Stabilization of the transition state for the transfer of tyrosine to tRNATyr by tyrosyl-tRNA synthetase

Aminoacylation of tRNATyr involves two steps: (1) tyrosine activation to form the tyrosyl-adenylate intermediate; and (2) transfer of tyrosine from the tyrosyl-adenylate intermediate to tRNATyr. In Bacillus stearothermophilus tyrosyl-tRNA synthetase, Asp78, Tyr169, and Gln173 have been shown to form hydrogen bonds with the α-ammonium group of the tyrosine substrate during the first step of the aminoacylation reaction. Asp194 and Gln195 stabilize the transition state complex for the first step of the reaction by hydrogen bonding with the 2′-hydroxyl group of AMP and the carboxylate oxygen atom of tyrosine, respectively. Here, the roles that Asp78, Tyr169, Gln173, Asp194, and Gln195 play in catalysis of the second step of the reaction are investigated. Pre-steady-state kinetic analyses of alanine variants at each of these positions shows that while the replacement of Gln173 by alanine does not affect the initial binding of the tRNATyr substrate, it destabilizes the transition state complex for the second step of the reaction by 2.3 kcal/mol. None of the other alanine substitutions affects either the initial binding of the tRNATyr substrate or the stability of the transition state for the second step of the aminoacylation reaction. Taken together, the results presented here and the accompanying paper are consistent with a concerted reaction mechanism for the transfer of tyrosine to tRNATyr, and suggest that catalysis of the second step of tRNATyr aminoacylation involves stabilization of a transition state in which the scissile acylphosphate bond of the tyrosyl-adenylate species is strained. Cleavage of the scissile bond on the breakdown of the transition state alleviates this strain.