The Binding of Amino Acids To Transfer RNAs

The first stage of translation is the binding of tRNA molecules to their appropriate amino acids. As we have seen, although there may be several different tRNAs for a particular amino acid, each tRNA is specific for only one amino acid. 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.

The attachment of a tRNA to its appropriate amino acid, termed tRNA charging, requires energy, which is supplied by adenosine triphosphate (ATP):

amino acid + tRNA + ATP → aminoacyl-tRNA + AMP + PPi

The carboxyl group (COO−) of the amino acid is attached to the adenine nucleotide at the 3′ end of the tRNA (Figure 11.8). To identify the resulting aminoacylated tRNA, we write the three-letter abbreviation for the amino acid in front of the tRNA; for example, the amino acid alanine (Ala) attaches to its tRNA (tRNAAla), giving rise to its aminoacyl-tRNA (Ala-tRNAAla).

image
Figure 11.8: An amino acid attaches to the 3′ end of a tRNA. The carboxyl group (COO−) of the amino acid attaches to the oxygen of the 2′− or 3′-carbon atom of the final nucleotide at the 3′ end of the tRNA, in which the base is always adenine.

CONCEPTS

Amino acids are attached to specific tRNAs by aminoacyl-tRNA synthetases in a reaction that requires ATP.

image CONCEPT CHECK 4

Amino acids bind to which part of the tRNA?

  1. anticodon

  2. codon

  3. 3′ end

  4. 5′ end

c