Figure 5
Simplified model structure of arabinogalactan and reaction sites of enzymes. The cleavage sites of the hydrolases (exo-β-1,3-galactanase, endo-β-1,6-galactanase, α-arabinofuranosidase) used in this paper are indicated. Recombinant AtGALT29A produced Gal incorporated products susceptible to the treatment of endo-β-1,6- and exo-β-1,3-galactanases (Figure 6), therefore three possible sites (β1 → 6a, b and β1 → 3c) are conceivable as the candidate sites of reaction. Towards β-1,3-galactan acceptor, both β1 → 6b and β1 → 3c galactosyltransferase activities are possible, but the main compound released by the exo-β-1,3-galactanase treatment was galactobiose, and not galactose (inset TLC in Figure 6C, D), indicating a β1 → 6b activity rather than β1 → 3c activity. Together with the β1 → 6a activity indicated by the endo-β-1,6-galactanase treatment, it is concluded that, AtGALT29A possesses β-1,6-galactosyltransferase activities both on β-1,3- and β-1,6-galactan (β1 → 6a, b activities).