Formation of Aminoacylated tRNAs

Amino acids are linked to tRNA molecules by a covalent bond between the carboxyl group of the amino acid and a ribose hydroxyl group of the 3' adenosine of the acceptor stem of the tRNA. A tRNA molecule bonded to it amino acid is said to be "aminoacylated" and the enzymes that catalyze this reaction are called aminoacyl-tRNA synthetases.

E. coli has 20 aminoacyl-tRNA synthetases, each of which recognizes one particular amino acid and one or more tRNAs. There are two general classes of aminoacyl-tRNA synthetases (I and II). They 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. When the amino acid is attached, the amino acid is referred to as "charged."

Some bacteria employ an unusual pathway in which glutamine is incorporated into gln-tRNA without a glutamine-specific aminoacyl tRNA-synthetase. In this case, the synthetase for glutamate is used and glutamine is created by transamination, as follows:

> Glu + tRNAGln <=> Glu-tRNAGln + AMP + PPi

> Gln + Glu-tRNAGln + ATP <=> Gln-tRNAGln + ADP + Pi + Glu

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.

See also: Structure of tRNAs, The Genetic Code, Translation Overview

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