Threonyl-tRNA synthetase, active site

Threonyl-tRNA synthetase can be incubated with tUNA hr that has been covalently linked with serine (Ser-tRNAThr) the tRNA has been "mischarged." The reaction is immediate a rapid hydrolysis of the aminoacyl-tRNA forms serine and free tRNA. In contrast, incubation with correctly charged Thr-tRNAThr results in no reaction. Thus, threonyl-tRNA synthetase contains an additional functional site that hydrolyzes Ser-tRNA hr but not Thr-tHNA hr This editing site provides an opportunity for the synthetase to correct its mistakes and improve its fidelity to less than one mistake in 10. The results of structural and mutagenesis studies revealed that the editing site is more than 20 A from the activation site (Figure 29.9). This site readily accepts and cleaves Ser-tRNAThr but does not cleave Thr-tRNA hr The discrimination of serine from threonine is relatively easy because threonine contains an extra methyl group a site that conforms to the structure of serine will sterically exclude threonine. 

R. Sankaranarayanan, A.C. Dock-Bregeon, P. Romby, J. Caillet, M. Springer, B. Rees, C. Ehresmann, B. Ehresmann, and D. Moras. 1999. The structure of threonyl-tRNA synthetase-tRNA(Thr) complex enlightens its repressor activity and reveals an essential zinc ion in the achve site Cell 97 371-381. (PubMed)... 

Figure 30.7 Active site of threonyl-tRNA synthetase. Notice that the amino acid-binding site includes a zinc ion (green ball) that coordinates threonine through its amino and hydroxyl groups.

Active site of threonyl-tRNA synthetase Figure 30.7... 

Coordination of the threonine hydroxyl by an active site Zn in the threonyl-tRNA synthetase allows discrimination between threonine and the isosteric valine (Sankaranarayanan et al., Nat. Struct. Biol. 7[2000] 461-465). Given the similarity of serine and threonine (Ser lacks only the methyl group of Thr), if this is the only mechanism for amino acid discrimination available, threonyl-tRNA synthetase mistakenly couples Ser to threonyl-tRNA at a rate several-fold higher than it does threonine. Since this would lead to unacceptably high error rates in translation, how it is it avoided ... 

Threonyl-tRNA synthetase has a proofreading mechanism. Any Ser-tRNA that is mistakenly formed is hydrolyzed by an editing site 20 A from the activation site. The decision to hydrofyze the aminoacyl-tRNA appears to depend on the size of the amino acid substituent, ff it is smaffer than the correct amino acid, the amino acid fits into the hydrolytic site and is cleaved. If it is the same size as the correct amino acid, it does not fit and is not destroyed. Discrimination between amino acids that are larger than the correct one or are not isoelectronic with it occurs at the aminoacylation step. 

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