UDP-GalA, a major sugar donor for pectin synthesis, is made by (i) the salvage pathway, by the phosphorylation of D-GalA to GalA-1-P in the presence of ATP and kinase activity (GalAK). The GalA-1-P pyrophosphorylase catalyzes the conversion of GalA1P and UTP to UDP-GalA. (ii) UDP-GlcA 4-epimerase (UGlcAE), a reversible 4-epimerase that converts UDP-GlcA to UDP-GalA.

1 Feeding experiments with radioactive GalA demonstrated that the label is readily incorpo­rated into pectin (459). Soluble enzyme preparations from a 1-day-old germinating seeds of mungbean have high GalAK activity (468), while no GalAK is observed in a 4-day-old etiolated seedlings (411). The collective biochemical data suggest that the soluble GalA kinase differs from the membrane-associated kinase activity that phosphorylates GlcA. A functional gene encoding GalAK activity was recently discovered in Arabidopsis [Ting and Bar-Pled, 2006, unpublished]. Proton NMR analysis confirm that pure reecombinant GalAK phosphoylates D-GalA in the presence of ATP to GalA-1-P. Currently however, it is difficult to predict the relative amount of GalA that is recycled back to UDP-GalA. Cleaarly, high activities of pectin derading enzymes during pollen germination could pro­vide suffcient substrate for AalAK. Indeed, the non-specific UDP-sugar pyrophosphorlase (fondly named SLOPPY in Arabidopsis (414) and also identified in pea (469)) that con­verts GalA-1-P with UTP to UDP-GalA is highly expressed during pollen germination. The relative amount of GalA recycled from pectins as free sugars in other plant tissues is unknown, although the “fate” of free GalA could be tissue specific. For example, during strawberry fruit maturation, an increase in GalA-reductase (GalUR) activity was shown to direct GalA, released from pectin, for the synthesis of ascorbic acid (470). Thus, the relative contribution of the GalA-salvage pathway for synthesis of wall polymers remains unclear. Hopefully, current analyses of GalAK mutants in Arabidopsis will shed light on its physiological functioon (i. e. in the wall and in ascorbic acid metabolism).

2 UDP-GlcA 4-epimerase (UGlcAE) is a membrane-bound enzyme that reversibly catalyzes the 4-epimerization of UDP-GlcA to UDP-GalA. In Arabidopsis, six distinct functional genes (UGlcAe) encode isoforms having UDP-GlcA 4-epimerase activity (450, 471, 472). The isoforms can be divided into three evolutionary clades: Type A, B, and C. Members of UGlcAE are predicted to be Type II membrane proteins, suggesting that their catalytic

domain faces the lumen of an endomembrane. The topology of the catalytic domain was not validated experimentally. Mohnen’s laboratory has shown that the activity of UGlcAE co-fractionated with the Golgi on sucrose-gradients (230) and expressing UGlcAE1-GFP in plants demonstrates that the chimeric fusion targets UGlcAEl to the Golgi as well (Gu and Bar-Peled, unpublished data). Multiple UGlcAE isoforms are common and found in other plants, for example, rice and maize. Biochemical analyses of Arabidopsis, rice, and maize UGlcAE isoforms showed that the reversible UGlcAE epimerase has a preference (2:1) to form UDP-GalA and is inhibited by both UDP-Ara and UDP-Xyl. Recent studies (Gu and coworkers, submitted) demonstrated that the maize UGlcAE is especially sensitive to, and strongly inhibited by, UDP-Xyl when compared with Arabidopsis UGlcAE2. The authors suggest that in maize, the relative low amount of pectin (when compared to dicots) could be due to inhibition of UGlcAE by UDP-Xyl. Hence, this may provide an explanation as to why maize has less pectin and more xylan when compared to dicots. The role of multiple UGlcAE isoforms is currently being addressed. One possible explanation is that different isoforms localize to distinct endomembranes, as was suggested by Pattathil and coworkers (473) for the different Uxs isoforms.