GLYCYL-TRANSFER-RNA SYNTHETASE

Glycyl-tRNA synthetase, a class II aminoacyl-tRNA synthetase, catalyze s the synthesis of glycyl-tRNA, which is required to insert glycine in to proteins. In a side reaction the enzyme also synthesizes dinucleosi de polyphosphates, which probably participate in regulation of cell fu nctions. Glycine is the smallest amino acid occurring in natural prote ins, probably established as a protein component very early in evoluti on. Besides the amino and the carboxyl groups there is no functional g roup in the molecule. Alanine, the amino acid which is structurally mo st similar to glycine, possesses an additional methyl group as 'side c hain'. Glycyl-tRNA synthetase is one of the few synthetases which exhi bit different oligomeric structures in different organisms (alpha(2) b eta(2) and alpha(2)), The alpha(2) beta(2) enzymes exhibit similaritie s to PheRS (also an alpha(2) beta(2) enzyme). The alpha(2) forms belon g to the subclass Ila enzymes with regard to sequence homologies. In e ukaryotes the polypeptide is weakly associated with multienzyme comple xes consisting of aminoacyl-tRNA synthetases. In the aminoacylation re action a 'half-of-the-sites' mechanism as found for GlyRS from Bombyx mori is probably used by all glycyl-tRNA synthetases under in vivo con ditions, Essentially, tRNA(Gly) is recognized by GlyRS through standar d identity elements in the anticodon region and in the acceptor stem, The last three facts may indicate that GlyRS is an enzyme which still possesses properties of a primordial aminoacyl-tRNA synthetase. Nine g enes of glycyl-tRNA synthetases from six organisms have been sequenced . They encode synthetase subunits of chain lengths ranging from 300 - 700 amino acids. One crystal structure, that of the alpha(2) enzyme fr om Thermus thermophilus, has also been determined, The two subunits ea ch possess three domains: the active site resembling that of aspartyl and seryl enzymes, a C-terminal anticodon recognition domain, and one domain which almost certainly interacts with the acceptor stem of tRNA (Gly). Antibodies against glycyl-RNA synthetase occur in the sera of p atients suffering from polymyositis and interstitial lung disease.