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.
|Superclasses:||Biosynthesis → Carbohydrates Biosynthesis → Sugars Biosynthesis → Sugar Nucleotides Biosynthesis → UDP-sugar Biosynthesis|
L-Rhamnose is a component of the plant cell wall pectic polysaccharides rhamnogalacturonan I and rhamnogalacturonan II and is also present in diverse secondary metabolites including anthocyanins, flavonoids and triterpenoids. Rhamnosyl transferases have been shown to utilize UDP-β-L-rhamnose as the donor substrate [BarPeled91, Watt04], although in mung bean (Vigna radiata), both dTDP-β-L-Rhamnose and UDP-β-L-rhamnose were reported to act as sugar donors for the rhamnosylation of flavonoids [Barber61]. In general UDP-D-glucose is used in plants rather than dTDP-D-glucose as a precursor in the de novo synthesis of L-rhamnose.
L-Rhamnose residues have also been reported to be present in fungal glycoproteins, exopolysaccharides, and the cell wall, although their physiological role remains to be fully elucidated [Martinez12, Pettolino09, Molinaro02]. Genes encoding a UDP-glucose 4,6-dehydratase, and a bifunctional UDP-L-rhamnose synthase with 3,5-epimerase/-4-reductase activities have been identified in the fungal plant pathogens Magnaporthe oryzae (referred to in [Martinez12] as "Magnaporthe grisea (Magnaporthe oryzae)") and Botrytis cinerea [Martinez12].
About This Pathway
Although the biosynthesis of UDP-L-rhamnose from UDP-D-glucose in Arabidopsis is thought to occur in one step through trifunctional enzymes (see reaction UDP-α-D-glucose + NADPH + H+ = UDP-β-L-rhamnose + NADP+ + H2O) [Reiter01a], a second route has been postulated which presumes two enzymatic steps: first the conversion of UDP-D-glucose into UDP-4-keto-6-deoxy-L-mannose via an unidentified UDP-D-glucose-4,6-dehydratase followed by a final conversion to UDP-L-rhamnose via NRS/ER, an enzyme which has been shown to be able to catalyse in vitro the conversion of UDP-4-keto-6-deoxy-L-mannose into UDP-L-rhamnose [Watt04]. To date RHM1 (a paralog of MUM4 which has been postulated to have the trifunctional activity mention hereabove) has been shown to possess a dehydratase domain that can catalyze the conversion of UDP-D-glucose into the reaction intermediate UDP-4-dehydro-6-deoxy-D-glucose; the enzyme could not however be fully expressed in Escherichia coli [Diet06].
Superpathways: UDP-sugars interconversion
Unification Links: AraCyc:PWY-3261
BarPeled91: Bar-Peled M, Lewinsohn E, Fluhr R, Gressel J (1991). "UDP-rhamnose:flavanone-7-O-glucoside-2''-O-rhamnosyltransferase. Purification and characterization of an enzyme catalyzing the production of bitter compounds in citrus." J Biol Chem 266(31);20953-9. PMID: 1939145
Diet06: Diet A, Link B, Seifert GJ, Schellenberg B, Wagner U, Pauly M, Reiter WD, Ringli C (2006). "The Arabidopsis root hair cell wall formation mutant lrx1 is suppressed by mutations in the RHM1 gene encoding a UDP-L-rhamnose synthase." Plant Cell 18(7);1630-41. PMID: 16766693
Martinez12: Martinez V, Ingwers M, Smith J, Glushka J, Yang T, Bar-Peled M (2012). "Biosynthesis of UDP-4-keto-6-deoxyglucose and UDP-rhamnose in pathogenic fungi Magnaporthe grisea and Botryotinia fuckeliana." J Biol Chem 287(2);879-92. PMID: 22102281
Molinaro02: Molinaro A, Piscopo V, Lanzetta R, Parrilli M (2002). "Structural determination of the complex exopolysaccharide from the virulent strain of Cryphonectria parasitica." Carbohydr Res 337(19);1707-13. PMID: 12423948
Pettolino09: Pettolino F, Sasaki I, Turbic A, Wilson SM, Bacic A, Hrmova M, Fincher GB (2009). "Hyphal cell walls from the plant pathogen Rhynchosporium secalis contain (1,3/1,6)-beta-D-glucans, galacto- and rhamnomannans, (1,3;1,4)-beta-D-glucans and chitin." FEBS J 276(14);3698-709. PMID: 19496815
Usadel04: Usadel B, Kuschinsky AM, Rosso MG, Eckermann N, Pauly M (2004). "RHM2 is involved in mucilage pectin synthesis and is required for the development of the seed coat in Arabidopsis." Plant Physiol 134(1);286-95. PMID: 14671019
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