InterPro domain: IPR002315

The aminoacyl-tRNA synthetases (also known as aminoacyl-tRNA ligases) catalyse the attachment of an amino acid to its cognate transfer RNA molecule in a highly specific two-step reaction [ 1 , 2 ]. These proteins differ widely in size and oligomeric state, and have limited sequence homology [ 3 ]. 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 [ 4 ]. Class II aminoacyl-tRNA synthetases share an anti-parallel beta-sheet fold flanked by alpha-helices [ 5 ], and are mostly dimeric or multimeric, containing at least three conserved regions [ 6 , 7 , 8 ]. 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, valine, and some lysine synthetases (non-eukaryotic group) belong to class I synthetases. The synthetases specific for alanine, asparagine, aspartic acid, glycine, histidine, phenylalanine, proline, serine, threonine, and some lysine synthetases (non-archaeal group), belong to class-II synthetases. 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 [ 9 ].

In eubacteria, glycine-tRNA ligase( 6.1.1.14 ) is an alpha2/beta2 tetramer composed of 2 different subunits [ 10 , 11 , 12 ]. In some eubacteria,in archaea and eukaryota, glycine-tRNA ligase is an alpha2 dimer, this family. It belongs to class IIc and is one of the most complex ligases. What is most interestingis the lack of similarity between the two types: divergence at the sequencelevel is so great that it is impossible to infer descent from common genes. The alpha (see IPR002310 ) and beta subunits (see IPR015944 ) also lack significant sequence similarity.However, they are translated from a single mRNA [ 13 ], and a single chain glycine-tRNA ligase from Chlamydia trachomatis has been found to have significant similarity with both domains, suggesting divergence from a single polypeptide chain [ 13 ].

The sequence and crystal structure of the homodimeric glyccine-tRNA ligase from Thermus thermophilus, shows that each monomer consists of an active site strongly resembling that of the aspartyl and seryl enzymes, a C-terminal anticodon recognition domain of 100 residues and a third domain unusually inserted between motifs 1 and 2 almost certainly interacting with the acceptor arm of tRNA(Gly). The C-terminal domain has a novel five-stranded parallel-antiparallel beta-sheet structure with three surrounding helices. The active site residues most probably responsible for substrate recognition, in particular in the Gly binding pocket, can be identified by inference from aspartyl-tRNA ligase due to the conserved nature of the class II active site [ 13 , 14 ].

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