Figure 2. A) Active site comparison showing homologous residues interacting with amino acid and adenosine moiety in B.

Figure 2

A) Active site comparison showing homologous residues interacting with amino acid and adenosine moiety in B. stearothermophilus TrpRS (black) and B. stearothermophilus TyrRS (gray) (adapted from ref. 12). Interactions in reversed contrast are not observed in TyrRS. Hydrogen bonded distances are given for both enzymes. The grey patch includes homologous interactions to the amino acid. The tyrosine moiety is not shown for TyrRS. Interactions of homologous residues Asp 132 and Asp 176 help to distinguish between the two side chains, as the former is positioned further into the site to form a hydrogen bond to the indole heterocyclic nitrogen, while the latter is positioned further away, in order to form a hydrogen bond to the phenolic hydroxyl group. Mutational analysis of amino acid specificity-determining sidechains is discussed in ref. 30 Interactions with the pyrophosphate leaving group of ATP are described below and shown in Figure 10. Corresponding residues observed in prokaryotic and eukaryotic Trp- and TyrRSs are indicated for amino acids at position 5 (β2) and 132 (α8) in Bst TrpRS, identified by Yang et al, as crucial to the speciation of specificity along the pro- and eukaryotic lineages of the two enzymes. The chemical roles of the two italicized residues are inversed in human (eukaryotic) TrpRS. B) Ribbon-stick drawing of the Bst TrpRS tryptophan-binding site, showing structural relationships involved in specificity determination.

From: Trytophanyl-tRNA Synthetases

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