Peculiar Inhibition of Human Mitochondrial Aspartyl-Trna Synthetase by Adenylate Analogs

Human mitochondrial aminoacyl-tRNA synthetases (mt-aaRSs), the enzymes which esterify tRNAs with the cognate specific amino acid, form mainly a different set of proteins than those involved in the cytosolic translation machinery. Many of the mt-aaRSs are of bacterial-type in regard of sequence and modular structural organization. However, the few enzymes investigated so far do have peculiar biochemical and enzymological properties such as decreased solubility, decreased specific activity and enlarged spectra of substrate tRNAs (of same specificity but from various organisms and kingdoms), as compared to bacterial aaRSs. Here the sensitivity of human mitochondrial aspartyl-tRNA synthetase (AspRS) to small substrate analogs (non-hydrolysable adenylates) known as inhibitors of Escherichia coli and Pseudomonas aeruginosa AspRSs is evaluated and compared to the sensitivity of eukaryal cytosolic human and bovine AspRSs. l-aspartol-adenylate (aspartol-AMP) is a competitive inhibitor of aspartylation by mitochondrial as well as cytosolic mammalian AspRSs, with Ki values in the micromolar range (4–27μM for human mt- and mammalian cyt-AspRSs). 5′-O-[N-(l-aspartyl)sulfamoyl]adenosine (Asp-AMS) is a 500-fold stronger competitive inhibitor of the mitochondrial enzyme than aspartol-AMP (10nM) and a 35-fold lower competitor of human and bovine cyt-AspRSs (300nM). The higher sensitivity of human mt-AspRS for both inhibitors as compared to either bacterial or mammalian cytosolic enzymes, is not correlated with clear-cut structural features in the catalytic site as deduced from docking experiments, but may result from dynamic events. In the scope of new antibacterial strategies directed against aaRSs, possible side effects of such drugs on the mitochondrial human aaRSs should thus be considered.