Conformational and electrostatic analysis of S N 1 donor analogue glycomimetic inhibitors of ST3Gal-I mammalian sialyltransferase.

Mammalian sialyltransferases play a role in the metastasis of various cancers in humans. Inhibitors of these enzymes will in principle be able to directly inhibit aberrant sialylation in cancer. Inhibitors of ST3Gal-I resembling the donor component of S _N 1 Transition State structures were previously evaluated as part of a kinetics study. Here, using classical dynamics simulations and free energy perturbation calculations, we rationalize the performance of three of these donor analogue ST3Gal-I enzyme inhibitors. We find to inhibit the mammalian ST3Gal-I enzyme a donor analogue requires configurationally limited functionality. This is mediated by the binding of the inhibitor to the enzyme. The inhibitor's ability to interact with Y194 and T272 through a charged group such as a carboxylate is especially important. Furthermore, a conformational rigid form approximating the donor substrate is central. Here this is achieved by an intramolecular hydrogen bond formed between the carboxylate group and one of the ribose hydroxyl groups of the cytidine monophosphate (CMP) leaving group. This intramolecular interaction results in the donor substrate conformer complimenting the form of the catalytic binding site. Finally the carboxylate charge is essential for electrostatic pairing with the binding site. Substituting this group for an alcohol or amide results in severe weakening of the ligand binding. The carboxylate thus proves an to be an irreplaceable functional group and an essential pharmacophore.
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