Recognition of β-d-Gal p-(1 → 3)-β-d-Glc pNAc-OR acceptor analogues by the Lewis α-(1 → 34)-fucosyltransferase from human milk

The Lewis α-(1 → 34)-fucosyltransferase (E.C. 2.4.1.65) transfers l-fucose from GDP-fucose to OH-4 of the Glc pNAc residue in the disaccharide β-d-Gal p-(1 → 3)-β-d-Glc pNAc-OR [R = (CH2)8COOMe] (1) to give the Lewis-A blood group determinant β-d-Gal p-(1 → 3)-[α-l-Fuc p-(1 → 4)]-β-d-Glc pNAc-OR. Five deoxy analogues of 1, as well as its N-propionyl derivative, were chemically synthesized and kinetically evaluated as both substrates and inhibitors for the enzyme from human milk. The unmodified acceptor 1 had Km = 640 μM with Vmax set arbitrarily to 100. The 6-deoxy (Km = 400 μM, Vmax = 90) and N-propionyl compounds (Km = 330 μM, Vmax = 170) remained excellent substrates while the 4-deoxy compound was a very weak competitive inhibitor with Ki = 9 mM. Deoxygenation of OH-2′ and OH-4′ (of the Gal residue) in 1 had little effect on the activity. The OH-6 group of the Gal residue proved to be critical for recognition by the enzyme since substitution of this group with hydrogen led to an inactive compound.