Arg side-chain-backbone interactions evidenced in model peptides by 17O-NMR spectroscopy

Phosphorylation-dependent conformational transition of the cardiac specific N-extension of troponin I in cardiac troponin

We present here the solution structure for the bisphosphorylated form of the cardiac N-extension of troponin I (cTnI(1-32)), a region for which there are no previous high-resolution data. Using this structure, the X-ray crystal structure of the cardiac troponin core, and uniform density models of the troponin components derived from neutron contrast variation data, we built atomic models for troponin that show the conformational transition in cardiac troponin induced by bisphosphorylation. In the absence of phosphorylation, our NMR data and sequence analyses indicate a less structured cardiac N-extension with a propensity for a helical region surrounding the phosphorylation motif, followed by a helical C-terminal region (residues 25-30). In this conformation, TnI(1-32) interacts with the N-lobe of cardiac troponin C (cTnC) and thus is positioned to modulate myofilament Ca2+-sensitivity. Bisphosphorylation at Ser23/24 extends the C-terminal helix (residues 21-30) which results in weakening interactions with the N-lobe of cTnC and a re-positioning of the acidic amino terminus of cTnI(1-32) for favorable interactions with basic regions, likely the inhibitory region of cTnI. An extended poly(L-proline)II helix between residues 11 and 19 serves as the rigid linker that aids in re-positioning the amino terminus of cTnI(1-32) upon bisphosphorylation at Ser23/24. We propose that it is these electrostatic interactions between the acidic amino terminus of cTnI(1-32) and the basic inhibitory region of troponin I that induces a bending of cTnI at the end that interacts with cTnC. This model provides a molecular mechanism for the observed changes in cross-bridge kinetics upon TnI phosphorylation.

The (17)O-NMR shielding range and shielding time scale and detection of discrete hydrogen-bonded conformational states in peptides

The (17)O-NMR shielding range and shielding time scale due to hydrogen-bonding interactions in peptides are critically evaluated relative to those of (1)H-NMR. Furthermore, the assumptions and conclusions in previous (17)O-NMR studies on the detection of discrete conformational states in peptides (V. Tsikaris et al., Biopolymers, 2000, Vol. 53, pp. 135-139) are reconsidered. Consistent examination of the method demonstrates that although (17)O shieldings of peptide oxygens are very sensitive to hydrogen bonding interactions, the (17)O-NMR shielding time scale is not advantageous compared to that of (1)H-NMR, and thus it is not suitable for the detection of discrete hydrogen-bonded conformational states in peptides. (17)O-NMR spectroscopy is prone to interpretation errors due to the formation of (17)O-labeled impurities during the synthetic procedures (A. Steinschneider et al., International Journal of Peptide and Protein Research, 1981, Vol. 18, pp. 324-333).

Cooperative helix stabilization by complex Arg-Glu salt bridges

Among the interactions that stabilize the native state of proteins, the role of electrostatic interactions has been difficult to quantify precisely. Surface salt bridges or ion pairs between acidic and basic side chains have only a modest stabilizing effect on the stability of helical peptides or proteins: estimates are roughly 0.5 kcal/mol or less. On the other hand, theoretical arguments and the occurrence of salt bridge networks in thermophilic proteins suggest that multiple salt bridges may exert a stronger stabilizing effect. We show here that triads of charged side chains, Arg(+)-Glu(-)-Arg(+) spaced at i,i+4 or i,i+3 intervals in a helical peptide stabilize alpha helix by more than the additive contribution of two single salt bridges. The free energy of the triad is more than 1 kcal/mol in excess of the sum of the individual pairs, measured in low salt concentration (10 mM). The effect of spacing the three groups is severe; placing the charges at i,i+4 or i,i+3 sites has a strong effect on stability relative to single bridges; other combinations are weaker. A conservative calculation suggests that interactions of this kind between salt bridges can account for much of the stabilization of certain thermophilic proteins.