Galactosyl transferase substrate specificity

The disaccharide O-glycan (II) accumulates in sheep salivary gland, which contains only low activity of specific galactosyl transferase. II is not a substrate for other transferases. 

It should be noted that the sialyl- and fucosyltransferases of pork hver have identical substrate specificities, i.e., both enzymes require a terminal galactosyl residue Unked (fi, l- 4) to a penultimate N-acetylglucosa-mine residue. Thus the ratio of siaUc acid to fucose in plasma glycoproteins is due at least in part to a competition between the two transferases for available galactosyl-()3, 1->4)-N-acetylglucosamine sites. There is no evidence that structures internal to this disaccharide play a major role in either fucose or siahc acid incorporation. 

UDP-galactosyl (UDP-Gal) transferase is the best-studied transferase in terms of specificity for the acceptor sugar. It has been demonstrated that this enzyme catalyzes the transfer of UDP-Gal to a remarkable range of acceptor substrates of the carbohydrate-type [1622, 1632-1635]. Other glycosyl transferases, although less well-studied than UDP-Gal transferase, also appear to tolerate various acceptors as substrates [1636-1639] (Table 2.9). 

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