Inhibitors of bacterial tyrosyl tRNA synthetase: synthesis of carbocyclic analogues of the natural product SB-219383

Aminoacyl-tRNA synthetases as drug targets

Aminoacyl-tRNA synthetases (AARSs) have been considered very attractive drug-targets for decades. This interest probably emerged with the identification of differences in AARSs between prokaryotic and eukaryotic species, which provided a rationale for the development of antimicrobials targeting bacterial AARSs with minimal effect on the homologous human AARSs. Today we know that AARSs are not only attractive, but also valid drug targets as they are housekeeping proteins that: (i) play a fundamental role in protein translation by charging the corresponding amino acid to its cognate tRNA and preventing mistranslation mistakes [1], a critical process during fast growing conditions of microbes; and (ii) present significant differences between microbes and humans that can be used for drug development [2]. Together with the vast amount of available data on both pathogenic and mammalian AARSs, it is expected that, in the future, the numerous reported inhibitors of AARSs will provide the basis to develop new therapeutics for the treatment of human diseases. In this chapter, a detailed summary on the state-of-the-art in drug discovery and drug development for each aminoacyl-tRNA synthetase will be presented.

Novel 3-arylfuran-2(5H)-one-fluoroquinolone hybrid: design, synthesis and evaluation as antibacterial agent

3-Arylfuran-2(5H)-one, a novel antibacterial pharmacophore targeting tyrosyl-tRNA synthetase (TyrRS), was hybridized with the clinically used fluoroquinolones to give a series of novel multi-target antimicrobial agents. Thus, twenty seven 3-arylfuran-2(5H)-one-fluoroquinolone hybrids were synthesized and evaluated for their antimicrobial activities. Some of the hybrids exhibited merits from both parents, displaying a broad spectrum of activity against resistant strains including both Gram-negative and Gram-positive bacteria. The most potent compound (11) in antibacterial assay shows MIC50 of 0.11μg/mL against Multiple drug resistant Escherichia coli, being about 51-fold more potent than ciprofloxacin. The enzyme assays reveal that 11 is a potent multi-target inhibitor with IC50 of 1.15±0.07μM against DNA gyrase and 0.12±0.04μM against TyrRS, respectively. Its excellent inhibitory activities against isolated enzymes and intact cells strongly suggest that 11 deserves to further research as a novel antibiotic.

Role of aminoacyl-tRNA synthetases in infectious diseases and targets for therapeutic development

Aminoacyl-tRNA synthetases (AARSs) play a pivotal role in protein synthesis and cell viability. These 22 "housekeeping" enzymes (1 for each standard amino acid plus pyrrolysine and o-phosphoserine) are specifically involved in recognizing and aminoacylating their cognate tRNAs in the cellular pool with the correct amino acid prior to delivery of the charged tRNA to the protein synthesis machinery. Besides serving this canonical function, higher eukaryotic AARSs, some of which are organized in the cytoplasm as a multisynthetase complex of nine enzymes plus additional cellular factors, have also been implicated in a variety of non-canonical roles. AARSs are involved in the regulation of transcription, translation, and various signaling pathways, thereby ensuring cell survival. Based in part on their versatility, AARSs have been recruited by viruses to perform essential functions. For example, host synthetases are packaged into some retroviruses and are required for their replication. Other viruses mimic tRNA-like structures in their genomes, and these motifs are aminoacylated by the host synthetase as part of the viral replication cycle. More recently, it has been shown that certain large DNA viruses infecting animals and other diverse unicellular eukaryotes encode tRNAs, AARSs, and additional components of the protein-synthesis machinery. This chapter will review our current understanding of the role of host AARSs and tRNA-like structures in viruses and discuss their potential as anti-viral drug targets. The identification and development of compounds that target bacterial AARSs, thereby serving as novel antibiotics, will also be discussed. Particular attention will be given to recent work on a number of tRNA-dependent AARS inhibitors and to advances in a new class of natural "pro-drug" antibiotics called Trojan Horse inhibitors. Finally, we will explore how bacteria that naturally produce AARS-targeting antibiotics must protect themselves against cell suicide using naturally antibiotic resistant AARSs, and how horizontal gene transfer of these AARS genes to pathogens may threaten the future use of this class of antibiotics.

IA, database of known ligands of aminoacyl-tRNA synthetases

The IA database contains 240 structures of known inhibitors of aminoacyl-tRNA synthetases. Structures can be downloaded in different file formats (mol, sdf, smile, png). The search engine offers possibility of searching for the ligands with a given functional group. Additionally, one can search for ligands that act on selected synthetases and from particular references. The data include information which synthetase a given ligand inhibits together with the inhibition constant (IC50) if known.

Aminoacyl-tRNA synthetases: essential and still promising targets for new anti-infective agents

The emergence of resistance to existing antibiotics demands the development of novel antimicrobial agents directed against novel targets. Historically, bacterial cell wall synthesis, protein, and DNA and RNA synthesis have been major targets of very successful classes of antibiotics such as beta-lactams, glycopeptides, macrolides, aminoglycosides, tetracyclines, rifampicins and quinolones. Recently, efforts have been made to develop novel agents against validated targets in these pathways but also against new, previously unexploited targets. The era of genomics has provided insights into novel targets in microbial pathogens. Among the less exploited--but still promising--targets is the family of 20 aminoacyl-tRNA synthetases (aaRSs), which are essential for protein synthesis. These targets have been validated in nature as aaRS inhibition has been shown as the specific mode of action for many natural antimicrobial agents synthesized by bacteria and fungi. Therefore, aaRSs have the potential to be targeted by novel agents either from synthetic or natural sources to yield specific and selective anti-infectives. Numerous high-throughput screening programs aimed at identifying aaRS inhibitors have been performed over the last 20 years. A large number of promising lead compounds have been identified but only a few agents have moved forward into clinical development. This review provides an update on the present strategies to develop novel aaRS inhibitors as anti-infective drugs.

Catalysis of tyrosyl-adenylate formation by the human tyrosyl-tRNA synthetase

Although the active site residues in the Bacillus stearothermophilus and human tyrosyl-tRNA synthetases are largely conserved, several differences exist between the two enzymes. In particular, three amino acids that stabilize the transition state for the activation of tyrosine in B. stearothermophilus tyrosyl-tRNA synthetase (Cys-35, His-48, and Lys-233) are not present in the human enzyme. This raises the question of whether the activation energy for the tyrosine activation step is higher for the human tyrosyl-tRNA synthetase than for the B. stearothermophilus enzyme. In this paper, we demonstrate that intrinsic fluorescence changes can be used to monitor the pre-steady state kinetics of human tyrosyl-tRNA synthetase. In contrast to the B. stearothermophilus enzyme, catalysis of the tyrosine activation step is potassium-dependent in the human tyrosyl-tRNA synthetase. Specifically, potassium increases the forward rate constant for tyrosine activation 260-fold in the human tyrosyl-tRNA synthetase. Comparison of the forward rate constants for catalysis of tyrosine activation by the human and B. stearothermophilus enzymes indicates that despite differences in their active sites and the potassium requirement of the human enzyme, the activation energies for tyrosine activation are identical for the two enzymes. The results of these investigations suggest that differences exist between the active sites of the bacterial and human tyrosyl-tRNA synthetases that could be exploited to design antimicrobials that target the bacterial enzyme.