Monitoring the conformational flexibility of cytochrome c at low ionic strength by 1H-NMR spectroscopy

Horse heart cytochrome c at pH 7 and low ionic strength is present as two conformers, as evidenced by H-1-NMR spectroscopy. The two structures have been calculated using NOE and pseudocontact shift constraints. They have the same folding patterns and are essentially equal, within the rmsd of the families. The two average structures have rmsd values of 0.049 nm and 0.093 nm for the backbone and the heavy atoms, respectively. Such a difference has been analyzed through a detailed analysis of the NOEs. It appears that the species at low ionic strength differs from the species present at high ionic strength by the displacement of some external residues, such as Gln16, Ile81 and Glu90. Other changes are monitored by the chemical shifts but they cannot be quantified at the present level of resolution. Ionic-strength-dependent structural rearrangements may be relevant with respect to the problem of molecular recognition.