Aminoacyl-tRNA synthetases action

Amino acids must be activated by the formation of aminoacyl-tRNAs by the action of aminoacyl-tRNA synthetases. 

The importance of the phosphoenzyme in the mechanism of action of succinyl-CoA synthetase in reactions (27a)-(27c) is also unknown. The mechanisms of action of aminoacyl-tRNA synthetases and of acyl-CoA synthetases do not include covalent enzymic intermediates. The fact that succinyl-CoA synthetase involves succinyl phosphate as the activated substrate, whereas the others involve acyl adenylates, does not explain the difference. There is no chemical catalytic basis for the mechanisms of the formation of these intermediates to vary in this way. Moreover, acetate kinase produces acetyl phosphate without the intermediate formation of a phosphoenzyme, so that at least acetate kinase has the capacity to catalyze direct phosphorylation of a carboxylate group. 

FtmdlOH. tRNA is esterified with its specific amino add by the action of Aminoacyl-tRNA synthetase (see). The resulting aminoacyl-tRNA becomes bound to the acceptor site of the SOS-subunit of a ribosome, where antiparallel base pairing occurs between the anticodon of the tRNA and the complementary codon of the assodated mRNA. The spedficity of this base pairing insures that the amino add is incorporated into the correct position in the growing polypeptide chain. During translation the deacylat tRNA is released from the ribosome and becomes available for recharging with its amino add. 

Figure 29-9 Selected views of aminoacyl-tRNA sjmthetase stmcture and action. (A) Alpha-carbon trace of the type IE. coli glutaminyl-tRNA synthetase. The phosphate backbone of tRNA " is shown in black ATP is shown in the active-site cleft. The canonical dinucleotide fold domain near the N terminus is shaded. Two structural motifs (black), proposed to link the active site with regions of the protein-RNA interface involved in tRNA discrimination, are indicated. The a helix (top) connects tRNA recognition in the minor groove of the acceptor stem with binding of the ribose group of ATP. The large loop (center) connects anticodon recognition by the two P-barrel domains (bottom) with sequences flanking the MSK sequence motif, which interacts with the phosphates of ATP. From Perona et Courtesy of Thomas A. Steitz. (B) The active site...

Interactions between each tRNA species and its own aminoacyl synthetase are among the most specific of all such interactions, since the synthetase distinguishes both the tRNA and its amino acid. The fidelity of protein synthesis depends upon this step, since the action oftRNA-aminoacyl synthetase results in translation of the nucleotide code into the amino acid sequence. Movement of the tRNAs within the ribosomes is mediated by certain ribosomal proteins. 

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