Family-wide analysis of aminoacyl-sulfamoyl-3-deazaadenosine analogues as inhibitors of aminoacyl-tRNA synthetases

Structural Basis of Cysteine Ligase MshC Inhibition by Cysteinyl-Sulfonamides

Mycothiol (MSH), the major cellular thiol in Mycobacterium tuberculosis (Mtb), plays an essential role in the resistance of Mtb to various antibiotics and oxidative stresses. MshC catalyzes the ATP-dependent ligation of 1-O-(2-amino-2-deoxy-α-d-glucopyranosyl)-d-myo-inositol (GlcN-Ins) with l-cysteine (l-Cys) to form l-Cys-GlcN-Ins, the penultimate step in MSH biosynthesis. The inhibition of MshC is lethal to Mtb. In the present study, five new cysteinyl-sulfonamides were synthesized, and their binding affinity with MshC was evaluated using a thermal shift assay. Two of them bind the target with EC₅₀ values of 219 and 231 µM. Crystal structures of full-length MshC in complex with these two compounds showed that they were bound in the catalytic site of MshC, inducing dramatic conformational changes of the catalytic site compared to the apo form. In particular, the observed closure of the KMSKS loop was not detected in the published cysteinyl-sulfamoyl adenosine-bound structure, the latter likely due to trypsin treatment. Despite the confirmed binding to MshC, the compounds did not suppress Mtb culture growth, which might be explained by the lack of adequate cellular uptake. Taken together, these novel cysteinyl-sulfonamide MshC inhibitors and newly reported full-length apo and ligand-bound MshC structures provide a promising starting point for the further development of novel anti-tubercular drugs targeting MshC.

Partitioning of the initial catalytic steps of leucyl-tRNA synthetase is driven by an active site peptide-plane flip

To correctly aminoacylate tRNA^Leu, leucyl-tRNA synthetase (LeuRS) catalyzes three reactions: activation of leucine by ATP to form leucyl-adenylate (Leu-AMP), transfer of this amino acid to tRNA^Leu and post-transfer editing of any mischarged product. Although LeuRS has been well characterized biochemically, detailed structural information is currently only available for the latter two stages of catalysis. We have solved crystal structures for all enzymatic states of Neisseria gonorrhoeae LeuRS during Leu-AMP formation. These show a cycle of dramatic conformational changes, involving multiple domains, and correlate with an energetically unfavorable peptide-plane flip observed in the active site of the pre-transition state structure. Biochemical analyses, combined with mutant structural studies, reveal that this backbone distortion acts as a trigger, temporally compartmentalizing the first two catalytic steps. These results unveil the remarkable effect of this small structural alteration on the global dynamics and activity of the enzyme.

Aminoacyl-tRNA Synthetases as Valuable Targets for Antimicrobial Drug Discovery

Aminoacyl-tRNA synthetases (aaRSs) catalyze the esterification of tRNA with a cognate amino acid and are essential enzymes in all three kingdoms of life. Due to their important role in the translation of the genetic code, aaRSs have been recognized as suitable targets for the development of small molecule anti-infectives. In this review, following a concise discussion of aaRS catalytic and proof-reading activities, the various inhibitory mechanisms of reported natural and synthetic aaRS inhibitors are discussed. Using the expanding repository of ligand-bound X-ray crystal structures, we classified these compounds based on their binding sites, focusing on their ability to compete with the association of one, or more of the canonical aaRS substrates. In parallel, we examined the determinants of species-selectivity and discuss potential resistance mechanisms of some of the inhibitor classes. Combined, this structural perspective highlights the opportunities for further exploration of the aaRS enzyme family as antimicrobial targets.

Synthesis and structure-activity studies of novel anhydrohexitol-based Leucyl-tRNA synthetase inhibitors

Leucyl-tRNA synthetase (LeuRS) is a clinically validated target for the development of antimicrobials. This enzyme catalyzes the formation of charged tRNA^Leu molecules, an essential substrate for protein translation. In the first step of catalysis LeuRS activates leucine using ATP, forming a leucyl-adenylate intermediate. Bi-substrate inhibitors that mimic this chemically labile phosphoanhydride-linked nucleoside have proven to be potent inhibitors of different members of the aminoacyl-tRNA synthetase family but, to date, they have demonstrated poor antibacterial activity. We synthesized a small series of 1,5-anhydrohexitol-based analogues coupled to a variety of triazoles and performed detailed structure-activity relationship studies with bacterial LeuRS. In an in vitro assay, K_i^app values in the nanomolar range were demonstrated. Inhibitory activity differences between the compounds revealed that the polarity and size of the triazole substituents affect binding. X-ray crystallographic studies of N. gonorrhoeae LeuRS in complex with all the inhibitors highlighted the crucial interactions defining their relative enzyme inhibitory activities. We further examined their in vitro antimicrobial properties by screening against several bacterial and yeast strains. While only weak antibacterial activity against M. tuberculosis was detected, the extensive structural data which were obtained could make these LeuRS inhibitors a suitable starting point towards further antibiotic development.

Synthesis and Biological Evaluation of 1,3-Dideazapurine-Like 7-Amino-5-Hydroxymethyl-Benzimidazole Ribonucleoside Analogues as Aminoacyl-tRNA Synthetase Inhibitors

Aminoacyl-tRNA synthetases (aaRSs) have become viable targets for the development of antimicrobial agents due to their crucial role in protein translation. A series of six amino acids were coupled to the purine-like 7-amino-5-hydroxymethylbenzimidazole nucleoside analogue following an optimized synthetic pathway. These compounds were designed as aaRS inhibitors and can be considered as 1,3-dideazaadenine analogues carrying a 2-hydroxymethyl substituent. Despite our intentions to obtain N¹-glycosylated 4-aminobenzimidazole congeners, resembling the natural purine nucleosides glycosylated at the N⁹-position, we obtained the N³-glycosylated benzimidazole derivatives as the major products, resembling the respective purine N⁷-glycosylated nucleosides. A series of X-ray crystal structures of class I and II aaRSs in complex with newly synthesized compounds revealed interesting interactions of these “base-flipped” analogues with their targets. While the exocyclic amine of the flipped base mimics the reciprocal interaction of the N³-purine atom of aminoacyl-sulfamoyl adenosine (aaSA) congeners, the hydroxymethyl substituent of the flipped base apparently loses part of the standard interactions of the adenine N¹ and the N⁶-amine as seen with aaSA analogues. Upon the evaluation of the inhibitory potency of the newly obtained analogues, nanomolar inhibitory activities were noted for the leucine and isoleucine analogues targeting class I aaRS enzymes, while rather weak inhibitory activity against the corresponding class II aaRSs was observed. This class bias could be further explained by detailed structural analysis.

Synthesis and structural insights into the binding mode of the albomycin δ1 core and its analogues in complex with their target aminoacyl-tRNA synthetase

Despite of proven efficacy and well tolerability, albomycin is not used clinically due to scarcity of material. Several attempts have been made to increase the production of albomycin by chemical or biochemical methods. In the current study, we have synthesized the active moiety of albomycin δ1 and investigated its binding mode to its molecular target seryl-trna synthetase (SerRS). In addition, isoleucyl and aspartyl congeners were prepared to investigate whether the albomycin scaffold can be extrapolated to target other aminoacyl-tRNA synthetases (aaRSs) from both class I and class II aaRSs, respectively. The synthesized analogues were evaluated for their ability to inhibit the corresponding aaRSs by an in vitro aminoacylation experiment using purified enzymes. It was observed that the diastereomer having the 5'S, 6'R-configuration (nucleoside numbering) as observed in the crystal structure, exhibits excellent inhibitory activity in contrast to poor activity of its companion 5'R,6'S-diasteromer obtained as byproduct during synthesis. Moreover, the albomycin core scaffold seems well tolerated for class II aaRSs inhibition compared with class I aaRSs. To understand this bias, we studied X-ray crystal structures of SerRS in complex with the albomycin δ1 core structure 14a, and AspRS in complex with compound 16a. Structural analysis clearly showed that diastereomer selectivity is attributed to the steric restraints of the active site of SerRS and AspRS.

Discovery of indolyl-containing peptides as novel antibacterial agents targeting tryptophanyl-tRNA synthetase

Background: There is an urgent need for antibiotics with novel structures and unexploited targets to counteract bacterial resistance. Methodology & results: Novel tryptophanyl-tRNA synthetase inhibitors were discovered based on virtual screening, surface plasmon resonance binding, enzymatic activity assay and antibacterial activity evaluation. Of the 29 peptide derivatives tested for antibacterial activity, some inhibited the growth of both Staphylococcus aureus and Staphylococcus epidermidis. A13 and A15 exhibited antibacterial activity against methicillin-resistant S. aureus NRS384 at an 8 μg/ml minimum inhibitory concentration. A13 snugly docked into the active site, explaining its improved inhibitory activity. Conclusion: Our results provide us with new structural clues to develop more potent tryptophanyl-tRNA synthetase inhibitors and lay a solid foundation for future drug design efforts.

Structural Insights into the Binding of Natural Pyrimidine-Based Inhibitors of Class II Aminoacyl-tRNA Synthetases

The pyrimidine-containing Trojan horse antibiotics albomycin and a recently discovered cytidine-containing microcin C analog target the class II seryl- and aspartyl-tRNA synthetases (serRS and aspRS), respectively. The active components of these compounds are competitive inhibitors that mimic the aminoacyl-adenylate intermediate. How they effectively substitute for the interactions mediated by the canonical purine group is unknown. Employing nonhydrolyzable aminoacyl-sulfamoyl nucleosides substituting the base with cytosine, uracil, and N3-methyluracil the structure-activity relationship of the natural compounds was evaluated. In vitro using E. coli serRS and aspRS, the best compounds demonstrated IC₅₀ values in the low nanomolar range, with a clear preference for cytosine or N3-methyluracil over uracil. X-ray crystallographic structures of K. pneumoniae serRS and T. thermophilus aspRS in complex with the compounds showed the contribution of structured waters and residues in the conserved motif-2 loop in defining base preference. Utilizing the N3-methyluracil bound serRS structure, MD simulations of the fully modified albomycin base were performed to identify the interacting network that drives stable association. This analysis pointed to key interactions with a methionine in the motif-2 loop. Interestingly, this residue is mutated to a glycine in a second serRS (serRS2) found in albomycin-producing actinobacteria possessing self-immunity to this antibiotic. A comparative study demonstrated that serRS2 is poorly inhibited by the pyrimidine-containing intermediate analogs, and an equivalent mutation in E. coli serRS significantly decreased the affinity of the cytosine congener. These findings highlight the crucial role of dynamics and solvation of the motif-2 loop in modulating the binding of the natural antibiotics.

Structure-Guided Enhancement of Selectivity of Chemical Probe Inhibitors Targeting Bacterial Seryl-tRNA Synthetase

Aminoacyl-tRNA synthetases are ubiquitous and essential enzymes for protein synthesis and also a variety of other metabolic processes, especially in bacterial species. Bacterial aminoacyl-tRNA synthetases represent attractive and validated targets for antimicrobial drug discovery if issues of prokaryotic versus eukaryotic selectivity and antibiotic resistance generation can be addressed. We have determined high-resolution X-ray crystal structures of the Escherichia coli and Staphylococcus aureus seryl-tRNA synthetases in complex with aminoacyl adenylate analogues and applied a structure-based drug discovery approach to explore and identify a series of small molecule inhibitors that selectively inhibit bacterial seryl-tRNA synthetases with greater than 2 orders of magnitude compared to their human homologue, demonstrating a route to the selective chemical inhibition of these bacterial targets.

Synthesis and Biological Evaluation of Lipophilic Nucleoside Analogues as Inhibitors of Aminoacyl-tRNA Synthetases

Emerging antibiotic resistance in pathogenic bacteria and reduction of compounds in the existing antibiotics discovery pipeline is the most critical concern for healthcare professionals. A potential solution aims to explore new or existing targets/compounds. Inhibition of bacterial aminoacyl-tRNA synthetase (aaRSs) could be one such target for the development of antibiotics. The aaRSs are a group of enzymes that catalyze the transfer of an amino acid to their cognate tRNA and therefore play a pivotal role in translation. Thus, selective inhibition of these enzymes could be detrimental to microbes. The 5′-O-(N-(L-aminoacyl)) sulfamoyladenosines (aaSAs) are potent inhibitors of the respective aaRSs, however due to their polarity and charged nature they cannot cross the bacterial membranes. In this work, we increased the lipophilicity of these existing aaSAs in an effort to promote their penetration through the bacterial membrane. Two strategies were followed, either attaching a (permanent) alkyl moiety at the adenine ring via alkylation of the N⁶-position or introducing a lipophilic biodegradable prodrug moiety at the alpha-terminal amine, totaling eight new aaSA analogues. All synthesized compounds were evaluated in vitro using either a purified Escherichiacoli aaRS enzyme or in presence of total cellular extract obtained from E. coli. The prodrugs showed comparable inhibitory activity to the parent aaSA analogues, indicating metabolic activation in cellular extracts, but had little effect on bacteria. During evaluation of the N⁶-alkylated compounds against different microbes, the N⁶-octyl containing congener 6b showed minimum inhibitory concentration (MIC) of 12.5 µM against Sarcina lutea while the dodecyl analogue 6c displayed MIC of 6.25 µM against Candidaalbicans.

Acylated sulfonamide adenosines as potent inhibitors of the adenylate-forming enzyme superfamily

The superfamily of adenylate-forming enzymes all share a common chemistry. They activate a carboxylate group, on a specific substrate, by catalyzing the formation of a high energy mixed phosphoanhydride-linked nucleoside intermediate. Members of this diverse enzymatic family play key roles in a variety of metabolic pathways and therefore many have been regarded as drug targets. A generic approach to inhibit such enzymes is the use of non-hydrolysable sulfur-based bioisosteres of the adenylate intermediate. Here we compare the activity of compounds containing a sulfamoyl and sulfonamide linker respectively. An improved synthetic strategy was developed to generate inhibitors containing the latter that target isoleucyl- (IleRS) and seryl-tRNA synthetase (SerRS), two structurally distinct representatives of Class I and II aminoacyl-tRNA synthetases (aaRSs). These enzymes attach their respective amino acid to its cognate tRNA and are indispensable for protein translation. Evaluation of the ability of the two similar isosteres to inhibit serRS revealed a remarkable difference, with an almost complete loss of activity for seryl-sulfonamide 15 (SerSoHA) compared to its sulfamoyl analogue (SerSA), while inhibition of IleRS was unaffected. To explain these observations, we have determined a 2.1 Å crystal structure of Klebsiella pneumoniae SerRS in complex with SerSA. Using this structure as a template, modelling of 15 in the active site predicts an unfavourable eclipsed conformation. We extended the same modelling strategy to representative members of the whole adenylate-forming enzyme superfamily, and were able to disclose a new classification system for adenylating enzymes, based on their protein fold. The results suggest that, other than for the structural and functional orthologues of the Class II aaRSs, the O to C substitution within the sulfur-sugar link should generally preserve the inhibitory potency.

Comparative analysis of pyrimidine substituted aminoacyl-sulfamoyl nucleosides as potential inhibitors targeting class I aminoacyl-tRNA synthetases

Aminoacyl-tRNA synthetases (aaRSs) catalyse the ATP-dependent coupling of an amino acid to its cognate tRNA. Being vital for protein translation aaRSs are considered a promising target for the development of novel antimicrobial agents. 5'-O-(N-aminoacyl)-sulfamoyl adenosine (aaSA) is a non-hydrolysable analogue of the aaRS reaction intermediate that has been shown to be a potent inhibitor of this enzyme family but is prone to chemical instability and enzymatic modification. In an attempt to improve the molecular properties of this scaffold we synthesized a series of base substituted aaSA analogues comprising cytosine, uracil and N³-methyluracil targeting leucyl-, tyrosyl- and isoleucyl-tRNA synthetases. In in vitro assays seven out of the nine inhibitors demonstrated K_i^app values in the low nanomolar range. To complement the biochemical studies, X-ray crystallographic structures of Neisseria gonorrhoeae leucyl-tRNA synthetase and Escherichia coli tyrosyl-tRNA synthetase in complex with the newly synthesized compounds were determined. These highlighted a subtle interplay between the base moiety and the target enzyme in defining relative inhibitory activity. Encouraged by this data we investigated if the pyrimidine congeners could escape a natural resistance mechanism, involving acetylation of the amine of the aminoacyl group by the bacterial N-acetyltransferases RimL and YhhY. With RimL the pyrimidine congeners were less susceptible to inactivation compared to the equivalent aaSA, whereas with YhhY the converse was true. Combined the various insights resulting from this study will pave the way for the further rational design of aaRS inhibitors.

Progress and challenges in aminoacyl-tRNA synthetase-based therapeutics

Aminoacyl-tRNA synthetases (ARSs) are universal enzymes that catalyze the attachment of amino acids to the 3' ends of their cognate tRNAs. The resulting aminoacylated tRNAs are escorted to the ribosome where they enter protein synthesis. By specifically matching amino acids to defined anticodon sequences in tRNAs, ARSs are essential to the physical interpretation of the genetic code. In addition to their canonical role in protein synthesis, ARSs are also involved in RNA splicing, transcriptional regulation, translation, and other aspects of cellular homeostasis. Likewise, aminoacylated tRNAs serve as amino acid donors for biosynthetic processes distinct from protein synthesis, including lipid modification and antibiotic biosynthesis. Thanks to the wealth of details on ARS structures and functions and the growing appreciation of their additional roles regulating cellular homeostasis, opportunities for the development of clinically useful ARS inhibitors are emerging to manage microbial and parasite infections. Exploitation of these opportunities has been stimulated by the discovery of new inhibitor frameworks, the use of semi-synthetic approaches combining chemistry and genome engineering, and more powerful techniques for identifying leads from the screening of large chemical libraries. Here, we review the inhibition of ARSs by small molecules, including the various families of natural products, as well as inhibitors developed by either rational design or high-throughput screening as antibiotics and anti-parasitic therapeutics.