NMR investigation of the alkaline-like conformational transition of horse heart cytochrome c in the presence of exogenous thiazole

The conformational transition of horse heart cyt c in the presence of exogenous thiazole is investigated by NMR spectroscopy. Surprisingly, besides the native form and the ligand-bound form, another species (species A) exists at neutral pH. Titration of thiazole at alkaline condition indicates that this form has Lys as the axial ligands and in fact is one of alkaline cyt c conformers. At a high concentration of thiazole, species A has a dominant population at a pH of approximately 7. It is the first time that a pure alkaline conformer of cyt c is obtained at neutral pH. The observed NOEs show that species A maintains the overall protein fold although large structural rearrangements are expected in the distal pocket. Furthermore, the different effect of imidazole, pyridine and thiazole on the alkaline transformation of cyt c is discussed.