It is noteworthy that different ARSases possess different specificity with regard to the position of the ribose hydroxyl participating in the transacylation reaction (Table 3.1). All class I ARSases catalyze the coupling of amino acids to the 2'-position of the ribose of the 3'-terminal adenosine residue. TyrRS and CysRS, however, may catalyze the reaction with both the 2'- and the 3'-hydroxyl groups. At same time class II synthetases catalyze the reaction of the 3'-hydroxyl with the amino acid residue; the only exception among them is PheRS that ligates the amino acid to the 2'-position of tRNA. This is of no great importance to the subsequent fate of the aminoacyl-tRNA formed because in an aqueous solution the aminoacyl residue spontaneously migrates between the 2'- and 3'-positions (through the formation of 2', 3'-derivative), and eventually the two forms are in equilibrium.

Thus, an ARSase uses three substrates of a different chemical nature: ATP, an amino acid, and tRNA. Correspondingly, it must possess three different substrate-binding sites. ATP is the universal substrate for all ARSases, whereas for the amino acid and tRNA, each ARSase displays high specificity.

As has already been mentioned, in many cases ARSases are dimers or pseudodimers, and, correspondingly, they possess two sets of substrate-binding sites. The substrate-binding sites both within each subunit (or the equivalent domain) and on different subunits (or domains) are interdependent. Frequently synergism is observed: the binding of one substrate molecule facilitates the binding of the other. On the other hand, there is a negative cooperativity in the binding of two tRNA molecules: the

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