| Abstract
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The structure and dynamic properties of OMTKY3 at pH 2.0 have been determined by NMR spectroscopy and compared with those at pH 4.0. H, C, and N NMR resonances of OMTKY3 at pH 2.0 were assigned by multi-dimensional homonuclear and heteronuclear NMR techniques. Differences in the proton, nitrogen, and carbon chemical shifts of OMTKY3 at pH 4.0 and pH 2.0 indicate pH-induced changes in the secondary structure. The solution structure of OMTKY3 at pH 2.0 was determined by a distance geometry/simulated annealing approach on the basis of 1009 distance constraints, 33 (J{\rm N\sp\alpha}) coupling constants, 17 1 dihedral angles, and 16 hydrogen-bond constraints. Stereospecific assignments were determined for 15 usd\betausd-methylene pairs. OMTKY3 at pH 2.0 retains the global fold and overall secondary structure of the structure at pH 4.0, but exhibits significant differences in certain backbone torsion angles and hydrogen-bonds. The pH dependence of H and N signals of OMTKY3 was determined from two-dimensional NMR spectra collected at pH values between 1 and 10. The results indicate the presence of four clusters of residues whose structure is perturbed when various side chains are protonated. The pH titration results are consistent with observed changes in the structure of OMTKY3 between pH 4.0 and 2.0. OMTKY3 has been found to exist as two interconverting conformers at low pH. At pH 2.0, the relative populations of the major and minor forms are 90% and 10%. Since 50% of the backbone NMR signals show doubling, the conformational change affects much of the structure, but since the shift differences are small, the conformations are expected to be quite similar. The residues that show the largest chemical shift changes are not clustered near any of the three proline residues or near any of the three disulfide bridges; rather, they involve residues whose side chains point into the hydrophobic core of the molecule which includes Tyr 31 and Phe 37. Thus, it is possible that the source of the heterogeneity is two alternative hydrogen-bonding patterns for the hydroxyl of Tyr 31, rather than cis/trans isomerization about Xxx-Pro peptide bond, or alternative conformations for a disulfide bridge.
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