Threonine--tRNA ligase

UniProtKB accession:  P0A8M3

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UniProtKB description:  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) (PubMed:15079065, PubMed:10881191, PubMed:18997014). The rate-limiting step is amino acid activation in the presence of tRNA (PubMed:18997014). 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 (PubMed:18997014). The zinc ion in the active site discriminates against charging of the isosteric amino acid valine (PubMed:10881191). Also activates L-serine, but does not detectably transfer it to tRNA(Thr) (PubMed:15079065). Edits incorrectly charged L-seryl-tRNA(Thr) via its editing domain (PubMed:15079065, PubMed:11136973, PubMed:15525511), in a post-transfer reaction probably via water-mediated hydrolysis (PubMed:15525511).