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 [PMID:2203971]. The 20 aminoacyl-tRNA synthetases are divided into two classes, I and II. Class I aminoacyl-tRNA synthetases contain a characteristic Rossman fold and are mostly monomeric [PMID:10673435], while class II aminoacyl-tRNA synthetases share an anti-parallel beta-sheet formation, flanked by alpha-helices [PMID:8364025], and are mostly dimeric or multimeric. 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. The synthetases specific for alanine, asparagine, aspartic acid, glycine, histidine, lysine, phenylalanine, proline, serine, and threonine belong to class-II synthetases [PUB00015156].

The 10 class I synthetases are considered to have in common the catalytic domain structure based on the Rossmann fold, which is totally different from the class II catalytic domain structure. The class I synthetases are further divided into three subclasses, a, b and c, according to sequence homology. tRNA binding involves an alpha-helical structure that is conserved between class I and class II synthetases.

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 [PMID:2203971]. The 20 aminoacyl-tRNA synthetases are divided into two classes, I and II. Class I aminoacyl-tRNA synthetases contain a characteristic Rossman fold and are mostly monomeric [PMID:10673435], while class II aminoacyl-tRNA synthetases share an anti-parallel beta-sheet formation, flanked by alpha-helices [PMID:8364025], and are mostly dimeric or multimeric. 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. The synthetases specific for alanine, asparagine, aspartic acid, glycine, histidine, lysine, phenylalanine, proline, serine, and threonine belong to class-II synthetases [PUB00015156].

The 10 class I synthetases are considered to have in common the catalytic domain structure based on the Rossmann fold, which is totally different from the class II catalytic domain structure. The class I synthetases are further divided into three subclasses, a, b and c, according to sequence homology. tRNA binding involves an alpha-helical structure that is conserved between class I and class II synthetases.

Class-II tRNA synthetases do not share a high degree of similarity, however at least three conserved regions are present [PMID:8274143, PMID:2053131, PMID:1852601].

Class-II tRNA synthetases do not share a high degree of similarity, however at least three conserved regions are present [PMID:8274143, PMID:2053131, PMID:1852601].

Phenylalanyl-tRNA synthetase (EC: 6.1.1.20) is an alpha2/beta2 tetramer composed of 2 subunits that belongs to class IIc. In eubacteria, a small subunit (pheS gene) can be designated as beta (E. coli) or alpha subunit (nomenclature adopted in InterPro). Reciprocally the large subunit (pheT gene) can be designated as alpha (E. coli) or beta (see INTERPRO: IPR004531 and INTERPRO: IPR004532). In all other kingdoms the two subunits have equivalent length in eukaryota, and can be identified by specific signatures. The enzyme from Thermus thermophilus has an alpha 2 beta 2 type quaternary structure and is one of the most complicated members of the synthetase family. Identification of phenylalanyl-tRNA synthetase as a member of class II aaRSs was based only on sequence alignment of the small alpha-subunit with other synthetases [PMID:8199244].