A Biosynthetic Control on Structures Serving as Ligands for Selectins: The Precursor Structures, 3-Sialyl/Sulfo Galβ1,3/4GlcNAcβ-0-R, Which Are High Affinity Substrates for α1,3/4-L-Fucosyltransferases, Exhibit the Phenomenon of Substrate Inhibition

The present study reports a control on the biosynthesis of fucosylated structures, serving as ligands for selectins by demonstrating the potential of 3-sialyl or 3-sulfo Galβ1,3/4GlcNAcβ-containing glycoconjugates as high affinity substrates for α1,3/4-L-fucosyltransferases and as substrate inhibitors at higher concentrations. The synthetic sulfated saccharides and the triantennary sialoglycopeptide from fetuin were potent competitive inhibitors of the transfer of fucose to non-anionic saccharide acceptors and the corresponding triantennary asialoglycopeptide respectively catalyzed by a partially purified α1,3/4-L-fucosyltransferase preparation from Colo 205 (specific activity:transfer of 113.1 nmol Fuc to 2′-FucosylLacNAc per h per mg protein); Ki for the inhibitions by triantennary sialoglycopeptide, 3-SulfoGalβ1,3GlcNAcβ-O-Allyl and a copolymer from 3-SulfoGalβ1,3GlcNAcβ-O-Allyl and acrylamide were 51.9 μM, 500 μM and 67.0 μM, respectively. Further, the α1,3-specific anionic acceptor, 3′-SulfoLacNAc, also inhibited the α1,4- activity; Km for the α1,4-specific acceptor, 2-methylGalβ1,3GlcNAcβ-O-Bn increased from 0.40 mM to 1.35 mM in presence of 3.0 mM 3′-sulfoLacNAc, whereas Ki for the mutual inhibition of α1,3-activity by the former was found to be high (3.64 mM). Furthermore, the phenomenon of substrate inhibition, serving as acceptors at lower concentrations and as inhibitors at higher concentrations, was exhibited by the anionic acceptors; the Hill plots gave the Ki values 342.7 μM, 13.03 mM and 13.36 mM respectively for fetuin triantennary sialo glycopeptide, 3′-sulfoLacNAc and 3-sulfoGalβ1,3GlcNAcβ-O-Allyl.