Aa + ATP + tRNA^=Aa-tRNA + AMP + PPi, where Aa is the amino acid, Aa-tRNA is the aminoacyl-tRNA, and PPi. is the inorganic pyrophosphate. It has been demonstrated that the enzyme catalyzes two different reactions comprising two consecutive steps of the above process.

A reaction proceeding at the first stage that is catalyzed by ARSase is the so-called amino acid activation, where the carboxyl group of the amino acid attacks the bond between the a- and b-phosphates of ATP, resulting in the formation of a mixed anhydride aminoacyl adenylate and inorganic pyrophosphate (Fig. 3.12):

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