Fucosylation of exogenous xyloglucans by pea microsomal membranes.

Microsomal membrane preparations from growing regions of etiolated pea stems catalyzed the transfer of [14C]fucosyl units from GDP-[U-14C]-L-fucose into exogenously added xyloglucan acceptors, as well as into endogenous xyloglucan. The transfer was more effective using nonfucosylated tamarind seed xyloglucan than with pea wall xyloglucan in which almost all galactose units are already fucosylated. Hydrolysis of products by endo-1,4-beta-D-glucanase yielded in each case radioactive nonasaccharide as the main fucosylated product. UDP-galactose enhanced the fucosylation of endogenous primer but it had little effect on fucosyl transfer to exogenously added xyloglucans. Low-molecular-weight nonfucosylated oligosaccharide fragments up to the octasaccharide Glc4Xyl3Gal (obtained by endoglucanase action on tamarind seed xyloglucan) were ineffectual as fucosyl acceptors but inhibited the fucosylation of endogenous as well as of added xyloglucan. With octasaccharide, the inhibition was competitive in relation to the xyloglucan acceptor (Ki = 70 microM) and noncompetitive in relation to the donor GDP-fucose (Ki = 210 microM). It is concluded that fucosyltransferase acts independently and in a noncoordinated manner from other glycosyltransferases that are required to synthesize xyloglucan. Its active site recognizes a fragment longer than the galactosylated octasaccharide unit before transfucosylation will ensue.