Tryptophanyl-tRNA synthetase from Bacillus subtilis. Characterization and role of hydrophobicity in substrate recognition.
The tryptophanyl-tRNA synthetase from Bacillus subtilis was purified to homogeneity and characterized. It has an alpha 2 subunit structure and a molecular weight of 77,000. Tryptophanyl-tRNA synthetase does not catalyze any significant proofreading. It activates tryptophan as well as the three fluorinated analogues, DL-4-fluoro-, DL-5-fluoro-, or DL-6-fluorotryptophan (4F-, 5F-, and 6F-Trp), in the ATP-pyrophosphate exchange reaction. In the aminoacylation reaction, the fluorotryptophans act as competitive inhibitors of Trp. Their relative activities follow the same order in both reactions: Trp greater than 4F-Trp greater than 6F-Trp greater than 5F-Trp. This order is the inverse of the order of relative hydrophobicities of these compounds, pointing to the importance of hydrophobic interactions in the selective recognition by tryptophanyl-tRNA synthetase among this group of substrates. To define the physical basis of the relative hydrophobicities, the crystallographic structure of 4F-Trp was determined and compared to that of trptophan. Charge distributions calculated for tryptophan and its different fluoroanalogues on the basis of molecular structures were supported by their carbon-13 NMR spectra. Correlations between charge distributions and relative hydrophobicities suggest that the polarity of the C-F bond represents an underlying factor determining the hydrophobicities of 4F-, 5F-, and 6F-Trp, thus relating tryptophanyl-tRNA synthetase selectivity toward tryptophan and its fluoroanalogues directly to their electronic configurations.
Xu
Z J
ZJ
Department of Biochemistry, University of Toronto, Canada.
Love
M L
ML
Ma
L Y
LY
Blum
M
M
Bronskill
P M
PM
Bernstein
J
J
Grey
A A
AA
Hofmann
T
T
Camerman
N
N
Wong
J T
JT
D000249:Adenosine Monophosphate / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000595:Amino Acid Sequence; D000604:Amino Acyl-tRNA Synthetases / Q000378:metabolism*; D001412:Bacillus subtilis / Q000201:enzymology*; D001667:Binding, Competitive; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008969:Molecular Sequence Data; D008970:Molecular Weight; D010710:Phosphates / Q000378:metabolism; D012364:RNA, Transfer, Trp / Q000378:metabolism; D014364:Tryptophan / Q000031:analogs & derivatives / Q000378:metabolism; D014369:Tryptophan-tRNA Ligase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014961:X-Ray Diffraction
D010710:Phosphates; D012364:RNA, Transfer, Trp; C039390:4-fluorotryptophan; C026927:5-fluorotryptophan; C001244:6-fluorotryptophan; D000249:Adenosine Monophosphate; D014364:Tryptophan; D000255:Adenosine Triphosphate; D000604:Amino Acyl-tRNA Synthetases; D014369:Tryptophan-tRNA Ligase