title: Aminoacyl thioester chemistry of class II aminoacyl-tRNA synthetases.
abstract: Lysyl-tRNA synthetase, a class II enzyme, edits homocysteine by converting it into homocysteine thiolactone. In a similar reaction, the enzyme converts homoserine into homoserine lactone. Other class II enzymes, aspartyl-tRNA synthetase and seryl-tRNA synthetase, do not edit any of the amino acids tested. However, all three class II aminoacyl-tRNA synthetases catalyze AMP- and pyrophosphate-independent deacylation of cognate aminoacyl-tRNA in the presence of thiols, mimicking editing of homocysteine. Thiol-dependent deacylations exhibit saturation kinetics with respect to concentration of thiols, suggesting the presence of a thiol binding site on each enzyme. 3-Mercaptopropionate-, N-acetyl-L-cysteine-, and dithiothreitol-dependent deacylations of aminoacyl-tRNA yield corresponding aminoacyl thioesters. Cysteine-dependent enzymatic deacylations of aminoacyl-tRNA by these class II enzymes yield dipeptides, N-(aminoacyl)cysteine. The formation of N-(aminoacyl)cysteine involves thioester intermediates S-(aminoacyl)-L-cysteine, which are not observed because of the facile transacylation of the aminoacyl residue from the sulfur to the alpha-amino group of cysteine to form a stable peptide bond. These data indicate that class II aminoacyl-tRNA synthetases possess unique thiol-binding subsites within their active sites. That the thiol-binding subsite exists also in AspRS and SerRS, which do not need editing function, suggests that these class II enzymes possess vestigial editing functions.
authors.affiliation: Department of Microbiology and Molecular Genetics, University of Medicine and Dentistry of New Jersey-New Jersey Medical School, 185 South Orange Avenue, Newark, New Jersey 07103, USA. jakubows@umdnj.edu
authors.forename: H
authors.initials: H
authors.lastname: Jakubowski
mesh_terms: D000249:Adenosine Monophosphate / Q000378:metabolism; D000604:Amino Acyl-tRNA Synthetases / Q000737:chemistry* / Q000378:metabolism; D001226:Aspartate-tRNA Ligase / Q000378:metabolism; D001665:Binding Sites; D002384:Catalysis; D006710:Homocysteine / Q000378:metabolism; D006714:Homoserine / Q000378:metabolism; D008250:Lysine-tRNA Ligase / Q000737:chemistry / Q000378:metabolism; D010957:Plasmids / Q000378:metabolism; D012714:Serine-tRNA Ligase / Q000378:metabolism; D013438:Sulfhydryl Compounds / Q000378:metabolism
chemical_list: D013438:Sulfhydryl Compounds; D006710:Homocysteine; D000249:Adenosine Monophosphate; D006714:Homoserine; D000604:Amino Acyl-tRNA Synthetases; D012714:Serine-tRNA Ligase; D001226:Aspartate-tRNA Ligase; D008250:Lysine-tRNA Ligase