High-Affinity Inhibitors of tRNA-Guanine Transglycosylase Replacing the Function of a Structural Water Cluster

Abstract

The tRNA-modifying enzyme tRNA–guanine transglycosylase (TGT) is essential for the pathogenic mechanism of Shigella flexneri, the causing agent of the bacterial diarrheal disease shigellosis. Herein, the synthesis of a new class of rationally designed 6-amino-imidazo[4,5-g]quinazolin-8(7H)-one- (lin-benzoguanine) based inhibitors of TGT are reported. In order to accommodate a small hydrophobic crevice opening near the binding site of ribose-34, 2-aminoethyl substituents were introduced in position 4 of the heterocyclic scaffold. For this purpose, a synthetic sequence consisting of iodination, Suzuki cross-coupling, hydroboration, Mitsunobu reaction, and Gabriel synthesis was employed, furnishing a primary amine that served as a common intermediate for the preparation of a series of derivatives. The resulting ligands displayed very low inhibition constants, down to Ki=2 nM. Substantial additional inhibitory potency is gained by interaction of terminal lipophilic groups attached to the substituent at position 4 with the hydrophobic crevice shaped by Val45 and Leu68. At the same time, the secondary ammonium center in the substituent displaces a cluster of water molecules, solvating the catalytic residues Asp102 and Asp280, without loss in binding affinity. In addition, a synthetic intermediate with an unusual 3,6,7,8,9,10-hexahydroimidazo[4,5-g][1,3]benzodiazepine core, as confirmed by X-ray analysis, is reported.