Aminoacyl tRNA synthetase

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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 (PP_i). 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:

amino acid + ATP → aminoacyl-AMP + PPi
aminoacyl-AMP + tRNA → aminoacyl-tRNA + AMP

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

[edit] 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 tRNA^Phe.