The human alanyl-tRNA synthetase (AARS) belongs to a family of tRNA synthases,of the class II enzymes.Class II tRNA synthases evolved early in evolution and are highly conserved.This is reflected by the fact that 498 of the 968-residue polypeptide human AARS shares 41% identity witht the E.coli protein.tRNA synthases are the enzymes that interpret the RNA code and attach specific aminoacids to the tRNAs that contain the cognate trinucleotide anticodons.They consist of a catalytic domain which interacts with the amino acid acceptor-T psi C helix of the tRNA, and a second domain which interacts with the rest of the tRNA structure.