The foodborne illness shigellosis is caused by Shigella bacteria that secrete the highly cytotoxic Shiga toxin, which is also formed by the closely related enterohemorrhagic Escherichia coli (EHEC). It has been shown that tRNA-guanine transglycosylase (TGT) is essential for the pathogenicity of Shigella flexneri. Herein, the molecular recognition properties of a guanine binding pocket in Zymomonas mobilis TGT are investigated with a series of lin-benzohypoxanthine- and lin-benzoguanine-based inhibitors that bear substituents to occupy either the ribose-33 or the ribose-34 pocket. The three inhibitor scaffolds differ by the substituent at C(6) being H, NH(2), or NH-alkyl. These differences lead to major changes in the inhibition constants, pK(a) values, and binding modes. Compared to the lin-benzoguanines, with an exocyclic NH(2) at C(6), the lin-benzohypoxanthines without an exocyclic NH(2) group have a weaker affinity as several ionic protein-ligand hydrogen bonds are lost. X-ray cocrystal structure analysis reveals that a new water cluster is imported into the space vacated by the lacking NH(2) group and by a conformational shift of the side chain of catalytic Asp102. In the presence of an N-alkyl group at C(6) in lin-benzoguanine ligands, this water cluster is largely maintained but replacement of one of the water molecules in the cluster leads to a substantial loss in binding affinity. This study provides new insight into the role of water clusters at enzyme active sites and their challenging substitution by ligand parts, a topic of general interest in contemporary structure-based drug design.

Apart from competitive active-site inhibition of protein function, perturbance of protein-protein interactions by small molecules in oligodomain enzymes opens new perspectives for innovative therapeutics. tRNA-guanine transglycosylase (TGT), a potential target to treat shigellosis, is active only as the homodimer. Consequently, disruption of the dimer interface by small molecules provides a novel inhibition mode. A special feature of this enzyme is the short distance between active site and rim of the dimer interface. This suggests design of expanded active-site inhibitors decorated with rigid, needle-type substituents to spike into potential hot spots of the interaction interface. Ligands with attached ethinyl-type substituents have been synthesized and characterized by Kd measurements, crystallography, noncovalent mass spectrometry, and computer simulations. In contrast to previously determined crystal structures with nonextended active-site inhibitors, a well-defined loop-helix motif, involved in several contacts across the dimer interface, falls apart and suggests enhanced flexibility once the spiking ligands are bound. Mass spectrometry indicates significant destabilization but not full disruption of the complexed TGT homodimer in solution. As directed interactions of the loop-helix motif obviously do not determine dimer stability, a structurally conserved hydrophobic patch composed of several aromatic amino acids is suggested as interaction hot spot. The residues of this patch reside on a structurally highly conserved helix-turn-helix motif, which remains unaffected by the bound spiking ligands. Nevertheless, it is shielded from solvent access by the loop-helix motif that becomes perturbed upon binding of the spiking ligands, which serves as a possible explanation for reduced interface stability.

Bacterial tRNA-guanine transglycosylase (Tgt) catalyses the exchange of the genetically encoded guanine at the wobble position of tRNAsHis,Tyr,Asp,Asn by the premodified base preQ1, which is further converted to queuine at the tRNA level. As eucaryotes are not able to synthesise queuine de novo but acquire it through their diet, eucaryotic Tgt directly inserts the hypermodified base into the wobble position of the tRNAs mentioned above. Bacterial Tgt is required for the efficient pathogenicity of Shigella sp, the causative agent of bacillary dysentery and, hence, it constitutes a putative target for the rational design of anti-Shigellosis compounds. Since mammalian Tgt is known to be indirectly essential to the conversion of phenylalanine to tyrosine, it is necessary to create substances which only inhibit bacterial but not eucaryotic Tgt. Therefore, it seems of utmost importance to study selectivity-determining features within both types of proteins. Homology models of Caenorhabditis elegans Tgt and human Tgt suggest that the replacement of Cys158 and Val233 in bacterial Tgt (Zymomonas mobilis Tgt numbering) by valine and accordingly glycine in eucaryotic Tgt largely accounts for the different substrate specificities. In the present study we have created mutated variants of Z. mobilis Tgt in order to investigate the impact of a Cys158Val and a Val233Gly exchange on catalytic activity and substrate specificity. Using enzyme kinetics and X-ray crystallography, we gained evidence that the Cys158Val mutation reduces the affinity to preQ1 while leaving the affinity to guanine unaffected. The Val233Gly exchange leads to an enlarged substrate binding pocket, that is necessary to accommodate queuine in a conformation compatible with the intermediately covalently bound tRNA molecule. Contrary to our expectations, we found that a priori queuine is recognised by the binding pocket of bacterial Tgt without, however, being used as a substrate.

The enzyme tRNA-guanine transglycosylase has been identified as a drug target for the foodborne illness shigellosis. A key challenge in structure-based design for this enzyme is the filling of the polar ribose-34 pocket. Herein, we describe a novel series of ligands consisting of furanoside-appended lin-benzoguanines. They were designed to replace a conserved water cluster and differ by the functional groups at C(2) and C(3) of the furanosyl moiety being either OH or OMe. The unfavorable desolvation of Asp102 and Asp280, which are located close to the ribose-34 pocket, had a significant impact on binding affinity. While the enzyme has tRNA as its natural substrate, X-ray co-crystal structures revealed that the furanosyl moieties of the ligands are not accommodated in the tRNA ribose-34 site, but at the location of the adjacent phosphate group. A remarkable similarity of the position of the oxygen atoms in these two structures suggests furanosides as a potential phosphate isoster.

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 Kd down to the picomolar range were measured for the bifunctionalized lin-benzoguanine-based ligands and Kd 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.