The heme iron coordination of unfolded ferric and ferrous cytochrome c in neutral and acidic urea solutions. Spectroscopic and electrochemical studies.
The heme iron coordination of unfolded ferric and ferrous cytochrome c in the presence of 7-9 M urea at different pH values has been probed by several spectroscopic techniques including magnetic and natural circular dichroism (CD), electrochemistry, UV-visible (UV-vis) absorption and resonance Raman (RR). In 7-9 M urea at neutral pH, ferric cytochrome c is found to be predominantly a low spin bis-His-ligated heme center. In acidic 9 M urea solutions the UV-vis and near-infrared (NIR) magnetic circular dichroism (MCD) measurements have for the first time revealed the formation of a high spin His/H(2)O complex. The pK(a) for the neutral to acidic conversion is 5.2. In 9 M urea, ferrous cytochrome c is shown to retain its native ligation structure at pH 7. Formation of a five-coordinate high spin complex in equilibrium with the native form of ferrous cytochrome c takes place below the pK(a) 4.8. The formal redox potential of the His/H(2)O complex of cytochrome c in 9 M urea at pH 3 was estimated to be -0.13 V, ca. 100 mV more positive than E degrees ' estimated for the bis-His complex of cytochrome c in urea solution at pH 7.
Fedurco
Milan
M
Department of Chemistry, University of Geneva, 30 quai Ernest Ansermet, CH-1211 Geneva, Switzerland. milan_fedurco@freesurf.ch
Augustynski
Jan
J
Indiani
Chiara
C
Smulevich
Giulietta
G
Antalík
Marián
M
Bánó
Mikulás
M
Sedlák
Erik
E
Glascock
Mary C
MC
Dawson
John H
JH
D000818:Animals; D002942:Circular Dichroism; D003574:Cytochrome c Group / Q000737:chemistry* / Q000378:metabolism*; D004563:Electrochemistry; D005290:Ferric Compounds / Q000737:chemistry; D005296:Ferrous Compounds / Q000737:chemistry; D006418:Heme / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry; D006736:Horses; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000737:chemistry*; D007700:Kinetics; D010084:Oxidation-Reduction; D011489:Protein Denaturation; D012996:Solutions / Q000494:pharmacology; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman; D014508:Urea / Q000494:pharmacology*; D014867:Water / Q000737:chemistry
D003574:Cytochrome c Group; D005290:Ferric Compounds; D005296:Ferrous Compounds; D012996:Solutions; D014867:Water; D006418:Heme; D006639:Histidine; D014508:Urea; D007501:Iron