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:||Biosynthesis → Carbohydrates Biosynthesis → Sugars Biosynthesis → Sugar Nucleotides Biosynthesis → UDP-sugar Biosynthesis|
|Degradation/Utilization/Assimilation → Carbohydrates Degradation → Sugars Degradation → Galactose Degradation|
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 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).
Unification Links: AraCyc:PWY-3821
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
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.
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
Barber06a: 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
Burke93a: 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
Dormann96: Dormann P, Benning C (1996). "Functional expression of uridine 5'-diphospho-glucose 4-epimerase (EC 18.104.22.168) from Arabidopsis thaliana in Saccharomyces cerevisiae and Escherichia coli." Arch Biochem Biophys 327(1);27-34. PMID: 8615692
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
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
Kang75: Kang UG, Nolan LD, Frey PA (1975). "Uridine diphosphate galactose-4-epimerase. Uridine monophosphate-dependent reduction by alpha- and beta-D-glucose." J Biol Chem 250(18);7099-105. PMID: 1100620
Ketley73: Ketley JN, Schellenberg KA (1973). "Substrate stereochemical requirements in the reductive inactivation of uridine diphosphate galactose 4-epimerase by sugar and 5'-uridine monophosphate." Biochemistry 12(2);315-20. PMID: 4345584
Kim11f: Kim HJ, Kang SY, Park JJ, Kim P (2011). "Novel activity of UDP-galactose-4-epimerase for free monosaccharide and activity improvement by active site-saturation mutagenesis." Appl Biochem Biotechnol 163(3);444-51. PMID: 20717852
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