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.
Aminoacyl tRNA synthetases
Aminoacylation are highly specific, as the correct amino acids must be linked to 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 any combination of the three . 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 incorrect amino acids, which are too large to fit in the site accepting the correct amino acid, and smaller incorrect amino acids. In order to then distinguish between the correct amino acid and smaller amino acids, the editing site will only hydrolytically cleave amino acids, which are smaller than the correct one. This proofreading mechanism greatly improves protein synthesis fidelity so mistakes are less than 1 in 10,000 amino acids.