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alaS | alanyl-tRNA synthetase; Catalyzes the attachment of L-alanine to tRNA(Ala) in a two- step reaction: L-alanine is first activated by ATP to form Ala-AMP and then transferred to the acceptor end of tRNA(Ala). AlaRS also incorrectly activates the sterically smaller amino acid glycine as well as the sterically larger amino acid L-serine; generates 2-fold more mischarged Gly than Ser. These mischarged amino acids occur because the of inherent physicochemical limitations on discrimination between closely related amino acids (Ala, Gly and Ser) in the charging step. Attaches Ala to transfer-me [...] (876 aa) | ||||
thrS | threonyl-tRNA synthetase; Catalyzes the attachment of threonine to tRNA(Thr) in a two- step reaction: L-threonine is first activated by ATP to form Thr-AMP and then transferred to the acceptor end of tRNA(Thr). The rate-limiting step is amino acid activation in the presence of tRNA. The 2'-OH of the acceptor base (adenine 76, A76) of tRNA(Thr) and His-309 collaborate to transfer L-Thr to the tRNA; substitution of 2'-OH of A76 with hydrogen or fluorine decreases transfer efficiency 760 and 100-fold respectively. The zinc ion in the active site discriminates against charging of the isost [...] (642 aa) |