title: Tryptophanyl-tRNA synthetase crystal structure reveals an unexpected homology to tyrosyl-tRNA synthetase.
abstract: BACKGROUND
  Tryptophanyl-tRNA synthetase (TrpRS) catalyzes activation of tryptophan by ATP and transfer to tRNA(Trp), ensuring translation of the genetic code for tryptophan. Interest focuses on mechanisms for specific recognition of both amino acid and tRNA substrates.
  
  
  RESULTS
  Maximum-entropy methods enabled us to solve the TrpRS structure. Its three parts, a canonical dinucleotide-binding fold, a dimer interface, and a helical domain, have enough structural homology to tyrosyl-tRNA synthetase (TyrRS) that the two enzymes can be described as conformational isomers. Structure-based sequence alignment shows statistically significant genetic homology. Structural elements interacting with the activated amino acid, tryptophanyl-5'AMP, are almost exactly as seen in the TyrRS:tyrosyl-5'AMP complex. Unexpectedly, side chains that recognize indole are also highly conserved, and require reorientation of a 'specificity-determining' helix containing a conserved aspartate to assure selection of tryptophan versus tyrosine. The carboxy terminus, which is disordered and therefore not seen in TyrRS, forms part of the dimer interface in TrpRS.
  
  
  CONCLUSIONS
  For the first time, the Bayesian statistical paradigm of entropy maximization and likelihood scoring has played a critical role in an X-ray structure solution. Sequence relatedness of structurally superimposable residues throughout TrpRS and TyrRS implies that they diverged more recently than most aminoacyl-tRNA synthetases. Subtle, tertiary structure changes are crucial for specific recognition of the two different amino acids. The conformational isomerism suggests that movement of the KMSKS loop, known to occur in the TyrRS transition state for amino acid activation, may provide a basis for conformational coupling during catalysis.
authors.affiliation: Department of Biochemistry and Biophysics, University of North Carolina at Chapel Hill 27599-7260, USA.
authors.forename: S
authors.initials: S
authors.lastname: Doublié
authors.forename: G
authors.initials: G
authors.lastname: Bricogne
authors.forename: C
authors.initials: C
authors.lastname: Gilmore
authors.forename: C W
authors.initials: CW
authors.lastname: Carter
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D003001:Cloning, Molecular; D003196:Computer Graphics; D018360:Crystallography, X-Ray; D001411:Geobacillus stearothermophilus / Q000201:enzymology; D046911:Macromolecular Substances; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry; D017386:Sequence Homology, Amino Acid; D014369:Tryptophan-tRNA Ligase / Q000737:chemistry*; D014448:Tyrosine-tRNA Ligase / Q000737:chemistry*
chemical_list: D046911:Macromolecular Substances; D011994:Recombinant Proteins; D014448:Tyrosine-tRNA Ligase; D014369:Tryptophan-tRNA Ligase