Heidi Salminen

The Centrally Acting β1,6N-Acetylglucosaminyltransferase (GlcNAc to Gal) FUNCTIONAL EXPRESSION, PURIFICATION, AND ACCEPTOR SPECIFICITY OF A HUMAN ENZYME INVOLVED IN MIDCHAIN BRANCHING OF LINEAR POLY-N-ACETYLLACTOSAMINES

In the present experiments the cDNA coding for a truncated form of the β1,6N-acetylglucosaminyltransferase responsible for the conversion of linear to branched polylactosamines in human PA1 cells was expressed in Sf9 insect cells. The catalytic ectodomain of the enzyme was fused to glutathione S-transferase, allowing effective one-step purification of the glycosylated 67–74-kDa fusion protein. Typically a yield of 750 μg of the purified protein/liter of suspension culture was obtained. The purified recombinant protein catalyzed the transfer of GlcNAc from UDP-GlcNAc to the linear tetrasaccharide Galβ1–4GlcNAcβ1–3Galβ1–4GlcNAc, converting the acceptor to the branched pentasaccharide Galβ1–4GlcNAcβ1–3(GlcNAcβ1–6)Galβ1–4GlcNAc as shown by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, degradative experiments, and 1H NMR spectroscopy of the product. By contrast, the recombinant enzyme did not catalyze any reaction when incubated with UDP-GlcNAc and the trisaccharide GlcNAcβ1–3Galβ1–4GlcNAc. Accordingly, we call the recombinant β1,6-GlcNAc transferase cIGnT6 to emphasize its action atcentral rather than peridistal galactose residues of linear polylactosamines in the biosynthesis of blood group I antigens. Taken together this in vitro expression of I-branching enzyme, in combination with the previously cloned enzymes, β1,4galactosyltransferase and β1,3N-acetylglucosaminyltransferase, should allow the general synthesis of polylactosamines based totally on the use of recombinant enzymes.

Enzymatic synthesis of site-specifically (α 1–3)-fucosylated polylactosamines containing either a sialyl Lewis x, a VIM-2, or a sialylated and internally difucosylated sequence

By using two different reaction pathways, we generated enzymatically three sialylated and site-specifically alpha 1-3-fucosylated polylactosamines. Two of these are isomeric hexasaccharides Neu5Ac(alpha 2-3)Gal(beta 1-4)GlcNAc(beta 1-3)Gal(beta 1-4)[Fuc(alpha 1-3)] GlcNAc and Neu5Ac(alpha 2-3)Gal(beta 1-4)[Fuc(alpha 1-3)]GlcNAc(beta 1-3)Gal(beta 1-4) GlcNAc, containing epitopes that correspond to VIM-2 and sialyl Lewis (x), respectively. The third one, nonasaccharide Neu5Ac(alpha 2-3)Gal(beta 1-4)GlcNAc(beta 1-3)Gal(beta 1-4)[Fuc(alpha 1-3)] GlcNAc(beta 1-3)Gal(beta 1-4)[Fuc(alpha 1-3)]GlcNAc, is a sialylated and internally difucosylated derivative of a trimeric N-acetyllactosamine. All three oligosaccharides have one fucose-free N-acetyllactosaminyl unit and can be used as acceptors for recombinant alpha 1-3-fucosyltransferases in determining the biosynthesis pathways leading to polyfucosylated selectin ligands.

Improved enzymatic synthesis of a highly potent oligosaccharide antagonist of L-selectin

The polylactosamine sLexβ1–3′(sLexβ1–6′)LacNAcβ1–3′(sLexβ1–6′)LacNAcβ1–3′(sLexβ1–6′)LacNAc (7) (where sLex is Neu5Acα2–3Galβ1–4(Fucα1–3)GlcNAc and LacNAc is Galβ1–4GlcNAc) is a nanomolar L‐selectin antagonist and therefore a potential anti‐inflammatory agent (Renkonen et al. (1997) Glycobiology, 7, 453). Here we describe an improved synthesis of 7. The octasaccharide LacNAcβ1–3′LacNAcβ1–3′LacNAcβ1–3′LacNAc (4) was converted into the triply branched undecasaccharide LacNAcβ1–3′(GlcNAcβ1–6′)LacNAcβ1–3′(GlcNAcβ1–6′)LacNAcβ1–3′(GlcNAcβ1–6′)LacNAc (5) by incubation with UDP‐GlcNAc and the midchain β1,6‐GlcNAc transferase activity of rat serum. Glycan 5 was enzymatically β1,4‐galactosylated to LacNAcβ1–3′(LacNAcβ1–6′)LacNAcβ1–3′(LacNAcβ1–6′)LacNAcβ1–3′(LacNAcβ1–6′)LacNAc (6). Combined with the enzymatic conversion of 6 to 7 (Renkonen et al., loc. cit.) and the available chemical synthesis of 4, our data improve the availability of 7 for full assessment of its anti‐inflammatory properties.