The twenty aminoacyl-tRNA synthetases catalyse the attachment of an amino acid to its cognate transfer RNA (tRNA) molecule in a highly specific two-step reaction. All of these proteins fall into one of two classes comprised of ten enzymes each: class 1 (Arg, Cys, Glu, Gln, Ile, Leu, Met, Tyr, Trp and Val) and class 2 (Ala, Asn, Gly, His, Lys, Phe, Pro, Ser, and Thr). Class 1 enzymes are mostly monomeric, and contain a characteristic Rossman binding fold that bind the tRNA acceptor stem from the minor groove side, using two highly conserved sequences. In contrast, class 2 enzymes share an anti-parallel beta-sheet formation that binds to the major groove side of the acceptor stem. Based on their mode of binding to the tRNA acceptor stem, both classes of tRNA synthetases have been subdivided into three subclasses, designated 1a, 1b, 1c and 2a, 2b, 2c. Class 1a (Arg, Cys, Ile, Leu, Met, Val) possess an RNA-binding domain with an alpha-helix-bundle fold; the binding of the anticodon of tRNA to the RNA-binding domain induces a conformation change in the catalytic domain of the enzyme .