InterPro: IPR004522 Asparaginyl-tRNA synthetase, class IIb

The aminoacyl-tRNA synthetases (EC:6.1.1.) catalyse the attachment of an amino acid to its cognate transfer RNA molecule in a highly specific two-step reaction. These proteins differ widely in size and oligomeric state, and have limited sequence homology [1]. The 20 aminoacyl-tRNA synthetases are divided into two classes, I and II. Class I aminoacyl-tRNA synthetases contain a characteristic Rossman fold catalytic domain and are mostly monomeric [2]. Class II aminoacyl-tRNA synthetases share an anti-parallel beta-sheet fold flanked by alpha-helices [3], and are mostly dimeric or multimeric, containing at least three conserved regions [4, 5, 6]. However, tRNA binding involves an alpha-helical structure that is conserved between class I and class II synthetases. In reactions catalysed by the class I aminoacyl-tRNA synthetases, the aminoacyl group is coupled to the 2'-hydroxyl of the tRNA, while, in class II reactions, the 3'-hydroxyl site is preferred. The synthetases specific for arginine, cysteine, glutamic acid, glutamine, isoleucine, leucine, methionine, tyrosine, tryptophan and valine belong to class I synthetases; these synthetases are further divided into three subclasses, a, b and c, according to sequence homology. The synthetases specific for alanine, asparagine, aspartic acid, glycine, histidine, lysine, phenylalanine, proline, serine, and threonine belong to class-II synthetases [7].

AsparaginyltRNA synthetase (EC:6.1.1.22) is an alpha2 dimer that belongs to class IIb. There is a striking similarity between asparaginyl-tRNA synthetases and archaeal/eukaryotic type aspartyl-tRNA synthetases (IPR004523) and a striking divergence of bacterial type aspartyl-tRNA synthetases (IPR004524). This family, AsnS, represents asparaginyl-tRNA synthetases from the three domains of life. Some species lack this enzyme and charge tRNA(asn) by misacylation with Asp, followed by transamidation of Asp to Asn.