title: Ion binding to cytochrome c studied by nuclear magnetic quadrupole relaxation.
abstract: The enhancement of the 35Cl- transverse relaxation rate on binding of chloride ions to oxidized and reduced cytochrome c has been studied under conditions of variable sodium chloride concentration, temperature, pH, sodium phosphate, iron hexacyanide, and sodium cyanide concentration. The results revealed the presence of a strong binding site(s) for chloride in both oxidized and reduced cyt c, with a higher affinity in ferrocytochrome c. Competition experiments suggest that these sites also bind iron hexacyanide and phosphate. Cyanide binding to the iron in ferricytochrome c at alkaline and neutral pH was shown to decrease the binding of chloride. The pH dependence of the 35Cl- relaxation rate has been fitted by using literature pK values for ionizable groups. No indications of Na+ binding to oxidized and reduced cytochrome c have been observed by using 23Na+ NMR. Our results suggest that chloride is bound near the exposed heme edge and that the surface structure or dynamics in this region are different in the two oxidation states.
mesh_terms: D002712:Chlorides; D003486:Cyanides; D003574:Cytochrome c Group; D006863:Hydrogen-Ion Concentration; D007501:Iron; D009682:Magnetic Resonance Spectroscopy; D008433:Mathematics; D011485:Protein Binding; D011487:Protein Conformation; D013696:Temperature
chemical_list: D002712:Chlorides; D003486:Cyanides; D003574:Cytochrome c Group; D007501:Iron