Lacunae of understanding exist concerning the active site organization during the charging step of the aminoacylation reaction. We present here a molecular dynamics simulation study of the dynamics of the active site organization during charging step of subclass IIa dimeric SerRS from Thermus thermophilus (ttSerRS) bound with tttRNASer and dimeric ThrRS from Escherichia coli (ecThrRS) bound with ectRNAThr. The interactions between the catalytically important loops and tRNA contribute to the change in dynamics of tRNA in free and bound states, respectively. These interactions help in the development of catalytically effective organization of the active site. The A76 end of the tttRNASer exhibits fast dynamics in free State, which is significantly slowed down within the active site bound with adenylate. The loops change their conformation via multimodal dynamics (a slow diffusive mode of nanosecond time scale and fast librational mode of dynamics in picosecond time scale). The active site residues of the motif 2 loop approach the proximal bases of tRNA and adenylate by slow diffusive motion (in nanosecond time scale) and make conformational changes of the respective side chains via ultrafast librational motion to develop precise hydrogen bond geometry. Presence of bound Mg2+ ions around tRNA and dynamically slow bound water are other common features of both aaRSs. The presence of dynamically rigid Zinc ion coordination sphere and bipartite mode of recognition of ectRNAThr are observed.
Keywords: active site; aminoacyl tRNA synthetase (RS); dynamics; seryl tRNA synthetase (SerRS); threonyl tRNA synthetase (ThrRS); transfer RNA (tRNA).