The key to specificity between an amino acid and its tRNA is a set of enzymes called aminoacyl-tRNA synthetases. A cell has 20 different aminoacyl-tRNA synthetases, one for each of the 20 amino acids. Each synthetase recognizes a particular amino acid, as well as all the tRNAs that accept that amino acid. Recognition of the appropriate amino acid by a synthetase is based on the different sizes, charges, and R groups of the amino acids. The tRNAs, however, are all similar in tertiary structure. How does a synthetase distinguish among tRNAs?
The recognition of tRNAs by a synthetase depends on the differing nucleotide sequences of tRNAs. Researchers have identified which nucleotides are important in recognition by altering different nucleotides in a particular tRNA and determining whether the altered tRNA is still recognized by its synthetase. The results of these studies revealed that the anticodon loop, the DHU-loop, and the acceptor stem are particularly critical for the identification of most tRNAs (< Figure 15.17).
The attachment of a tRNA to its appropriate amino acid (termed tRNA charging) requires energy, which is supplied by adenosine triphosphate (ATP):
aminoacyl-tRNA + AMP + PPi
Two phosphates are cleaved from ATP, producing adenosine monophosphate (AMP) and pyrophosphate (PPi), as well as the aminoacylated tRNA (the tRNA with its attached
In the first step, the amino acid reacts with ATP,.
I .producing aminocyl-AMP and PP;
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