tRNA Activation

Image depicting an aminoacyl-tRNA, or activated tRNA

Core Concepts

In this article, you will learn about tRNA activation, the enzymes that facilitate it, and the steps in the tRNA activation reaction.

Concepts Covered in Other Articles


Transfer RNA molecules, also called tRNAs, have a crucial role in translation, the process of converting mRNA into a protein. tRNAs carry amino acids to their corresponding mRNA codons to build the amino acid chain. Before translation can occur, tRNAs have to bind with the appropriate amino acid. A process called tRNA activation, or tRNA charging, matches tRNAs with their corresponding amino acids. In this article, we will learn about the tRNA activation reaction and the enzymes that mediate this reaction.

Aminoacyl-tRNA Synthetase

There are two classes of aminoacyl-tRNA synthetases. Class I synthetases aminoacylate (or attach the amino acid) at the 3’ hydroxyl group of the tRNA’s 3’ end. Class II synthetases aminoacylate at the 2’ hydroxyl group of the tRNA’s 3’ end. Both classes attach the amino acid’s carboxyl group to the tRNA’s hydroxyl group.

Reaction Steps of tRNA Activation

tRNA activation, also known as tRNA charging or tRNA aminoacylation, is a two-step exergonic reaction.

tRNA Activation Step One

In the first step, the aminoacyl-tRNA synthetase binds its target amino acid as well as an ATP molecule. ATP releases pyrophosphate, leaving behind AMP. Then, the released pyrophosphate is hydrolyzed. This pyrophosphate hydrolysis releases a large amount of energy, which drives the other reactions  and makes them energetically favorable. Once pyrophosphate has been released, AMP can covalently bind to the amino acid’s carboxyl group by its phosphate group. This creates a 5’ aminoacyl adenylate intermediate.

Step one of the reaction is as follows: 

The first step of tRNA Activation
A graphic representation of the first step in tRNA activation

tRNA Activation Step Two

In the second step, the aminoacyl adenylate intermediate undergoes nucleophilic attack by tRNA’s 2’ or 3’ hydroxyl group and displaces AMP. Then, the aminoacyl-tRNA synthetase releases aminoacyl-tRNA, the final product of the reaction. The ester bond between the amino acid and the tRNA molecule stores energy, which is used to form peptide bonds between amino acids during translation.

Step two of the reaction is as follows:

Step 2 in tRNA Activation
A graphic representation of the second step in tRNA activation

Overall Reaction

The overall, net reaction is as follows:

The net reaction of tRNA activation

More on Aminoacyl-tRNA Synthetases- Expanding the Genetic Code

Image showing how orthogonal tRNA synthetases are used to expand the genetic code.

Aminoacyl tRNA synthetases have become a key component in the field of genetic code expansion technology. Scientists are working to create unnatural amino acids (UAAs), partially by engineering new aminoacyl tRNA synthetases. These UAAs have a wide range of applications. They can help organisms to synthesize helpful molecules, which has aided in development of vaccines, protein visualization, and small molecule regulation.

The field is expansive, and many different projects are being tackled. Read more about the applications of expanding the genetic code in this review article. You can also read more about the chemistry of this process in this article.