Arabinose (Ara) is an important sugar in plant walls and with a few exceptions, the pre­dominant form of Ara in plant glycans is the furanose configuration (Ara). However, some polysaccharides, RG-II, for example, carry both forms of the arabinose moiety, i. e. Ara — furanose and Ara-pyranose. UDP-Ara (pyranose form) was identified in all plant extracts and is synthesized by (i) the sequential phosphorylation of L-Ara at its C-1 by a membrane associated L-arabinokinase (412) followed by a pyrophosphorylase that converts L-Ara-1- P and UTP to UDP-Arap, and (ii) a membrane-bound UDP-Xyl-4 epimerase (UXE) that converts UDP-D-Xyl to UDP-L-Arap.

1 Neufeld and coworkers (411, 412, 468) isolated sugar-kinase activities from different sources of tissues. A membrane fraction from bean was shown to catalyze the C-1 phos­phorylation of L-Ara to p-L-Ara-1-P. The same membrane preparation phosphorylated D-Gal to a — D-Gal-1-P. However, the GalK kinase and the AraK kinase are different enzymes since AraK requires divalent ion for activity (Mg[1]+, Mn2+) whereas, the GalK kinase re­quires no additional divalent ion for activity (412). In addition, treatment of membranes with digitonin solubilizes the L-AraK activity but not the D-GalK activity. The AraK is specific for the L-form since D-Ara (that is common in prokaryotes) is not a substrate.

The aral mutant from Arabidopsis, in the At4g16130 locus, has reduced ability to metabolize arabinose and lacks Ara-1-P kinase activity (479). Bioinformatic analysis suggests that Ara1 belongs to a large family including galactokinase, homoserine kinase, mevalonate kinase, and phosphomevalonate kinase (GHMP kinases). The Ara1 protein is speculated to be a Type Ia membrane protein. If the topology is correct, it would be interesting to know whether the catalytic domain is facing the cytosol or the lumen. Direct biochemical assays and substrate specificity studies of the encoded Ara1 gene were not performed. The subsequent pyrophosphorylation of a-L-Ara-1-P to UDP-Ara can be mediated by “Sloppy,” the non­specific UDP-sugar pyrophosphorylase (413).

UDP-Xyl 4-epimerase) was shown to convert reversibly UDP-D-Xyl to UDP-Ara (480). Bioinformatic analysis suggests that Uxe1 is a Type II membrane protein whose catalytic domain is facing the lumen. A Uxe1-GFP chimera was localized to the Golgi apparatus (480). Two isoforms (UXE1, At1g30620 and UXE2, At2g34850) that share 83% aa sequence identity to each other exist in the Arabidopsis genome; two isoforms are in the rice genome and three UXE isoforms were isolated for barley (Hordeum vulgare) (Zhang and Fincher, unpublished). Since several GTs were able to transfer the Ara (pyranose) from UDP-Ara into plant glycans (271, 397); it remains a puzzle when the Ara acquires the furanose configuration. One can predict that the Ara/-donor has not yet been identified. However, this is unlikely since the mur4 mutant (involved in the synthesis of UDP-Ara pyranose), lacks glycan consisting of Ara/. This could imply that during the arabinosyltransferase catalyzed reaction the Arap is altered to the Ara/ form. A specific mutase may exist to convert the Arap to Ara/ on the glycan itself, similar to the conversion of GlcA to IdoA in proteoglycans (481). Alternatively, UDP-Ara (furanose) is made in plants as recently confirmed and described in the following section.