Aminoacyl tRNA synthetase

An aminoacyl tRNA synthetase (aaRS) is an enzyme that catalyzes the esterification of a specific amino acid to one of all its compatible cognate tRNAs to form an aminoacyl-tRNA. The synthetase first binds ATP and the cognate amino acid to form an aminoacyl-adenylate and release inorganic pyrophosphate (PPi). The adenylate-aaRS complex then binds the appropriate tRNA molecule, and the amino acid is transferred from the aa-AMP to either the 2'- or 3'-OH of the last tRNA base (A76) at the 3'-end. Some synthetases also mediate a proofreading reaction to ensure high fidelity of tRNA charging; if the tRNA is found to be improperly charged, the aminoacyl-tRNA bond is hydrolyzed.

Reaction:
 * 1) amino acid + ATP → aminoacyl-AMP + PPi
 * 2) aminoacyl-AMP + tRNA → aminoacyl-tRNA + AMP

Sum of 1 and 2: amino acid + tRNA + ATP → aminoacyl-tRNA + AMP + PPi

Classes
There are two classes of aminoacyl tRNA synthetase:
 * Class I has two common homologous peptide sequence motifs. It aminoacylates at the 2'-OH.
 * Class II has three common homologous peptide sequence motifs. It aminoacylates at the 3'-OH.

The one exception to the above rule is PheRS, a class II enzyme that attaches Phenylalanine to the 2'-OH of tRNAPhe.