Galactose is a major constituent of diverse pectic polysaccharides including RG-I. The sugar donor, UDP-Gal, is produced by (i) phosphorylation of C-1 of D-Gal by galactokinase (414) followed by pyrophosphorylase-catalyzed conversion of a-D-Gal-1-P and UTP to UDP — Gal, and by (ii) UDP-Glc-4 epimerase (UGE as described above) that reversibly converts UDP-Glc to UDP-Gal.

1 D-galactokinase activity was isolated by Neufeld and coworkers in 1960 from mung bean (412). The D-GalK is membrane bound and the activity, unlike L-AraK activity, could not be solubilized with digitonin. The galactokinase gene (GalK, Gal1, At3g06580) was cloned from Arabidopsis by functional complementation of yeast (446) and E. coli (445) galK mutants that are unable to metabolize galactose. While the Gal1/GalK clone was able to complement the yeast mutant, a definitive substrate specificity of the recombinant Arabidopsis enzyme (GalK) will provide information on whether other “recycled sugars” are substrates. Sequence alignment of various sugar kinase proteins shows that the Ara­bidopsis GalK shares amino acid sequence similarity (45%) to GalK2, a human kinase with phosphorylation preference to GalNAc (446). A meaningful alignment could not be obtained between the Arabidopsis GalK with the human GalK1 whose true substrate is Gal (447). Whether At3g06580 encodes a Gal-1-P kinase activity and/or GalNac-1-P kinase activity remains to be determined biochemically. The subsequent pyrophosphorylation of Gal-1-P to UDP-Gal is likely mediated via “Sloppy (416),” the non-specific UDP-sugar pyrophosphorylase (At5g52560).

2 The second route to form UDP-Gal is with UDP-Glc-4-epimerase (as described above). In humans and fungi, UDP-Gal is synthesized by uridylyltransferase activity (GalT). GalT transfers UMP from UDP-Glc onto Gal-1P forming Glc-1-P and UDP-Gal. Such activity and corresponding genes have not yet been described in plants.