Enzymes fucosyltransferases

This class of enzymes includes galactoside 3(4)-L-fucosyl-transferase, [EC 2.4.1.65] glycoprotein 6-a-L-fucosyl-transferase [EC 2.4.1.68], galactoside 2-ce-L-fucosyltran-sferase [EC 2.4.1.69], and galactoside 3-fucosyltransferase [EC 2.4.1.152]. 

The plasma fucosyltransferase to Gal produces a-L-Fuc-( 1—>-2)-Gal linkages, and is the H-dependent, blood-group-specific enzyme. Its selective inhibition by thiol-blocking agents has permitted its discrimination245 from other fucosyltransferases. 

Differences in fucolipid content as between normal and transformed cells, and changes on ontogenesis, have already been discussed, and these effects are presumably due to variations in fucosyltransferase activities, with possible specificity differences, namely, a-(l— 2) to Gal or (1— 3 or 4) to GlcNAc. However, the enzymes involved in these cells have not yet been separated and purified, and information on the separate enzyme-activities involved is almost nonexistent. 

Several fucosyltransferases have been isolated and used for in vitro synthesis. The Lewis A al,4-fucosyltransferase transfers unnatural fucose derivatives in from their GDP esters [23]. The enzyme al,3-fiicosyltransferase has been used to L-fucosylate the 3-position of the GlcNAc of A-acetyllactosamine and of sialyl a2,3-N-acetyllactosamine [24]. Several acceptor substrates with modifications in the GlcNAc residue could also be... 

Other examples of enzyme immobilization have also been reported. For example, the recombinant human al,3/l,4-fucosyltransferase were immobilized on Ni -agarose through a 6His tag and exhibited a remarkable stability. It was exploited in the synthesis of Le and Le trisaccharides (184). 

Once the number of antennae is established, further extension is possible through addition of backbone polymers and terminal structures similar to those found on mucin-type glycans. LacNAc polymers can be added to any of the aforementioned GlcNAcs (Fig. 4A, w-x). Similarly, these polymers can be elaborated with fucose, sulfate, and sialic acid added in the same linkages described for mucin-type structures (Fig. 4A, y-z). Most fucosyltransferases and sulfotransferases, as well as the sialyltransferases that cap polyLacNAc, are capable of modifying both O-linked and A-linked glycans some exhibit a preference for one or the other. These biases are likely a result of differences in enzyme localization. 

A variant of the GT-B fold is shown in Fig. 4e for an al,6-fucosyltransferase (17). The structure shows an N-terminal coiled-coil domain, a catalytic domain that is similar to GT-B fold enzymes, and a C-terminal SH3 domain whose biological significance is currently uncertain. 

Fig. 8. Enzymatic fucosylation with integrated cofactor regeneration (Enzymes vi cf. Fig. 2, xi GDP-fucose-pyrophosphorylase, xii al-3/4-fucosyltransferase) [72]... 

An associated enzyme, al-3-fucosyltransferase, has also been used in syntheses to provide the Lewis x and sialyl Lewis x structural motifs [65]. Increasing availability of fucosyltransferases via cloning techniques are expected to have a strong influence on oligosaccharide synthesis in the near future. 

Here again, it is as probes that carba-oligosaccharides find their best applications. Compounds 7 and 8 were both submitted to the action of a-(l-3/4)-fucosyltransferase in the presence of GDP-fucose. Only 7 was accepted and fucosylated to give the Lewis analog 9 showing that this enzyme has a different mode of action regarding the synthesis of Lewis s (Scheme 8.3) [16]. 

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