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Aminoacyl-tRNA synthetases (amino acid-tRNA ligases, EC 6.1.1.-) catalyze the aminoacylation of specific amino acids onto their cognate tRNAs with extraordinary accuracy. Recent reports, however, indicate that this class of enzymes may play other roles in cellular metabolism. Several aminoacyl-tRNA synthetases are herein shown to catalyze the AMP----ADP and ADP----ATP exchange reactions (in the absence of tRNAs) by utilizing a transfer of the gamma-phosphate of ATP to reactive AMP and ADP intermediates that are probably the mixed anhydrides of the nucleotide and the corresponding amino acid. AMP and ADP produce active intermediates with amino acids by entering the back-reaction of amino acid activation, reacting with labile covalent amino acid-enzyme intermediates. Gramicidin synthetases 1 and 2, which are known to activate certain amino acids through the formation of intermediate thiol-esters of the amino acids and the enzymes, catalyze the same set of reactions with similar characteristics. Several lines of evidence suggest that these activities are an inherent part of the enzymatic reactions catalyzed by the aminoacyl-tRNA synthetases and gramicidin synthetases and are not due to impurities of adenylate kinase, NDP kinase, or low levels of tRNAs bound to the enzymes. The covalent amino acid-enzyme adducts are likely intermediates in the aminoacylation of their cognate tRNAs. The use of gramicidin synthetases has thus helped to illuminate mechanistic details of amino acid activation catalyzed by the aminoacyl-tRNA synthetases.

Aminoacyl-tRNA synthetases catalyze AMP----ADP----ATP exchange reactions, indicating labile covalent enzyme-amino-acid intermediates.