Use of an altered sugar-nucleotide to unmask the transition state for alpha(2 -> 6) sialyltransferase

Rat liver alpha(2-->6) sialyltransferase catalyzes the formation of a glyco sidic bond between N-acetylneuraminic acid and the 6-hydroxyl group of a ga lactose residue at the nonreducing terminus of an oligosaccharide. This rea ction has been investigated through the use of the novel sugar-nucleotide d onor substrate UMP-NeuAc. A series of UMP-NeuAc radioisotopomers were prepa red by chemical deamination of the corresponding CMP-NeuAc precursors. Kine tic isotope effects (KIEs) on V/K were measured using mixtures of radiolabe led UMP-NeuAc's as the donor substrate and N-acetyllactosamine as the accep tor. The secondary beta-H-2 KIE was 1.218 +/- 0.010, and the primary C-14 K IE was 1.030 +/- 0.010. A large inverse H-3 binding isotope effect of 0.944 +/- 0.010 was measured at the terminal carbon of the NeuAc glycerol side c hain. These KIEs observed using UMP-NeuAc are much larger than those previo usly measured with CMP-NeuAc [Bruner, M., and Horenstein, B. A. (1998) Bioc hemistry 37, 289-297]. Solvent deuterium isotope effects of 1.3 and 2.6 on V/K and V-max were observed with CMP-NeuAc as the donor, and it is revealin g that these isotope effects vanished with use of the slow donor substrate UMP-NeuAc. Bell-shaped pH versus rate profiles were observed for V-max (pK( a) values = 5.5, 9.0) and V/KUMP-NeuAc (pK(a)values = 6.2, 9.0). The result s are considered in terms of a mechanism involving an isotopically sensitiv e conformational change which is independent of the glycosyl transfer step. The isotope effects reveal that the enzyme-bound transition state bears co nsiderable charge on the N-acetylneuraminic acid residue, and this and othe r features of this mechanism provide new directions for sialyltransferase i nhibitor design.