Elongation and Termination Reactions in the trans-Golgi

By the time the first Gal residue is introduced to the oligosaccharide chain, the glycoprotein has reached the franj-Golgi in which the elongation and termination 

In this concerted action of p3-GlcNAcT and p4-GalT polylactosaminoglycan extensions are formed which also may be sialylated, fucosylated, or a-galactosylated. These reactions can in principle take place with any terminally exposed lacNAc unit irrespective of whether it is presented on di-, tri- or tetra-antennary, hybrid, or bisected N-linked oligosaccharides or on 0-linked oligosaccharides (see later). 

Because of the substrate specificities of each of these enzymes, however, not all of the theoretically possible product structures are formed in nature in detectable amounts. By way of contrast, such structures may be formed in vitro where very active (recombinant) enzyme preparations can be used. In the next paragraphs the specificities of the enzymes are further described. 

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