pH-induced changes in Rhodospirillum rubrum cytochrome c2 and subsequent renaturation: an 15N NMR study

The 15N-enriched ferrocytochrome c2 from Rhodospirillum rubrum was studied by 15N NMR at different solvent pH values. The mobility and chemical shift of the N-terminal glutamic acid (335.4 ppm at pH 5.1) were found to depend on pH. It was least mobile between pH 8 and 9.0, which is explained in terms of pH-dependent conformational changes and formation of salt linkages and/or hydrogen bonds. The resonances of the lysine side chains are centered around 341.7 ppm at low pH and move upfield with pH by about 8.4 ppm with pKa values of 10.8. The exchange rates of the epsilon NH protons are lowest near their pKa values. The protein is very stable in the pH range between 4.9 and 10.0 but unfolds abruptly at pH 10.5-11. Denaturation was verified by the measurement of several parameters by NMR. The renaturation of the protein demonstrates that the folding begins with reformation of heme coordination and establishment of a hydrophobic core, followed by positioning of side chains and peptide backbones linking the nucleation centers. The repositioning processes had time scales of minutes to hours in contrast to the reported values of seconds in some studies.