A water-mediated and substrate-assisted aminoacylation mechanism in the discriminating aminoacyl-tRNA synthetase GlnRS and non-discriminating GluRS.

Glutaminyl-tRNA synthetase (GlnRS) catalyzes the aminoacylation of glutamine to the corresponding tRNAGln . However, most bacteria and all archaea lack GlnRS and thus an indirect noncanonical aminoacylation is required. With the assistance of a non-discriminating version of Glutamyl-tRNA synthetases (ND-GluRS) the tRNAGln is misaminoacylated by glutamate. In this study, we have computationally investigated the aminoacylation mechanism in GlnRS and ND-GluRS employing Molecular Dynamics (MD) simulations, Quantum Mechanics (QM) cluster and Quantum Mechanics/Molecular Mechanics (QM/MM) calculations. Our investigations demonstrated the feasibility of a water-mediated, substrate-assisted catalysis pathway with rate limiting steps occurring at energy barriers of 25.0 and 25.4 kcal mol-1 for GlnRS and ND-GluRS, respectively. A conserved lysine residue participates in a second proton transfer to facilitate the departure of the adenosine monophosphate (AMP) group. Thermodynamically stable (-29.9 and -9.3 kcal mol-1 for GlnRS and ND-GluRS) product complexes are obtained only when the AMP group is neutral.