Formation of amino-acyl tRNA

Activation of amino acid
This reaction is brought about by the binding of an amino acid with ATP. The step is mediated by specific activating enzymes known as amino acyl RNA synthetases (amino acid is abbreviated as AA). As a result of this reaction between amino acid and adenosine triphosphate, mediated by specific enzyme, a complex (amino acyl-AMP-enzyme complex) is formed. It should be realized that amino acyl-RNA synthetases are specific with respect to amino acids. Therefore, for different amino acids, different enzymes would be required. Moreover a particular enzyme has a double specificity in so far as it can recognize its own amino acid on the one hand and its own tRNA on the other hand. This later specificity allows the second step outlined in the next section.


The transfer of amino acid to tRNA
The amino acyl-AMP-enzyme complex, formed during the step outlined above, reacts with a particular tRNA and transfers the amino acid to the tRNA. This reaction is accompanied with the liberation of AMP and the enzyme. It should be noticed in this reaction that with AA1; Enz1 and tRNA1, 1 is used as suffix to emphasize the specificity, which exists between them. A particular amino acid would require a particular enzyme and a particular species of tRNA. This would mean that for 20 amino acids, atleast 20 different enzymes and also atleast 20 different tRNA species would be required (while there are cases where more than one kind of tRNA may be available for the same amino acid, no cases are known, where the same tRNA can recognize two or more different amino acids).