Aminoacyl tRNA synthetase

 Aminoacyl tRNA synthetases are enzymes which catalyse tRNA molecules linking to their corresponding amino acid to create aminoacyl tRNAs (or charged tRNAs). They can also be referred to as activating enzymes.

Aminoacylation of tRNAs

tRNA molecules joined to their corresponding amino acid are called aminoacyl tRNAs, this reaction is called aminoacylation and is a 2 step reaction driven by ATP and catalysed by aminoacyl tRNA synthetases. Amino acids are activated by adenylation by reacting with ATP to form aminoacyl adenylate (aminoacyl AMP), a high energy intermediate, and pyrophosphate. The next step is the transfer of the aminoacyl group from aminoacyl adenylate to a tRNA molecule to make an aminoacyl tRNA and release AMP[1]. 

Aminoacyl tRNA synthetases

Aminoacylation must be highly specific as the correct amino acids must be on the correct tRNAs for a functional protein to be synthesised. Aminoacyl tRNA synthetases must be able to distinguish not only between approximately 40 similarly shaped tRNA molecules, but also very similar amino acids acids such as serine, threonine and valine. These enzymes use particular 'identity elements' in different tRNAs, which can be located in the anticodon region, amino acid acceptor stem, variable arm, or a mixture of them. In order to discriminate between different amino acids, aminoacyl tRNA synthetases have a proofreading mechanism. They have an acylation (activation) site and some also have an editing site (not all tRNA synthetases have this, only when the enzyme needs an additional mechanism to distinguish between amino acids). The acylation site rejects amino acids which are too large to fit in the site, and in doing so will accept the correct amino acid, but also other amino acids which are smaller than the correct one. Amino acids which fit into the editing site are hydrolytically cleaved as they are identified as being smaller than the correct amino acid, so only the correct amino acid will remain. This proofreading mechanism greatly improvesprotein synthesis fidelity so mistakes are less than 1 in 10,000 amino acids[2].