In Vitro Biosynthesis of Pectic β-1,4-Galactan by Membrane-bound Galactosyltransferase from Soybean (Glycine max Merr.) Seedlings

Abstract

We have investigated galacturonosyl- [1] and methyltransferase [2] participating in the synthesis and modification of pectic polymers of soybean (Glycine max Merr.). We report the properties of a galactosyltransferase (GalT) involved in the synthesis of β-1,4-galactan side chains of pectins. A membrane preparation from etiolated soybean hypocotyls transferred [¹⁴C]Gal from UDP-[¹⁴C]Gal into β-1,4-galactans as exogenous acceptor, while activity to endogenous acceptor was negligible. The reaction occurred maximally at pH 6.5 and 25ºC in the presence of 25 mM Mn²⁺ and 0.75% Triton X-100. Digestion of the radiolabeled product with endo-β-1,4-galactanase released mainly radioactive β-1,4-galactobiose and Gal, indicating that the transfer of [¹⁴C]Gal occurred through β-1,4-linkages. On HPLC analysis, the enzyme also catalyzed incorporation of Gal into pyridylaminated (PA) β-1,4-galactooligomers (DP=5-7). The chain elongation of Gal₇-PA occurred and led to the formation of Gal_8-10-PA.
[1] Biosci. Biotechnol. Biochem., 65 (2001) 1519-1527
[2] Planta, 210 (2000) 782-791