Keywords
Adenosine; Substrate analog; Ketone hydrate; tRNA-dependent amidotransferase
tRNA-dependent transamidation is a process in which the mis-aminoacylated tRNA(s), Glu-tRNAGln and/or Asp-tRNAAsn, are converted into the correctly aminoacylated tRNA(s), Gln-tRNAGln and/or Asn-tRNAAsn.1 This process is shared by all archaea and some bacteria, especially pathogens such as Staphylococcus aureus 2 and Helicobacter pylori. 3 The heterotrimeric enzyme, Glu-tRNAGln/Asp-tRNAAsn amidotransferase (GatCAB) is responsible for this biotransformation. Due to the absence of a GatCAB human homolog, this enzyme represents a potential target for antibacterial drug development. Recently, some transition state analogs as well as chloramphenicol derivatives were successfully tested in vitro as GatCAB inhibitors. 4 and 5
Due to the spontaneous acyl shift at the ester bond connecting an amino acid to the cognate tRNA6 (Scheme 1), the precise location of the amino acid during GatCAB catalysis is still unknown. Although it could be inferred from the crystal structure of the transamidosome7 that the 3′-hydroxyl group on the adenosine is the amino acid attachment site, Glu-tRNAGln, generated from glutamyl-tRNA synthetase 2 (GluRS2) with the amino acid on the 2′-hydroxyl group, is also one of the substrates for GatCAB.8 This intriguing substrate specificity in GatCAB prompted us to synthesize the four non-hydrolyzable substrate analogs shown in Figure 1. These target molecules resemble the accepting base of Glu-tRNAGln and Asp-tRNAAsn with Glu and Asp at the 2′ and 3′ positions of the adenosine, and connected through an amide bond rendering stability toward the acyl shift and hydrolysis. Investigation of the interactions between these small molecules and GatCAB should ultimately lead to a systematic design of small molecule inhibitors, which could be further developed into antibiotic drugs with novel mechanisms of action.