On the non‐liner Eadie plots of the tRNA kinetics and non‐liner Dixon plots of the PPi inhibition kinetics of the aminoacyl‐tRNA synthetases

A model of the aminoacyl-tRNA synthetase reaction was analyzed by deriving a rate equation, and by calculating the aminoacylation rates at various values of the rate and equilibrium constants. The model specially contained the possibilities that (1) the activation of the amino acid occurs either with bound or non-bound tRNA, and that (2) the transfer of the aminoacyl moiety from the aminoacyl adenylate to tRNA occurs either with bound or non-bound PPi. The analysis showed that the Eadie plots (tRNA as the variable substrate) are straight lines only if the rates of the activation reactions with bound and non-bound tRNA are equal. Otherwise the Eadie plots can be either curved upwards or downwards. The Dixon plots of the PPi inhibition are straight lines only if PPi must be dissociated from the enzyme before the transfer reaction. The conditions under which the Kiapp values are much lower than the dissociation constants for PPi are met if the transfer reaction is relatively slow and the reverse reaction of the activation (pyrophosphorolysis) is fast, and if the tRNA concentration is low.