| Popis: |
Internuclear distances obtained from nuclear Overhauser effects were used in combination with a distance geometry algorithm to determine the conformation of Mg(alpha,beta-methylene)ATP bound to the Escherichia coli truncated methionyl-tRNA synthetase (delta MTS) both in the absence and presence of cognate and noncognate amino acids. Mg(alpha,beta-methylene)ATP, a nonhydrolyzable analog of ATP, was used to prevent hydrolysis of the nucleotide in the presence of either cognate or noncognate amino acids. Kinetic analysis showed that Mg(alpha,beta-methylene)ATP was a linear competitive inhibitor with respect to ATP in the ATP-pyrophosphate exchange reaction with a Ki = 1.2 mM. The pattern of internuclear Overhauser effects on Mg(alpha,beta-methylene)ATP bound to delta MTS was qualitatively consistent only with an anti glycosidic torsional angle, suggesting that the adenosine portion of the nucleotide is uniquely oriented in the binary enzyme-nucleotide complex. Nearly identical patterns of nuclear Overhauser effects were also observed in ternary complexes containing either cognate L-methionine or noncognate L-homocysteine amino acids. Distance geometry calculations permitted the range and conformational space of the allowed adenine-ribose glycosidic torsional angles in each of the complexes to be better defined and compared. Average adenine-ribose glycosidic torsional angles for enzyme-bound Mg(alpha,beta-methylene)ATP of -106 +/- 9 degrees, -99 +/- 11 degrees, and -97 +/- 11 degrees were determined for the delta MTS.Mg(alpha,beta-methylene)ATP, delta MTS.Mg(alpha,beta-methylene)ATP.L-methionine, and delta MTS.Mg(alpha,beta-methylene)ATP.L-homocysteine complexes, respectively. Comparison of the three enzyme-bound conformations showed that a single nucleotide structure having an adenine-ribose glycosidic torsional angle of -98 degrees with a 3'-endo to O4'-exo ribose sugar pucker was, within error, consistent with the experimental internuclear distances obtained in all three complexes. The nearly identical anti glycosidic torsional angles observed in all three complexes demonstrates that the conformation of the adenosine moiety of the enzyme-bound nucleotide is not sensitive to the presence or the nature of the amino acid bound at the aminoacyladenylate site. Therefore, conformational changes known to occur in the methionyl-tRNA synthetase upon ligand binding appear not to alter the bound conformation of the nucleotide. Information on the conformation and arrangement of substrates bound at the aminoacyladenylate site of delta MTS is necessary for understanding the molecular mechanisms involved in amino acid activation and discrimination. |
