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#Unit 3. Fundamental Processes
ThrRS has the opposite problem: It must synthesize Thr–tRNAThr but not Val–tRNAThr. Specificity is conferred by the aminoacylation site, which contains a Zn2+ ion that is coordinated by the side chain OH group of threonine. Valine cannot coordinate the Zn2+ in this way and hence does not undergo adenylylation by ThrRS. A separate editing site deals with misacylated Ser–tRNAThr. TyrRS distinguishes between tyrosine and phenylalanine through hydrogen missing from the repertoire are GlnRS and asparaginyl-tRNA synthetase (AsnRS). To synthesize Gln-tRNAGln and Asn-tRNAAsn, these organisms possess distinct glutamyl-tRNA synthetase (GluRS) and aspartyl-tRNA synthetase (AspRS) enzymes that are nondiscriminating (ND). GluRSND synthesizes both Glu-tRNAGlu as well as misacylated Glu-tRNAGln; AspRSND synthesizes both Asp-tRNAAsp and misacylated Asp-tRNAAsn.