High-affinity inhibitors of Zymomonas mobilis tRNA-guanine transglycosylase through convergent optimization

The tRNA-modifying enzyme tRNA-guanine transglycosyl­ase (TGT) has been recognized as a drug target for the treatment of the foodborne illness shigellosis. The active site of TGT consists of three pockets: the central guanine/preQ₁ recognition site and the ribose-33 and ribose-34 pockets. In previous work, lin-benzoguanines and lin-benzohypo­xanthines, which differ by the presence of an exocyclic NH₂ group in the former and its absence in the latter, were used as central scaffolds that bind to the guanine/preQ₁ recognition site and allow suitable functionalization along exit vectors targeting the two ribose pockets. The substituents for both of these two pockets have been optimized individually. Here, a series of bifunctionalized inhibitors that occupy both ribose pockets are reported for the first time. Dissociation constants K_d down to the picomolar range were measured for the bifunctionalized lin-benzoguanine-based ligands and K_d values in the nanomolar range were measured for the corresponding lin-benzohypo­xanthine-based ligands. The binding mode of all inhibitors was elucidated by X-ray crystal structure analysis. A remarkable influence of the crystallization protocol on the solvation pattern in the solid state and the residual mobility of the bound ligands was observed.