Glutamine–tRNA ligase

Glutamine–RNA ligase (EC 6.1.1.18) is an enzyme that catalyzes the chemical reaction

ATP + L-glutamine + tRNA^Gln ${\displaystyle \rightleftharpoons }$ AMP + diphosphate + L-glutaminyl-tRNA^Gln

The 3 substrates of this enzyme are ATP, L-glutamine, and tRNA^Gln, whereas its 3 products are AMP, diphosphate, and L-glutaminyl-tRNA^Gln. The cycle of aminoacylation reaction is shown in the figure.

This enzyme belongs to the family of ligases, to be specific those forming carbon–oxygen bonds in aminoacyl-tRNA and related compounds. The systematic name of this enzyme class is L-glutamine:tRNA^Gln ligase (AMP-forming). Glutaminyl-tRNA synthetase or GlnRS is the primary name in use in the scientific literature. Other names that have been reported are:^[2]

• glutaminyl-transfer RNA synthetase,
• glutaminyl-transfer ribonucleate synthetase,
• glutamine-tRNA synthetase, and
• glutamate-tRNA ligase

In the eukaryotic cytoplasm and in some bacteria such as E. coli, glutaminyl-tRNA synthetase catalyzes glutamine-tRNA^Gln formation.^[3] However a two-step formation process is necessary for its formation in all archaebacteria and most eubacteria as well as most eukaryotic organelles.^[3] In these cases, a glutamyl-tRNA synthetase first mis-aminoacylates tRNA^Gln with glutamate. Glutamine-tRNA^Gln is then formed by transamidation of the misacylated glutamate-tRNA^Gln by the glutaminyl-tRNA synthase (glutamine-hydrolysing) enzyme.^[4] It is believed that glutaminyl-tRNA synethetases have evolved from the glutamyl-tRNA synthetase enzyme.^[5]

Aminoacyl tRNA synthetases are divided into two major classes based on their active site structure: class I and II.^[4] Glutaminyl-tRNA synthetase belongs to the class-I aminoacyl-tRNA synthetase family.

Of the glutaminyl-tRNA synthetases, the enzyme from E. coli is the most well studied structurally and biochemically.^[1] It is 553 amino acids long and is about 100 Å long. At the N-terminus, it has its catalytic active site with a Rossmann dinucleotide fold interacting with the 2′ OH of the final nucleotide of tRNA^Gln (A76), while the C-terminus interacts with the tRNAs anti-codon loop.^[1] The human glutaminyl-tRNA synthetase structure at N-terminus contains a two tandem nonspecific RNA binding regions, a catalytic domain, and two tandem anti-codon binding domains in the C-terminus.^[6]

The first crystal structure of a tRNA synthetase in complex with its cognate tRNA was that of the E. coli tRNA-Gln:GlnRS, determined in 1989 (PDB accession code (1GSG).^[7] This was also the first crystal structure of a nonviral protein:RNA complex.^[8] The purified enzyme was crystallized in complex with in vivo overexpressed tRNA^Gln.

As of late 2024, over 38 structures have been solved for this class of enzymes.^[9] Some of the PDB accession codes include PDB: 1EUQ​, PDB: 1EUY​, PDB: 1EXD​, PDB: 1GSG​, PDB: 1GTR​, PDB: 1GTS​, PDB: 1NYL​, PDB: 1O0B​, PDB: 1O0C​, PDB: 1QRS​, PDB: 1QRT​, PDB: 1QRU​, PDB: 1QTQ​, PDB: 1ZJW​, and PDB: 2HZ7​. The E. coli glutaminyl-tRNA synethetase structure complexed with its cognate tRNA, tRNA^Gln is depicted in the figure (accession number 1EUG.^[10]