Amino acyl tRNA synthetases are enzymes that catalyze the covalent joining of an amino acid to its specific tRNA molecule. E. coli has 20 aminoacyl-tRNA synthetases, each of which recognizes one particular amino acid and one or more tRNAs. The two general classes of aminoacyl-tRNA synthetases (I and II) differ in amino acid sequence, the ways in which they bind their cognate tRNAs, and in their quaternary structures.

Figure 27.10 shows the sequence of reactions that covalently links an amino acid to its tRNA. The reaction proceeds via an aminoacyl adenylate intermediate, and AMP is released as the amino acid is joined to the proper tRNA.

How is the proper tRNA recognized by the aminoacyl-tRNA synthetase? Interestingly, the anticodon can be all of, part of, or no part of the recognition site for the enzyme. As a result, other regions of the tRNA besides the anticodon must be involved in identifying a tRNA. Figure 27.11 shows the identity elements known for class I and class II tRNAs.

Aminoacyl tRNA-synthetases have a proofreading ability to double-check that an amino acid is linked with its proper tRNA. The proofreading ability of the enzyme and other proofreading steps (see here) in translation reduce the error frequency to less than 1 in 10,000.