SYNTHESIS OF CYSTEINE-CONTAINING DIPEPTIDES BY AMINOACYL-TRANSFER-RNA SYNTHETASES

Arginyl-tRNA synthetase (ArgRS) catalyses AMP- and PPi-independent deacylation of Arg-tRNA(Arg) in the presence of cysteine. A dipeptide, Arg-Cys, is a product of this deacylation reaction. Similar reaction with homocysteine yields Arg-Hcy. Arginine is a noncompetitive inhibitor of the cysteine-dependent deacylation which indicates that cysteine binds to the enzyme-Arg-tRNA(Arg) complex at a site separate from the arginine binding site. In the presence of arginine, [C-14]Arg-tRNA(Arg) is deacylated at a rate similar to the rate of its spontaneous deacylation in solution and [C-14]arginine is a product. Experiments with cysteine derivatives indicate that the -SH group is essential for the reaction whereas -NH2 and -COOH groups are not. Thioesters of arginine are formed with 3-mercaptopropionic acid, N-acetyl-L-cysteine and dithiothreitol. These data suggest that formation of the dipeptide Arg-Cys involves a thioester intermediate, S-(L-arginyl)-L-cysteine, which is not observed because of the rapid rearrangement to form a stable peptide bond. Facile intramolecular reaction results from the favorable geometric arrangement of the a-amino group of cysteine with respect to the thioester formed in the initial reaction. Similar reactions, yielding Ile-Cys and Val-Cys, are catalyzed by isoleucyl and valyl-tRNA synthetases, respectively.