STRUCTURES OF TRYPTOPHANYL-TRNA SYNTHETASE-LIGAND COMPLEXES Xin Huang and Charles W. Carter, Jr., Department of Biochemistry and Biophysics, CB 7260 University of North Carolina at Chapel Hill, Chapel Hill, NC 27514 USA

The crucial role of aminoacyl-tRNA synthetases (aaRSs) in maintaining the fidelity of the genetic code has motivated intense study of the sources of their specificity for cognate amino acids and tRNAs. Conclusions of the structural basis for coupling of specificity and catalysis of aaRSs can best be drawn by examining a series of different complexes involving the same enzyme. Bacillus stearothermophilus Tryptophanyl-tRNA synthetase (TrpRS) provides such a series. The success of the first structure in this series, that of TrpRS complexed with tryptophanyl-5'AMP has inspired us to study the structures of the complexes with other ligands such as Tryptophan (substrate) and 5'-O-[N-(L-tryptophanyl)sulfamoyl]adenosine (a stable analog of the tryptophanyl adenylate intermediate). We have synthesized 5'-O-[N-(L-tryptophanyl)sulfamoyl]adenosine and inhibition measurements showed that it is a strong inhibitor with K_i in the nanomolar range. Co-crystallization of this analog with TrpRS is underway. Translation and rotation searches are being carried out on the data from monoclinic crystals (space group P2₁) grown in the presence of tryptophan . Contrary to previous expectation (Carter, et al., 1990, Acta Cryst. A46:57-68), our initial results revealed that what we previously interpreted as a non-crystallographic three-fold axis is actually a three-fold screw axis. We hope to make progress in solving these two structures.

(This work supported by NIH GM48519-02)