Substrate Binding Drives Active-Site Closing of Human Blood GroupB Galactosyltransferase as Revealed by Hot-Spot Labeling and NMR Spectroscopy Experiments

Crystallography has shown that human blood groupA (GTA) and B (GTB) glycosyltransferases undergo transitions between open, semiclosed, and closed conformations upon substrate binding. However, the timescales of the corresponding conformational reorientations are unknown. Crystal structures show that the Trp and Met residues are located at conformational hot spots of the enzymes. Therefore, we utilized N-15 side-chain labeling of Trp residues and C-13-methyl labeling of Met residues to study substrate-induced conformational transitions of GTB. Chemical-shift perturbations (CSPs) of Met and Trp residues in direct contact with substrate ligands reflect binding kinetics, whereas the CSPs of Met and Trp residues at remote sites reflect conformational changes of the enzyme upon substrate binding. Acceptor binding is fast on the chemical-shift timescale with rather small CSPs in the range of less than approximately 20Hz. Donor binding matches the intermediate exchange regime to yield an estimate for exchange rate constants of approximately 200-300Hz. Donor or acceptor binding to GTB saturated with acceptor or donor substrate, respectively, is slow (<10Hz), as are coupled protein motions, reflecting mutual allosteric control of donor and acceptor binding. Remote CSPs suggest that substrate binding drives the enzyme into the closed state required for catalysis. These findings should contribute to better understanding of the mechanism of glycosyl transfer of GTA and GTB.