engleski

Role of serine ordering loop in atypical seryl-tRNA synthetase

Seryl-tRNA synthetases aminoacylate cognate tRNASer with serine. Recently determined crystal structure of methanogenic Methanosarcina barkeri SerRS revealed that binding of serine is accompanied by a notable conformational change in the enzyme active site bringing the serine ordering loop (SOL) in an ordered conformation. These induced fit rearrangements seem to be required for proper positioning of the carboxylate oxygen of serine (via direct contact with Gln400) for nucleophilic attack on the α – phosphate of ATP. We designed and kinetically characterized several mutants in SOL. Mutation of Gln400 did not have large effect on enzyme's efficiency suggesting that the additional contacts with SOL residues significantly participate in the positioning of serine. Surprisingly, mutation of His250 that is in indirect contact with serine mediated by Gln400 almost abolished enzyme's activity. However, gel-mobility shift analysis indicates that its importance might be in influencing binding of tRNA. Several mutants were designed to study dependence of enzyme's activity on the conformational flexibility of this region. We demonstrated that N-terminal α -helix has an important role in productive positioning of serine. Mutation of Pro395, located N-terminally to the helix and thus probably influencing its orientation, showed significant, but not drastic drop in enzyme's efficiency implying that there are other interactions involved in correct positioning of the helix.

flexible loop; seryl-tRNA synthetase; induced fit

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