Synthases Attach Amino Acids to tRNAs

A unique class of enzymes, called aminoacyl-tRNA synthases, attach amino acids to their cognate tRNAs. This attachment serves two functions (1) The linkage between 

All synthase reactions proceed in two separate steps (fig. 29.9). In the first step the synthase recognizes its corresponding amino acid and its second substrate, ATP, and forms a mixed anhydride bond between the carboxyl group of the amino acid and the phosphate of AMP with the release of PPj. 

The equilibrium constant for this reaction is about 1, so that the energy derived from the cleavage of the phosphate anhydride of ATP is conserved in the mixed anhydride. Aminoacyl-AMP remains tightly bound to the enzyme and, as we will soon show, this fact has allowed researchers to crystallize this important complex and analyze its structure. 

The second reaction catalyzed by the aminoacyl-tRNA synthases results in the attachment of the amino acid through an ester linkage to the 3 terminal ribose of tRNA  

Synthases differ with respect to their site of attachment to tRNA. Some synthases form the 2 ester, some form the 3 ester, and still others produce a mixture of the two. The specificity of the synthases was determined by analyzing their ability to act on tRNA derivatives lacking one or the other terminal hydroxyl group. Once esterified to the terminal ribose, the aminoacyl group can migrate between the vicinal 2 and 3 hydroxyl groups. Thus, in cells, amino-acyl-tRNAs are mixtures of 2 and 3 esters. Only the 3 derivative is a substrate for the subsequent transpeptidation reaction catalyzed by the ribosome. 

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Synthases Attach Amino Acids to tRNAs Each Synthase Recognizes a Specific Amino Acid and Specific Regions on Its Cognate tRNA Aminoacyl-tRNA Synthases Can Correct Acylation Errors... 

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