title: Differences in the magnesium dependences of the class I and class II aminoacyl-tRNA synthetases from Escherichia coli.
abstract: The magnesium dependences of the ATP/PPi exchange and tRNA aminoacylation of reactions were measured for six aminoacyl-tRNA synthetases (isoleucyl-, tyrosyl- and arginyl-tRNA synthetases from class I, and histidyl-, lysyl- and phenylalanyl-tRNA synthetases from class II). The measured values were subjected to best-fit analyses using sum square error calculations between the data and the calculated curves in order to find the mode of participation of the Mg2+ and to optimize the sets of the kinetic constants. The following four dependences were observed: the class II synthetases require three Mg2+ for the activation reaction (including the one in MgATP), but the class I synthetases require only one Mg2+ (in MgATP); in class II synthetases both MgPPi and Mg2PPi participate in the pyrophosphorolysis of the aminoacyl adenylate. Arginyl-tRNA synthetase from class I also shows a better fit if also Mg2PPi reacts, but in the isoleucyl- and tyrosyl-tRNA synthetases only MgPPi but not Mg2PPi is used in the pyrophosphorolysis. Different synthetases have different requirements for the tRNA-bound Mg2+ and spermidine, independent of the enzyme class. 1-4 Mg2+ or spermidines are required in the best fit models. At the end of the reaction in all the synthetases analysed the dissociation of Mg2+ from the product aminoacyl-tRNA essentially enhances the subsequent dissociation of the aminoacyl-tRNA from the enzyme. The binding of ATP to the E. aminoacyl-tRNA complex also speeds up the dissociation of the aminoacyl-tRNA from most of these enzymes.
authors.affiliation: Department of Biochemistry, University of Turku, Finland.
authors.forename: R K
authors.initials: RK
authors.lastname: Airas
mesh_terms: D000604:Amino Acyl-tRNA Synthetases / Q000145:classification / Q000302:isolation & purification / Q000378:metabolism*; D001126:Arginine-tRNA Ligase / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology*; D006643:Histidine-tRNA Ligase / Q000378:metabolism; D007533:Isoleucine-tRNA Ligase / Q000378:metabolism; D007700:Kinetics; D008250:Lysine-tRNA Ligase / Q000378:metabolism; D008274:Magnesium / Q000494:pharmacology*; D008956:Models, Chemical; D010652:Phenylalanine-tRNA Ligase / Q000378:metabolism; D013095:Spermidine / Q000494:pharmacology; D014448:Tyrosine-tRNA Ligase / Q000378:metabolism
chemical_list: D000604:Amino Acyl-tRNA Synthetases; D014448:Tyrosine-tRNA Ligase; D001126:Arginine-tRNA Ligase; D010652:Phenylalanine-tRNA Ligase; D006643:Histidine-tRNA Ligase; D007533:Isoleucine-tRNA Ligase; D008250:Lysine-tRNA Ligase; D008274:Magnesium; D013095:Spermidine