug 2014.

Burke93: Burke JR, Frey PA (1993). "The importance of binding energy in catalysis of hydride transfer by UDP-galactose 4-epimerase: a 13C and 15N NMR and kinetic study." Biochemistry 1993;32(48);13220-30. PMID: 8241177

Dai06: Dai N, Petreikov M, Portnoy V, Katzir N, Pharr DM, Schaffer AA (2006). "Cloning and expression analysis of a UDP-galactose/glucose pyrophosphorylase from melon fruit provides evidence for the major metabolic pathway of galactose metabolism in raffinose oligosaccharide metabolizing plants." Plant Physiol 142(1);294-304. PMID: 16829585

Dalrymple13: Dalrymple SA, Ko J, Sheoran I, Kaminskyj SG, Sanders DA (2013). "Elucidation of substrate specificity in Aspergillus nidulans UDP-galactose-4-epimerase." PLoS One 8(10);e76803. PMID: 24116166

Damerow10: Damerow S, Lamerz AC, Haselhorst T, Fuhring J, Zarnovican P, von Itzstein M, Routier FH (2010). "Leishmania UDP-sugar pyrophosphorylase: the missing link in galactose salvage?." J Biol Chem 285(2);878-87. PMID: 19906649

Dey83: Dey PM (1983). "Galactokinase of Vicia faba seeds." Eur J Biochem 136(1);155-9. PMID: 6617655

Dey84: Dey P.M. (1984). "UDP-galactose 4-epimerase from Vicia faba seeds." Phytochemistry 23:729-732.

Dormann96: Dormann P, Benning C (1996). "Functional expression of uridine 5'-diphospho-glucose 4-epimerase (EC from Arabidopsis thaliana in Saccharomyces cerevisiae and Escherichia coli." Arch Biochem Biophys 327(1);27-34. PMID: 8615692

Dutta97: Dutta S, Maiti NR, Bhattacharyya D (1997). "Reversible folding of UDP-galactose 4-epimerase from Escherichia coli." Eur J Biochem 244(2);407-13. PMID: 9119006

ElGaniny10: El-Ganiny AM, Sheoran I, Sanders DA, Kaminskyj SG (2010). "Aspergillus nidulans UDP-glucose-4-epimerase UgeA has multiple roles in wall architecture, hyphal morphogenesis, and asexual development." Fungal Genet Biol 47(7);629-35. PMID: 20211750

Estrela91: Estrela AI, Pooley HM, de Lencastre H, Karamata D (1991). "Genetic and biochemical characterization of Bacillus subtilis 168 mutants specifically blocked in the synthesis of the teichoic acid poly(3-O-beta-D-glucopyranosyl-N-acetylgalactosamine 1-phosphate): gneA, a new locus, is associated with UDP-N-acetylglucosamine 4-epimerase activity." J Gen Microbiol 1991;137 ( Pt 4);943-50. PMID: 1906927

Flentke90: Flentke GR, Frey PA (1990). "Reaction of uridine diphosphate galactose 4-epimerase with a suicide inactivator." Biochemistry 1990;29(9);2430-6. PMID: 2186813

Gulbinsky68: Gulbinsky JS, Cleland WW (1968). "Kinetic studies of Escherichia coli galactokinase." Biochemistry 7(2);566-75. PMID: 4868542

Guo06: Guo H, Li L, Wang PG (2006). "Biochemical characterization of UDP-GlcNAc/Glc 4-epimerase from Escherichia coli O86:B7." Biochemistry 45(46);13760-8. PMID: 17105195

Hoffmeister03: Hoffmeister D, Yang J, Liu L, Thorson JS (2003). "Creation of the first anomeric D/L-sugar kinase by means of directed evolution." Proc Natl Acad Sci U S A 100(23);13184-9. PMID: 14612558

Hoffmeister04: Hoffmeister D, Thorson JS (2004). "Mechanistic implications of Escherichia coli galactokinase structure-based engineering." Chembiochem 5(7);989-92. PMID: 15239057

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Please cite the following article in publications resulting from the use of MetaCyc: Caspi et al, Nucleic Acids Research 42:D459-D471 2014
Page generated by Pathway Tools version 19.5 (software by SRI International) on Fri Apr 29, 2016, biocyc14.