7.4.1: Explain that each tRNA molecule is recognized by a tRNA activating
enzyme that binds a specific amino acid to the tRNA, using ATP for energy.
The role or tRNA activating enzymes is to attach a specific amino acid to a specific
tRNA molecule. When an amino acid has been attached to the tRNA molecule, the
tRNA molecule is said to be activated.
There are 20 different amino acids and there is a specific tRNA activating enzyme
for each of the 20 amino acids. There are however 61 different tRNA molecules. The
differences are based on the different sequences of bases on the anticodon of each
tRNA molecule. So some amino acids will attach correctly to multiple tRNA
molecules. Each tRNA activating enzyme molecule specific for one amino acid must
recognise all the different tRNA molecules that will code for one specific amino acid.
The attachment of an amino acid to a tRNA molecule requires energy in the form of
ATP. As seen in Figure 1 each tRNA activating enzyme has three binding sites. One
site is for the amino acid another site for the tRNA molecule and the third site for the
attachment of ATP.
Figure 1: tRNA molecule showing the three binding sites.
Once an amino acid has been attached to the tRNA molecule the tRNA molecule
transfers the amino acid to a ribosome for the process of translation.