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L-Arabinose, the 4-epimer of D-xylose, is a monosaccharide found in the plant?s cell wall. The biosynthesis of UDP-L-arabinose occurs via two routes. The de novo synthesis of UDP-L-arabinose involves the epimerization of UDP-D-xylose by a UDP-D-xylose 4-epimerase. The reverse reaction (UDP-L-arabinose --> UDP-D-xylose) is also catalyzed by the same enzyme (Fan & Feingold 1970). In Arabidopsis, the orthologous MUR4 protein (At1g30620) is shown to be localized in the Golgi membranes (Burget et al. 2003). The alternative salvage pathway recycles free arabinose (a byproduct of cell wall degradation) into UDP-L-arabinose in two steps: first, L-arabinokinase phosphorylates ?-L-arabinopyranose, and then ?-L-arabinose-1-phosphate is converted to UDP-L-arabinose by a UDP-sugar pyrophosphorylase. In the plant cell wall, the majority of arabinose is found in a furanose (not pyranose) configuration. UDP-L-arabinose mutase (UAM) catalyzes the interconversion of UDP-L-arabinopyranose (UDP-Arap) and UDP-L-arabinofuranose (UDP-Araf) (Saqib et al. 2019). At thermodynamic equilibrium, in vitro, the ratio of the pyranose form over the furanose form is 90:10. The ratio of the pyranose to furanose forms reached a plateau at 93:7 from both pyranose- and furanose-forming directions. Native rice mutase is believed to exist as a complex composed of several different UAM monomers localized on the Golgi membranes (Konishi et al. 2010). UDP-Araf is transported from the cytosol into the Golgi lumen by UDP-Araf transporters localized in the Golgi membrane (Rautengarten et al. 2017), where it is utilized by glycosyltransferases to glycosylate various entities (i.e. proteins and signaling peptides) for subsequent cell wall biosynthesis. (This pathway replaces the arabinose biosynthesis I and II and contains additional reactions. Shayla Rao, an undergraduate at Oregon State University assisted in literature review for this pathway.)
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