Class II aminoacyl-tRNA synthetases aminoacylate the 3'OH of their cognate tRNAs.
The active site to HisRS contains a histidine binding pocket composed of highly conserved residues found in distinct sequences motifs. First, the LV/AAGGGLDYY loop (or HisA loop) forms one wall of the binding pocket. This HisA loop is highly conserved and extends over a part of the active site.<refname="aaRSbk">Francklyn, C., and Arnez, J.G. (2004) in Aminoacyl-tRNA Synthetases (Ibba, M.,Francklyn, C.,Cusack, S.. Eds.) Landes Publishing, Austin, TX</ref> Second, the glycine-rich β-strand (sequence AGGRYDGL preceding motif III) comprises the histidine binding pocket floor and wall. Finally, conserved side chains that make direct contact with histidine are Glu83 and Gly127 (motif II), which contact the α-amino and α-carbonyl functional groups, respectively, and Glu131 (motif II) and Tyr264, which make hydrogen bonds to the Nδ and Nε, respectively, of the imidazole ring.<refname="aaRS" />
